the latter being limited only by the practical speed
of the machine and ability to supply it with clay.
It has been found, according to the Scientific
American, that in practical working a speed that
will turn out sixty to seventy bricks per minute is
about what can be done with two men to shovel in
the clay, no attendance being required except
to shovel the clay into the pugmill and hack up
the bricks as they are carried away by an endless

apron.

The pugmill operates precisely like the old pugmill, tempering the clay before pressing it in the moulds. The pressure is adjustable to compress the bricks more or less, so that the proper density may be secured. The bricks may be pressed so as to be hacked up immediately. The commonest clay may be used, and it is claimed that as perfectly formed bricks are made on it as can be done with the same quality of clay on a hand machine, that on subsequent baking the poorer sorts of clay will yield on the average enough perfectly-faced bricks for the fronts of ordinary buildings, while with the finer clays bricks of the finest finish may be made at the same rate as coarser ones. The machine is driven by a powerful and heavy gear placed below and on the shaft of the pugmill. This gear is driven by a pinion on the pulley shaft, which takes its power from an engine or water-wheel. If the mill is impelled by a lever sweep from the top of the shaft no toothed gears are necessary.

In the body of the large gear are formed, at proper intervals, moulds which receive the tempered clay from the pugmill as it is forced down into them by a powerful propeller screw fastened to the shafts below the knives, which do the grinding and mixing. In each of these moulds-which are steel-faced-runs a steel or brass-faced follower, having at the bottom a roller wheel, which rolls up in a fixed track, elevated at the proper points, the action of which is to force the follower up, at the proper times, to compress the bricks, and also to thrust them out of the moulds when they have arrived at the proper point of the revolution. As the bricks are thrust up out of the moulds they pass on to an endless belt or apron, and are conveyed away to be hacked up. There is nothing whatever about the machine which can get out of repair. Its parts are few in number, and so constituted and arranged that they can be made of a weight and strength which insure their power of endurance, and no skilled attendance is required to run it. All the work can be done by boys except the shoveling of the clay. Any complication liable to interfere with the perfect and permanent operation of the machine or to lessen durability is avoided.

SCIENTIFIC SOCIETIES.

THE OBSERVING ASTRONOMICAL SOCIETY.
Report of Observations made during the period from
August 7th to September 6th, 1870, inclusive.

OLAR PHENOMENA.-Mr. Elger, of Bedford,

Maximum number of groups on disc = 13
(August 29, 21h. 15m.).
Minimum number of groups on disc = 4 (August
20, 4h. 15m.).

Mr. William F. Denning, of Bristol, observed the
sun with his 3in. refractor on August 28, and re-
ports that on this date four large groups of spots
were visible in the sun's northern hemisphere. In
the N.E. quadrant two large groups were per-
ceptible, lying just above the equator. In the N.W.
quadrant an irregular scattered group was seen near
the limb, and another group near the centre of the
disc was very conspicuous. The S.W. quadrant
was free from visible spots, while the S.E. quadrant
contained three small groups.

AURORA BOREALIS. Mr. H. Michell Whitley writes that on August 20th he observed a brilliant display of this phenomenon. From 11h. 30m. to 12h. it was very well defined. Straight beams of light ascended from the N. horizon to an altitude of about 35°. "These streamers faded, and appeared in other parts." Mr. Henry Ormestier, of Manchester, also witnessed the occurrence. He says: "I first observed it at 11h. 40m., but from its appearance it must have been visible for some time previons. I determined the extent of its base to be as far as W. by N. to N.E. by N. From between these points streamers shot forth in rapid succession to a very considerable altitude; a great many of them reaching to the zenith of my place of observation. Some of these streamers were very brilliant, particularly one which at 11h. 50m. shot forth from a point just beneath the Pointers, in a direction towards the polar star and on to the zenith. I should think this stream of light to have been of about five minutes' duration, during which time its colour changed from a dark straw to a yellowish tinge. At 12h. 10m. there was quite an auroral arch, whose centre was towards the magnetic pole, and extending from the before-mentioned points to an altitude of at least 40°. The brightness of this arch increased until about 12h. 14m., when it was exceedingly brilliant. During the whole of the time the sky was very clear with the exception of a reddish glow, of which the Aurora was the cause.

actions, and that our knowledge of the ultimate constitu tion of matter upon which those laws are based is but of the most elementary nature." That accords precisely with what I have said. "T. A." seems to insist on seeing in the views of men of science respecting force and matter a definiteness which has in reality no existence.

"T.A." offers an opinion about the zodiacal light without giving any evidence in its favour. I, in re turn, without assigning any reason, express my opinion that "T. A." is wholly mistaken. He now has his quis RICHARD A. PROCTOR. pro quo.

THE SATELLITES OF URANUS (4996), "OURS,"
CAUSE OF THE EARTH'S ROTATION.
[477] SIR,-"Etudiant" has not quite understood
me. I expect that all his notes are correct; and his
views respecting their meaning are also, I think,
correct. I believe that the discrepancy which perplexes
him is due simply to the fact that Mr. Lassell made, 10
1851, an unsuccessful attempt to determine the position
in which the satellites attained their greatest elonga
tion. The error which, undoubtedly, exists in Mr.
Lassell's results, assuming "Etudiant's" notes to be
correct, is not at all surprising, considering the nature
of the observation.

Is there any reason why "science should be made of such stern stuff" as "Inductorium suggests? For my own part (and I am glad to see that many of our most valued correspondents are of the same mind)! think a reference to our journal, our MECHANIC, and so on, neither silly nor puerile. A feeling of good fellowship is not an undesirable thing among mer however the case may stand with inductoria and machines generally.

A correspondent who modestly calls himself " Moke," is desirous that I should state my views res specting the cause of the earth's rotation. That is easily done. I do not know the cause of the earth' RICHARD A. PROCTOR. rotation. P.S.-I do not claim this opinion as altogether original.

SCREW POWER.

[478] SIR,-In answer to "J. K. P.'s" letter (441), METEORS.-Very few meteors were observed on it is probable his results are theoretically correct. I or about August 10th. Mr. Edmund Neison, of did not very carefully read his letter, having no great London, saw nine on the 10th, three on the 11th, liking for researches into the theoretical principles of and two on the 12th. The Rev. S. J. Johnson the screw, wedge, and some other mechanical power watched the sky from 10h. 45m. to 11h. 46m. on-simply because theory and practice happen to have in the same date, but only succeeded in discovering their case no bearing on each other. The remark alo one. Mr. H. Michell Whitley, of Penarth, Truro, the differentiation of tan i cot (i + p) caught my eve witnessed the appearance of two meteors' on the and as I have a strong belief that many who might evening of August 29th. The first was visible at make good mathematicians are deterred from the study 10h. 25m., and was accompanied with a faint train. of mathematics by a belief in altogether imaginary It passed downwards below Corona Borealis. The difficulties, I ventured to point out the real simplicity of the problem. second was seen at 10h. 30m. to the west of Aquarius. Both were equal in brilliancy to a 1st mag; star. On the 30th the Rev. S. J. Johnson observed the train of a very brilliant meteor. From the appearance of this train it was evident that the meteor must have become invisible a degree or so to the west of K. Draconis, and have ended a degree or two to the east of a Draconis.

LETTERS TO THE EDITOR.

[We do not hold ourselves responsible for the opinions
of our correspondents. The EDITOR respectfully
requests that all communications should be drawn
up as briefly as possible.]

All communications should be addressed to the
EDITOR of the ENGLISH MECHANIC, 31, Tavistock-
street, Covent Garden, W.C.

All Cheques and Post Office Orders to be made payable to J. PASSMORE EDWARDS.

It will be noticed that I wrote down (at p. 12) the differential co-efficient of tan i cot (i+p) without any steps at all. I cannot make any steps come in betwees the expression and its differential co-efficient. If, indeed, we go back to first principles-to the very de nition of a differential co-efficient-we may occupy & page of letter-paper with the solution of the problem But if we have gone beyond first principles, we should be able to write the co-efficient at once. We take the known differential of each part, multiply severally all the other part, and add the products. This invaiv merely writing down the expression given at p. 12. RICHARD A. PROCTOR

SCREW-PROPELLING.

[479] SIR,-Permit me to thank your corresponde (370, p. 612), for his information regarding Jos Ressel's claims as inventor of the propeller. Member of the ENGLISH MECHANIC Society" had reason to fear any offence being given to me by ta mention of Ressel's priority to Ericsson. The truth so gently and so modestly told, could not but be agres able.

E. KERNAN.

Sites. Observers of solar phenomena have seldom an opportunity of witnessing such a fine outbreak of spots as that which took place during the last fortnight of August. After the disappearance of the large group observed in the south hemisphere (about July 31st) a comparative lull in solar activity ensued, lasting thirteen days. The spots which appeared during this interval presented no remarkable features, and were mostly confined to the south hemisphere. On the 17th, in the north hemisphere, a large scattered group was observed, which a few days before had consisted of a congeries of minute specks. On the 18th it was 2'55" in diameter, and was followed by another group 2′26′′ in length; both these groups diminished very rapidly after the 19th. On the 20th the two largest groups on the disc were nearly central, one of them was 36" in diameter, the other 54". Cloudy days intervened between the 21st and 24th. On the latter date the first indications of the approaching outburst were remarked. At 4h. 30m. there were three immense groups in the north hemisphere, extending from the centre of the disc to the east limb. The preceding group, which was made up of very light and ill-defined penumbra, enclosing upwards of sixty separate black spots, measured 3'10" x 1'49"; the second group was 1′20′′ in length, the third was too near the limb to be satisfactorily measured. From the 26th to the end of the month the north maculose zone was completely when speaking of any Letter previously inserted will In order to facilitate reference, correspondents crowded with groups and isolated spots, while the oblige by mentioning the number of the Letter, as corresponding south zone contained only punc-well as the page on which it appears. By placing the cam-disc nearer the leading scre tures and small clusters. The following are the "J. K. P.'s" pins may certainly be shorter, bata lengths of the three largest groups observed on the there is but little strain on these pins and no reaso 29th,-3'6", 2′26′′, and 1'57". The spotted zone able limit to their size, I imagine the redaction could be seen with the naked eye, protected by an ordinary dark glass, at noon, on the 28th. It had Professor Roscoe's, which appeared either in the same [476] SIR,-I will simply quote certain remarks of their length is a very immaterial improvemen Under the circumstances that obtain in J. K. P. the appearance of a dusky belt parallel to the sun's MECHANIC as that containing my reply to "T. A.," or case it is decidedly a convenience to be able to remos equator. in the following:-"It will not take an observer long to the square plate and get access to the mechanism, b Fresh groups observed in the sun's north hemi-see," says Roscoe, "that in spite of the numerous im- the greatest objection to his inside cam-dise is, in sphere during August 11. portant and brilliant discoveries of which every year has opinion, that in order to clean it this removal of th Fresh groups observed in the sun's south hemi- to boast, we are really but very imperfectly acquainted whole front of his slide-rest should be rendered necessar with the fundamental laws which regulate chemical And as access for cleaning is the only object to 1

sphere during August = 15.

"I would have every one write what he knows, and
as much as he knows, but no more; and that not in [480]
this only, but in all other subjects: For such a person
may have some particular knowledge and experience of
the nature of such a person or such a fountain, that as
to other things, knows no more than what everybody
of his, will undertake to write the whole body of
does, and yet to keep a clutter with this little pittance
physicks: a vice from whence great inconveniences
derive their original."-Montaigne's Essays.

"T. A." (445).

SCREW CUTTING DIVIDING APPARATUS-
PLANING MACHINE-BOILER PRIMING, Erc
SIR,-Although I neither am unduly biasse
towards my own designs nor believe my projects to
perfect to be improved upon, I nevertheless fail to se
that the arrangement of clasp-nuts and leading scre
described by our obliging friend "J. K. P." is 0
an improvement upon the one I had previously describe
as he claims it to be. "J. K. P.'s" claim of superiont
is apparently based upon two points: 1st, that th
square plate carrying the mechanism can be remov
to allow of the latter being cleaned; and 2nd, that th
cam-disc is recessed on the inside of the square plate
instead of being on the outside, which inside arrang
ment admits of shorter pins being used for raising an
depressing the nuts.

tained, I cannot see what claim of superiority can be vanced for his arrangement upon this head over one hich permits cleaning without necessitating removal. nevertheless the inside camplate is the neater disposihon, and were it not for the disadvantage I mention as isting under "J. K. P.'s" arrangement of it, and which uld be easily obviated, I should be as readily disposed admit that "J. K. P's." plan is an improvement upon ane as he has been to claim it.

I

"Propeller;' "that is, when the ship is impelled by a
strong breeze, and the propeller is still connected with
the engines, but the engines not supplied with steam.
It is to obviate this that steam vessels intended to sail
occasionally have a clutch on the screw-shaft between
the screw and the engines which enables the two to be
disconnected at pleasure, so that when the vessel is
under sail only the screw alone may rotate.

W. H. N.

["W. H. N." would have increased the value of his communication had he simply stated the numbers of the letters to which he respectively refers. Then, not only the writers of the letters, but our readers generally, might, if they felt inclined, easily refer to the letters themselves. Now it must cost them some time in searching.-ED.]

TO "H. P."

the latter for many reasons. I find to dre at the rate
of five or six miles an hour is quite fast enough, and is
easier than walking, providing you walk up the steep
hills, but to spin along at the rate of eight or nine
miles an hour is work, and as for ten or twelve miles an
hour I shall believe it when I see it. I cannot imagine
how "Two Bicycle Riders," letter 375, p. 614, Vol. XI.,
could do forty miles in five hours, and "W. B." seventy-
five miles in eleven hours, unless they had supernatural
strength. How did their shirts feel after their journey?
Rather moist I should say. I have tried bicycles of differ-
ent sizes and make, but never one on which I could keep
up the speed of eight miles an hour. I have known
persons who have a great amount of walking to do, try
the bicycle, and give it up, preferring walking. I have
never heard any one personally praise it; the general
statement is they can do seven miles an hour, but it's
warm work. One thing will show the decline of the
of second-hand ones for sale ? But it has one great
three or four-wheeler.
advantage, it will run where it is impossible to run a

In my slide-rest, by removing the two screws that ss through the camplate, and screw into the clasp ts, the camplate comes away, and the lower nnt tops down; this enables me to pretty well clean the hole without further disarrangement of mechanism, hilst the saddle is materially stiffened by the solid ange, and the thrust of the traverse against a cut is en by a solid casting, instead of by two small screws in the ease of "J. K. P.'s" improved slide-rest. hould have quite concurred with him had he said that THE MICROSCOPE.-USE OF THE LIVE-BOX. bicycle. How is it there is such a numerous quantity I slides of a rest are better with the " fillet "bearing in Addition to the ordinary bearing surface. And if he poks to your engraving of my slide-rest he will see that avery slide of it is so provided, and I may add, both prfaces are scraped to a uniform contact. But a slide ving only this "fillet" bearing appears to me to be able to spring down in the unsupported middle against beary ent. The double bearing surfaces, however, 100siderably increase the cost of the slide-rest, and the peration of fitting them is one that I would not reommend a poor workman to attempt. There are other points about "J. K. P.'s" slide-rest and bed about which something could be said, but I atangine he does not give them in their entirety as good xamples. I concur with most of his advice to Joseph Moseley, but I don't like the V thread for a leading rew, as, except for very light work, the inclined Surface must tend to force open the nuts. I prefer and Be a square thread, with the sides eased off to give the read nearly the section of a wheel tooth. I ought erhaps to have confined myself to a defence of my wn slide-rest, but I have too high an opinion of venient for observation, which result is unattainable Among the many plans that I tried was ")

(J. K. P.'s" disposition and ability to fear his taking
by criticism in any but a friendly part.
CUTTING LEFT-HAND THREAD WITH RIGHT-HAND
DIES.-I see some of your correspondents are dis-
assing the practicability of this which is a manifest
mpossibility; but to cut a left-hand thread with a
ght hand chasing tool is neither a difficult nor an
usual operation.

DIVIDING APPARATUS.—I see "J. K. P." is thinking bringing out something neat in this way. Let de offer a suggestion. I have frequently when double counting "--which is at best but tedious pas ime-found some difficulty in seeing when to stop the inter, and have had the idea of supplementing the isual apparatus with a very small bell, which, by means f an electric current broken by the division holes, Lould strike and enable me to move round the re

quired number of holes without taking my eyes off the

Vurk. I have had no time to work this out, but any competent mechanic would see half a dozen ways of anng it. I can see the thing pretty well in my mind's ge, and believe its cost need be but a few shillings. PLANING MACHINE.-I have just fitted one to my the which I believe is the simplest contrivance for the purpose yet out. In addition to my ordinary chanism, I have needed but one small casting, and seced ten shillings; nor is it wanting in convenience. otal cost of the planing accommmodation will scarcely I can use either my ordinary slide-rest tools or rotating Frills or cutters, and I get a horizonal, a vertical, or an agular cut. I have endeavoured to get a photograph aken, but the light is too unfavourable; however, I tend trying again with the magnesium light, and ope for better success. Meanwhile, I shall be pleased show it to "J. K. P." if he is coming my way. BOILER PRIMING.-This is a very great annoyance, nd a cause of considerable danger and loss of heat. A Working Engineer" who asks some questions retive to it receives advice from "R. D." which does ot appear to me to be very satisfactory upon some oints. I have not had time to go through the calculaous, but I should think deficient surface for the disgagement of steam may, as suggested by "R. D.," one cause of the priming, although "R. D.'s" asseron that "the quantity of water carried over will be proportion to the required and given surface" is arcely correct. "Pure water, however cold, cannot oduce priming," looks very startling as it stands in 3. D.'s" letter, but if he means that the boiler in estion does not prime from the introduction of cold

For

[481] SIR,-Your worthy correspondent, "H. P.,"
in his excellent papers on "The Microscope-How
to Choose, and How to Use It," on page 4, letter 287,
says, respecting the live-box, that "he uses" it
his readers to spend their money upon one.
"once in three years," and does not recommend
my part I must differ from him, knowing from con-
siderable experience how very useful a piece of
apparatus is the "live-box," or animalcule-cage,
especially to beginners. In the first place, it en-
ables one to present any random-gathered object,
such as a small portion of a leaf, hair, or piece of
ekin, &c., perfectly flat for examination, which, unless
it is so, is certain to be anything but a pleasing object,
from different parts of the field being out of focus.
But with a well-constructed live-box, the view obtained
is very different. Then again, in the examination of
liquids it is indispensable, for by its means the drop
of fluid may be flattened out to the degree most con-
like to examine small objects, collected by themselves,
by any other means. I am well aware most beginners
with the lower powers, but very few succeed in obtain
ing anything like a correct idea of the object viewed,
from the fact of their not being able to make the object
sufficiently flat. I need scarcely add that the focus is
a mathematical plane, and that any object presented
must of necessity be perfectly flat, if anything like a
of apparatus which tends to secure this important
distinct view is to be obtained. Therefore any piece
result should not be set aside. Permit me, therefore.
that no microscopist should neglect to provide himself
to differ from our kind friend "H. P.," and suggest
with a live-box; but it must be a good one, with a
diameter at least 1in., the upper thin glass to fit the
lower thick one perfectly, and then I am certain that
for the purposes I have named, and many others be-
sides, it will be by no means money ill-laid out. In
able authority on such a subject. Dr. Carpenter, in his
confirmation of my opinion, I will quote an unanswer-
Microscope and its Revelations," says:-"This is an
appendage with which every microscope should be pro-
the box be well-constructed, and the glass discs be flat,
vided, so varied and so constant is its utility. ... If
they will come into such close contact that objects of
extreme thinness may be compressed between them;
(Entomostraca) be restrained from the active movements
hence not only may such small animals as water-fleas
which preclude any careful observation of their struc-
inflicted upon them-but much smaller creatures,
ture-and this without any permanent injury being
such as wheel animalcules (Rotifera) or Bryozoa, may
be flattened out so as to display their internal organi-
He
says that supposing the microscopist
possesses two live-boxes, one of the glass covers should
its rim, so that if the cover be made to slide equably
be made stronger than the other, and "firmly fixed in
on the box the instrument, in hands accustomed to
careful manipulation, may be made to answer the
purpose of a compressorium."

zation.

power;

H. T. R.

I have a back-wheel bicycle, which I prefer to the common one. It is made almost like the "Kilmarnock," which appeared in the MECHANIC some time back. The driving wheel is 3ft. diameter, cranks 9in. long, made chiefly of wood. I can stop anywhere and keep seated, the feet touching the ground on either side, and can stand over it, catch hold of the spring, lift it off the ground, and twist it round in any direction, it being very light. It runs easy on level ground, but like the common bicycle it is hard work up-hill. I have done seven miles an hour over bad hilly road with it, and have done three miles in fifteen minutes, but not with the greatest ease, as "S. James" said he did with his. I have never done over twenty miles in one journey on either bicycle. I have spent a great deal of time and study to find a substitute for cranks, in driving velocipedes. Last winter I made two or three different kinds of bicycles, to be driven by ratchets, but none answered. Leo's," which fifteen miles an hour, at which it was stated it could be appeared in the MECHANIC, No. 218, Vol. IX. The propelled, was indeed " a tempting bait" as a correspondent has before said. I said to myself if "Leo," with his 3ft. 8in. wheels can go fifteen miles an hour surely I could, with a 3ft. driving wheel, and the same size pulleys, and only two wheels instead of four. My bicycle was made like the "Kilmarnock; ratchets wheel, and a pulley ran loose on either side of the were fixed on both sides of the nave of hind driving ratchets, with pawls fastened to them; cords passed rubber springs were used to bring the pulleys back to from the pulleys to the swing bars in front, and indiatheir first position. Well I remember the first time I mounted my machine! From my house down the road is a rapid descent. It was evening, several of my neighbours were out, who stared with open-mouth astonishment at my new machine, and their astonishment was increased when I started off swinging along down the hill like mad, the pulleys flying back the moment the pressure was off the treadles, and the ratchets click-clicking, which caused the spectators to It went so swiftly down the hill, I was delighted. "No stare at each other and utter the word "clockwork." more cranks," thought I. But I was doomed to disand I soon came to a dead stop. I tried to mount the appointment. Arrived at the foot of the hill, the pressure required on the treadles began to increase wonderful, hill, but it was useless; it proved a complete failure, I found the ratchet could be used to advantage, and there was not power enough. There is only one way that is not suitable for the bicycle. I agree with Mr. J. A. Mays, who says in his letter, p. 256, No. 271, that if correspondents would come forward and confess their "failures," much good would be done by it. I have No. 236, p. 50. tried "Wilson's patent," which appeared in MECHANIC, I applied it to a tricycle, not as foot levers, but hand levers. It was a complete failure great power could be obtained, but it was too slow. I have known failures through having too high driving wheels, and I have known two failures, close by me, with the American "one wheeler," which was illustrated in the MECHANIC, No. 209, Vol. IX.

I

d-water under the conditions given by "A Working gineer," I should be disposed to agree with him, hough how "capillary attraction" "from the side the boiler" can affect the matter I don't undernd, nor can I understand what kind of steam chest it he recommends in which the globules of water rising h the steam and parted from it "would always be ot in suspension by the superior force of those direct contact with the road, they yield at once to miles, in a day, on a four-wheeled vertical tread veloce. sing from the surface along with the steam." A Working Engineer" can scarcely do wrong by -ting his feed-water, if he can do so by waste heat; the rubber tires as a point of scientific interest, I wheel, and 8in. cranks; he was certain the treadles did

THE VELOCIPEDE QUESTION. [482] SIR,-Perhaps you will allow me a few words I perfectly agree with Mr. Lambourne, that the as an attempt, if possible, to settle this much-debated vertical tread is best, and that a three or four-wheeler question. In the action of walking the body is raised is better than the bicycle. With the power applied at each step, and this involves a loss of mechanical direct on the crank the feet have to move through 36in. of space, with a 6in, crank, to every turn of the wheel, face. With the bicycle, on a moderately level surface, have seen a tricycle driven by the vertical tread; a clumsy and this must take place even on a level surwhile the vertical or oscillating motion only 24in. I the principal resistance to be overcome is the friction of the axles, and if well oiled this may be reduced to a affair, twice as heavy as a bicycle, made for two persons, minimum. I am, from the above considerations, of with separate treadles, but driven by only one person opinion that the labour of driving a bicycle with rubber up a hill, with only 3in. cranks and 3ft. 4in. wheels. tires on an ordinary road must be considerably less should very much like to see a bicycle, with the same these are essential to comfort, and they have the ad- home to the metropolis, a distance of eighty-four than that of walking. I mention the rubber tires as size driving wheel and cranks, ascend the same hill. Several years ago, two persons went from their vantage over every other form of spring, that being in inequalities. A rubber-tired wheel may in fact be said to lay down its own road. I give the above opinion on having otherwise no concern whatever in the matter.

I believe he would find priming lessened if he were interpose a perforated (plate between the water face and the half-circle pipe taking off the steam. I id ask him whether his engine has a large cylinder runs at a low speed. Priming is very often caused the steam being taken off by jerks, as it were, by ch means steam is generated only during the flow he steam when the pressure falls. On steam being off, the pressure of the boiler rises, and the evoluof steam is stopped. I have little doubt, however, the perforated plate I recommend would lessen priming, however caused. EGATIVE SLIP.-No doubt the engines would be n from the screw ander the conditions stated by

S. T. PRESTON.

VELOCIPEDES AND BICYCLE RIDING.
[483] SIR,-It is with pleasure that I read the
letters on "Bicycle Riding" in the ENGLISH MECHANIC,
and I hope you will always allow a place in your valu-
able paper for the discussions of velocipedists, which,
by the bye, is the only paper we velocipedists have.

I am surprised to see the bicycle praised as it is by
some. I am a bicycle rider, and have ridden both the
"common" and the "back wheel" bicycles. I prefer

A friend of mine was telling me he saw a four-wheeler vertical tread, not far from Norwich, with 3ft. driving

not swing like Mr. Lambourne says his do. Now I cannot see how any one could have Sin. cranks to a 3ft. wheel, unless the treadles swing. Have any brother readers seen a velocipede of this kind and size; if they have would they give a description? I believe handcranks are better than foot-cranks, because there are no dead points, you have power all round; with foot-cranks, you have power only when the cranks are at one certain point. Another advantage is, the cranks can be much longer. Has any reader tried " Samuel's patent velocipede," which was illustrated in the MECHANIC, No. 212, Vol. IX., and said to run at the extraordinary speed of twenty-five miles an hour?

FREDERIC W. SHEARING.

[487] SIR,-The subject of ventilation is a most important one, and knowing your willingness to assist in giving publicity to any scheme which may result in saving life and prolonging it, I have ventured (after several efforts to do so) to give a description of an arrangement which I have found useful, and to offer a few suggestions for bringing it into further practical use. I find, Mr. Editor, that a great many good and useful things remain unsaid, and the public deprived of the benefit of them because those who are in possession of them are afraid to say them lest they country it is easily and quickly made, and the enemy should commit some error in doing so. Such has been my condition. There are many things I should like to cannot readily bring guns to bear upon it. If opposed say, but I have been afraid to say them lest I should to infantry only, single rows of trunks of trees, eithe intrude upon space which might be more profitably em- upright or horizontal, make a very good block-honse. I ployed. However, after the many encouraging notices I opposed to artillery, double rows of trunks three fee have read in your very interesting and useful journal, I apart, with well-rammed earth between them. have summoned up courage to put my experience upon paper for the use and guide of others.

before him a method by which he may form the grooves in his polishing tool.

Having made the polisher in the usual way, by pouring melted pitch over the grinder, and given the figure by pressing the speculum upon it, mark the surface into squares with a point; taking then an old common table-knife, make it hot in a gas flame or over a fire, and cut the pitch where marked, drawing the

knife quite through, and wiping off what adheres to it with a piece of rag; the knife being hot (it should not be too hot though) cuts into the pitch without the least risk of breaking or chipping it. Continue warming and drawing the knife through the cuts until they are brought into grooves of the required width. It is likely that there will be a slight burr left on the edges of the facets, but this can be easily pressed down by the speculum, and if care has been taken the polisher will be finished without chip or blemish.

PERCY M. GROVE.

PROPELLING BOAT BY WINDMILL OR SCREW.
[490] SIR,-Permit me to return thanks to Mr. J.
Morrison (ENGLISH MECHANIC, September 9, letter
824), for his communication relative to propelling a
Mr. Morrison
boat by means of a windmill and screw.
would much oblige if he would say what the relative
sizes of the sails and the fan were in his model; also,
whether the wind sails were in the fore or after part of
the boat. I do not know how it would be possible
to keep the boat head to wind, and I suppose the
windsails would not work in any other position. In
the interesting sketch of "Windmill for Pumping"
(letter 401, No. 287), there appears no appliance for
adjusting the windmills on a change of wind.
T. C. BURton.

NEGATIVE SLIP.

[491] SIR-I think that considering "W. H. N." did not explain the reason of negative slip, he might

have refrained from calling the Rev. E. Kernan's ex-
114 When there is a thrust
planation "utter rubbish."
on the screw shaft, and the vessel outstrips the screw,
this is negative slip, and consequently the power of the
engines is not thrown away; but when there is a pull
on the shaft it is evident that the screw exerts a re-
tarding influence, this is not negative slip.

A. P. D.

* [This phrase escaped us at the time, or we should The Rev. E. have altered it or omitted it altogether. Kernan, who is studiously courteous, deserves courteous criticism, and particularly from "W. H. N.," who is eminently capable of arguing a point in the most gentlemanly manner.-ED.]

HEREPATHITE.

[492] SIR,-In answer to your correspondent, "Vulpecula," No. 427, I understand that suitable crystals of "Herepathite" are very difficult to make, but to those who wish to hear of a cheap polariscope, I may mention the arrangement which I use, and with which a great many of the splendid phenomena of polarized light may be very well seen.

It was pointed out some time ago by the Rev. J. B. Reade, in his papers on the "Use of the Common Prism for Microscope Illumination," that a bright beam of polarized light could be obtained by its means, when placed at a proper angle. The prism is mounted in any convenient way under the stage of the microscope, and the method of using it will be seen by the following extract from a pamphlet on the subject by Rotate the prism in its Mr. S. Highley, optician: fitting till it stands parallel with the source of light, with an angle towards the object; then, from the plane face nearest the light reflect rays up the axis of the microscope body, without allowing them to enter the prism, and a bright polarized beam will be obtained. The proper angle of polarization is readily secured by observing when the most intense tint is produced in a selenite plate, the analyzer of course being fitted to the body of the microscope." So much for the polarizer. The analyzer consists of about a dozen of the thin glasses used for covering microscopic objects. A hole is drilled through the axis of a cork, and a wide slit crossing the hole at an angle of about 33 is then This is held above the cut, and the glasses inserted. eyepiece, and is rotated with the fingers as occasion requires. I have only described the rough method I use myself, but any one with a little mechanical skill may easily hit on a better method of mounting and using the thin glasses serving for the analyzer.

BRIDGETON.

JORDAN'S WOOD CARVING MACHINE. [498] SIR,-In your number of Sept. 23rd, there is an article on Mr. Cunningham's wood carving machine, You will therefore, no which is damaging to me. doubt, do me the justice of giving equal publicity to the following facts in reply. Jordan's original carving machinery was invented in 1845, and received the gold medal of the Society of Arts in 1847. A number of the machines were put in action in 1846 at the Thames Bank Works, on the carved decoration of the Houses of Parliament, and they continued to be used during the whole progress of that work, to the entire satisfaction of the late Sir Chas. Barry and other authorities. The same machines are still in use by Messrs. George Trollope & Sons, in the same building. Others are in use by Messrs. Holland, and at the work in BelvedereThe inventor continues road by Messrs. Cox & Sons. to build these machines, and has recently introduced a

smaller one for amateur carving, which, like the original,
has the power of carving the entire round figure, or any
THOS. B. JORDAN.
extent of undercutting, which can be effected by hand
carving.

BOILER PRIMING.

[494] SIR,-I beg to call the attention of "A Work-
ing Engineer" and others intrusted with the care of
steam boilers to the true cause of boiler priming. I
may inform "R. D." that the want of water surface
or the construction of the boiler has nothing to do with
priming. (I attribute the causes of priming to three
things-namely, irregularity in firing; secondly, neg-
lecting to blow the boiler off at proper intervals;
My em.
thirdly, from carrying too low a pressure.)
ployer suffers greatly from dirty water, but would
suffer more from priming if I neglected the above
rules. We have almost to work out of thin mud. If
sometimes the engineer or attendant of the boiler will
follow the rules above, namely, not allow the steam to
vary more than is possible, and blow the boilers off at
proper times, and keep the steam if possible above
301b., he will not have one case of priming in six
WILLIAM H. LORD.
months.

affirming that "Kaufmann, of Dresden, introduced the
percussion," a fact I was not aware of, and should be
Also he states that "Debain, of Paris, first invented
glad to receive particulars of the action he devised.
might more correctly
the harmonium" (or
say, first brought into the notice of civilized Europe an
invention long previously known to the Chinese under
a ruder form). This instrument, bronght into notice.

one

I have always looked by Debain, would seem to be something more of the harmonium in character than any Chinese instrument with which I am acquainted. upon Debain as the improver of the class of instruhad been under the notice and acceptance of musicians L'Orgue ments which for many years previous to his operations as the "Physharmonica," "Seraphine,"

expressif," &c., &c. Still it is possible he may have had

a Chinese model; and it would be intersting if Mr. E. H. Jones would furnish us with a fuller description, HERMANN SMITH. and if possible with an illustration.

PICRIC ACID IN BEER.-TO "URBAN." [498] SIR,-I see that "Urban" (p. 35, ante) in replying to "An Associate of the Royal School of [Will Mr. Lord when he writes again mention the Mixes," describes a test I gave (p. 573) for detecting number of the letter he may refer to, so that any reader picric acid in beer as useless." Now I certainly regard without loss of time can get the full benefit of his in-Urban" as a great authority in chemical matters, formation ?-ED.]

AURORA BOREALIS.

24th and 25th ult.

aurora

boreales

[495] SIR,-Two rather fine
were seen here by several people on the nights of the
On the 24th the northern sky was
lit up by a bright bluish glow, like strong twilight;
one of my friends noticed it about 9.45, but it must
have begun earlier. At about 10.20 I saw a patch of
pink light and several good streamers, some of which
reached nearly to the zenith; from this time they
decreased in number, and at 11 seemed to have ceased,

but I have been told that it was seen well at 3 in the
morning. On the 25th, at 8, the north was lit up with
the same bright glow, and many streamers shot up
narrow, then broadened, and then gradually faded.
But about 8.20 there was a most beautiful sight, six
or seven streamers rose side by side, and these
remained for some little time-very much like a group
After a while they widened and changed
of fountains.
from a whitish to a beautiful pink colour, and then
this faded away. At 10 there was little or no sign of
an aurora; at 11 only two or three streamers.

May we not expect many more aurore while the
sun remains so spotted? There is an immense spot
on the sun now, easily visible by aid of a dark
glass; it presented a fine sight through a 3in. achro-
matic on the 23rd. Would some of our subscribers
be so good as to send sketches made from large instru-
ments on the 23rd, 24th, and 25th inst., if the Editor
can find room, as I think the change in shape, &c.,
H. W. B., Croydon.
would be very interesting to notice?

THE NATURE OF THE LATENT IMAGE.
[496] SIR,-It remains a question whether the
latent image is due to a chemical or physical action.
In the wet collodion process, I say the latter, as the
closest scrutiny fails to detect any difference before
and after exposure in the structure of the film. A few
years ago, M. Carey Lea, in a series of delicate experi-
ments, undertaken to determine this question, demon-
strated the possibility of dissolving out the developed
picture and re-developing it, proving thereby the phy-
sical character of the action, and that the visible image
principally, if not entirely, consisted of the silver pre-
cipitated by the developer. If this is accepted, then
the yellow ray does not continue a chemical action
commenced by the actinic, but a physical one, the
collodion film undergoing no change of structure
during the process.

The Editor of the Photographic News, No. 627, in
treating of the accelerating action of the red ray,
considers it simply due to traces of actinic light pass-
ing through the red medium. It may be so, but the
temperature greatly influences the result. At 60° Fahr.
an exposed plate does not fog in the ordinary yellow
light of the dark room; but when the thermometer
stands above 90°, it becomes exceeding difficult to obtain
a clear picture, even when there can be no suspicion of
In my opinion,
the presence of rays of actinism.
although considering the red, yellow, and violet rays,
vibrations of distinct forces, yet are they so closely
related that an action commenced by the one is cap-
able of being assisted and continned by the others,
particularly when aided by the still more powerful,
though non-luminous rays of heat.

"Urban" (letter 463) will perceive from these re-
marks there is no visible chemical reduction of the
bromo-iodide of silver contained in a wet collodion
T. A.
film exposed to the action of light.

P.S.-I enclose 7s. in postage stamps towards the "Lifeboat Fund." Sorry to see it progress so slowly.

CHINESE HARMONIUM AND KAUFMANN'S

PERCUSSION.

[497] SIR,-I am anxious to collect all the infor-
mation that is to be obtained concerning the history
of the harmonium. I therefore beg to ask Mr. Eustace
Will
Hinton Jones for more explicit statement of two parti-
culars mentioned in his letter (239, page 518).
he be good enough to refer me to his authority for

and on more than one occasion have I been thankful
for assistance he has rendered me through this
journal. But in this case he must excuse me if I am
unable to appreciate his condemnation of the above-
mentioned test, for it is not to be supposed that any
brewer using picric acid as a substitute for hops, would
make use of less quantity than sufficient to impart a
decided bitterness to the beer; and if it be present in
this quantity, I believe the ferric picramate test to be
practically reliable. Of course I don't mean to say
If they are, I wish
that chemicals could not be added, that-in addition of
potash and copperas-would impair the results or pro-
stances likely to be found there?
duce similar results; but the question is, are such sub-
"Urban "-for information's sake-would kindly name
CROW-TREES.
them.

READINGS FROM THE GLOBES.-XIII. [499] SIR,-The consideration of climate, in a popular sense, involves many collateral branches, and to do it justice it would lead to other subjects which can be conveniently studied in books professedly treating upon it; for in a popular sense, the term climate means the temperature of any particular latitude or country, as determined by the thermometer; and the vegetable productions which that country might produce, and the animals peculiar to it. As a general rule we may say that climate is affected by distance from the equator, and that places in the same latitude have the same climate; but this is by no means to be taken as a universal proposition, for the temperature of any country is affected by a variety of causes, the consideration of which would lead to the study of physical geography, which, however interesting it might be, would require more time and space than is at our disposal. We must therefore be content with merely alluding to certain causes which affect temperature. The elevation of the land above the level of the ocean exerts a great influence upon climate, and the more elevated any place is, the colder it is known to be. But this elevation varies in different latitudes, and to it has been given the name of congelation-curve (I hope none of your readers will object to its use, as I have not coined the word), as it assumes a curved shape, being high at the equator, and low near the poles. It has been calculated that the congelation-curve at the equator, or the height above the level of the sea where freezing takes place, is 15,207ft.; at the latitude of London, 5,808ft.; and at 89 latitude, 5ft.; thus it appears that a curve described proportionately from scale with a height at the equator of 15,207ft. and at the poles 0 will give some idea of the congelation-curve on our earth. Hence the cold of Siberia is accounted for by its elevation above the level of the sea, combined with its very high latitude. It is stated also that the vicinity of seas affects temperature, as the water, owing to its motion, will not absorb cold and heat so rapidly as land; and places in the vicinity of seas have been found to possess a climate less liable to extreme heat and cold than inland places in the same latitude. The quarter towards which a country slopes, its mountains, its soil, its cultivation, and its prevalent winds, have a considerable influence upon its climate; hence the hottest place in the summer time is said to be the western part of the desert of Sahara, because the winds blow over an extent of sand 4,000 miles in breadth, and contract the heat of those dry and sun-burnt plains, bringing with them a heat unIt is owing to these causes that the geobearable. grapher, Humboldt, has divided the belts of the globe by isothermal lines, or lines of equal heat, as determined by the productions which such belts furnish; thereby showing that cultivated lands in the same parallel have generally a milder temperature than those uncultivated, and the north than the southern hemiBut the climates chiefly connected with our sphere. subject are those which are illustrated and explained by the artificial globe. They are sixty in number, being thirty on each side of the equator; and they are distinguished by the difference in time of the longest day in each climate. According to this view, the first climate is that parallel where the longest day is 121 hours long, and extends to that parallel where the longest day is 13 hours long, which is the half-hours, until where the longest day is 24 hours beginning of the second climate; and they increase by long, making 24 on each side of the equator. Within

the sun has been greatly more diffused than now, the
sun's atmosphere may have retired and left the ejected
matter to describe a permanent orbit with a perihelion
more remote than Mercury-as, for instance, our
November shower-and even now may eject matter
which shall describe one or many more imperfect
T. S. USBORNE.
parabolic orbits.

P.S.-Since writing the above I have thought out
somewhat more, and will continue.

MEDICAL COILS.

[501] SIR, Though the information "V. W. X."
seeks has often been given in the pages of your paper,
yet, trusting to your kindness, I will sum up, and after
a few remarks, give a résumé. On pp. 513, No. 20, Vol.
X., and 381, No. 17, Vol. VIII., “Sigma" and Mr.
Moody give instructions for making medical coils.
Many excellent medical coils are brought into dis-
repute and ill-favour because their possessors, not re-
stricting their use to that for which they were intended
physiological effects upon a single body), fail in pro-
ducing startling effects when their numerous friends
and acquaintances form a circuit by joining hands;
but if V. W. X." desires to make an instrument
capable of producing beneficial therapeutic effects upon
the human organism, such coils are easily made, and
if carefully used, will not fail to prove
Their principle is thus briefly stated:-

the polar circles the climates increase by months, that
is, the first climate in the polar circle, or the 25th from
the equator, is where the longest day is one month long,
increasing until where the longest day is six months
long. These climates and their width may be found
both by the globe and trigonometry, and nearly all the
problems on the globes involve some little acquaintance
with spherical trigonometry. If then we wish to know
what climate any place is in, we must know the length
of the longest day in that parallel, and having deducted
12 therefrom, the number of half-hours in the re-
mainder will give the climate; thus, for instance, London
is in the ninth climate, because, if we deduct 12 from
161, the length of the longest day, the remainder is 44,
or nine half-hours, showing London to be in the ninth
climate; but Paris, whose latitude is 48° 50', is in
the eighth climate, the boundary of which is 49 2.
The width of each climate is ascertained thus: as the
longest day in each hemisphere is when the sun has
231 declination, we must elevate the pole 23}°, and in
that case the globe is supposed to rotate on its axis
in the same way as the earth: consequently by turning
the globe round we shall find the latitude answering to
The width of each climate
any particular climate.
may be found out by ascertaining what latitude has its
longest day, the same as in the latitude given; thus,
if we wish to find the width of the ninth climate, we
must find first what parallel has its longest day 16
hours long; and secondly, what parallel has its longest
day, 17 hours long; and the number of degrees
A battery current traversing the wire of a coil, itself
between these two parallels will be the width of the incapable of producing any striking physiological
ninth climate. The width of the climates may be effects, will induce in the same wire a momentary cur-
marked in any meridian. If, for the sake of illustrarent in the opposite direction every time it is closed or
tion, we take the meridian of Greenwich, and bring it increased, and one in the same direction every time it
to the east point of the horizon, then by turning the is broken or decreased. This action is easily re-
globe gently until one quarter of 15°, or 31°, get below membered by imagining the current to push back to
the horizon, the width of the first climate may be obtain a purchase when it starts, and to react in the
measured on the meridian of Greenwich, where it cuts other way when it stops.
the horizon; again turn the globe gently, until 74° of
the equator get below the horizon, and the width of
the second climate may be marked in like manner on
the same meridian, and continue in this way until all
the climates are marked; then by bringing the marks
to the brazen meridian, the width of each may be
known. But the simplest way of ascertaining the width
of the climates is by using the formula given in letter
XII., namely, given the ascensional difference to find the
latitude, which is expressed thus: tan 23-28: R: sine
ascensional difference: tan latitude. One example
will show the working. It is required to find the lati-
tude where the ninth climate ends; in this case the
ascensional difference is 24 hours, which converted into
degrees becomes 33° 45', then we have
= 10-0000000

R

sin 33 45 =

tan 23-28

9.7447390 19-7447390 9.6376106 10-1071284

which gives 51° 59' as the latitude where the ninth climate ends. In books of geography and the globes, there is generally given a table with the width of the several climates inserted, which has been worked out T. S. H. by the method given above.

P.S.-There are a few errata in some of the former letters, which I must beg the intelligent reader to correct for himself.

PROCTOR ON WILLIAMS'S THEORY. [500] SIR,-I doubt whether Mr. Proctor (p. 577, Vol. XI., note,) is justified in assuming it "inconceivable that a central body shall project matter so that it shall describe a permanent orbit."

It is evident that if it were possible to stand on the highest mountain of any equator in vacuo, and hurl a projectile horizontally with a certain given and fixed velocity, that the projectile would travel all the way round and hit the projector on the back of his head, or otherwise continue its circular orbit, but that if the initial velocity exceeded that given amount, it would

increase its orbit into infinite space.

It may also be taken for granted that if projected at any angle with the horizon at less than a velocity calculable for that angle, it would describe a parabola and return to the central body, but if with a greater velocity it would describe some hyperbola and not return. But there is in the case of the sun another coefficient which must not be altogether despised (I leave out the rotation of the sun on its own axis, as that would simply compound with the initial force), I mean the resistance of the sun's atmosphere. Now we know the visible atmosphere of the sun to be at the least 20,000 miles thick, and we know from Encke's comet that the invisible atmosphere extends pretty near to

the orbit of Mercury.

Now the ejected matter could only commence to describe a perfect parabola after it had passed outside the region of the sun's atmosphere. The next perihelion of its orbit will therefore be displaced, and as the original perihelion was the surface of the solid globe of the sun, the greater probability is-displaced outwards.

Now this displacement, if the ejected matter were solid, might be considered as small, were it not that it must inevitably become gaseous in passing through the visible atmosphere of the sun, including 10,000 miles of red-hot hydrogen, and afterwards encounter great resistance from the outer invisible atmosphere; but if the ejected matter be metallic (as our aërolites) it will cool during the first aphelion passage, and in its return, therefore, encounter less resistance in entering its perihelion orbit.

Now these causes could not of course throw the per helion outside the sun's atmosphere (i.e., say the orbit of Mercury); but if we imagine, with Laplace, that

efficacious.

The wave induced on opening, breaking, or decreasing, is the more powerful of the two, the other being partially neutralized by the battery current, and it is this large wave in the same direction as the battery that is used for medical purposes.

I

The circuit through the body and coil may include
or exclude the battery. In the former case, the body
is intruded into the circnit of the coil and wire, its
great resistance having the same effect as breaking the
current. In the latter case, the body is substituted for
the battery. The latter case is the only one possible
when the terminals are applied to adjacent parts of
the body where the resistance is little, but the former
is the best, and is suitable in nearly all cases.
mention these things because their proper comprehen-
sion will avoid all trouble or hesitation in making
connections at contact-breaker, &c. Cotton-covered
wire will do very well if well soaked in paraffine, by
passing it through a melted portion of it while wind-
ing, but silk-covered wire is better, occupying less
He will need about one pound of number 26,
room.
which, cotton-covered, will cost about 5s. 6d. ; silk, 11s.
The best way to insulate the layers is by a piece of
silk or tissue paper, saturated with paraffine; one
thickness is ample insulation between each layer; reel
may be made of paper with wood ends, or will cost

about 2s.

He says science and art have joints, back, and lines are impaired by age, he tells me I ought to progress. made wonderful strides in the past twenty years, but I remain still the same. "Well, master," I said, "what do you wish me to do?" He answered, "I want you to tell me the length of a link" (meaning the chain for measuring land). I said, "That is 7·92, sir." "Ah!" said he, "I know it, but you cannot point it out to me; I want to see where I may place the point of a needle, and say, that is the length of a link." Of course I was shut up. Again, he called me an old "flat," because I could not tell him how many square I said, "Master, if feet there were in a round table. you read the ENGLISH MECHANIC you will know how to square the circle mathematically." "Confound you," he said, "I want a more simple method; one cannot do anything with a circle unless he dives head and heels into an ocean of vulgar fractions and decimals." I said, "You will never dive far in those waters; your head is too light (empty)." I thought this would end the subject, but no! He then said, "Is the method which we adopt to measure round timber correct? said, "A proxy mate, or the buyer or seller would be "I am glad to hear it," he calling out before this." said, "for we avoid vulgar fractions; for, after all, we must practically call it feet, inches, and parts." Again, he said, on looking over the MECHANIC, Vol. X., p. 139, Query 589, "Solid Contents," "The querist gives the

I

size and shape of a room, and asks for a simple way
of measuring the same,' to which some gentlemen
reply, p. 193, 21348'9ft. and 20896ft.; and, again,
p. 239, 21348ft. 10 in., and 20896ft. 5in. Now, as you
say the method of measuring round timber is approxi-
mate, could not the cove there named be treated as a
I said,
circular solid, 30ft. long and 10ft. diameter?"
"Master, I think you want to find your diameter and
circumference with a chalk line, and extract your
roots with a shovel." He said, "I should like to see
some method adopted, by which a practical man could
treat a practical question in a practical manner, and
prove sufficiently approximate for any practical pur-
We work to 12ths, 8ths, and 16ths by rule; and
10ths, 100ths, and 1000ths on paper. Could not the
I said,
two be made to harmonize better together?"
"I'm afraid not, Master." Now, sir, you see the
trouble I am in. If any of your correspondents can do
anything to alleviate it they will much oblige
OLD TWO-FEET.

pose.

ELECTRICAL.

[503] SIR,-Your readers are greatly indebted to "Sigma" for his painstaking efforts to instruct, and I cannot but regard him as a studious and successful teacher. It is therefore with a feeling of respectful diffidence, that I venture to offer a little amendment to his paragraphs, 163 and 164.

The resistance of German silver wire, with a given difference of temperature varies less than copper; but as the galvanic coils are of copper wire, it is better that the shunts should also be of copper, so as to secure, or at any rate aim at, an equality of change between the two, and therefore the same shunting or multiplying value. What is meant by a perfect arrangement of astatism I know not, but it is clear that perfect astatism is undesirable; while different degrees are convenient for the different characters of work.

Varying in the direction of perfect astatism, we get increased sensibility, but with increased difficulty from instability and a varying zero. Varying in the opposite direction we get less sensibility with a more stable zero. The system is one of wide convenience and is found very useful in practice. Nor am I aware that the deflection on one side is always greater than on the other. I know little from a scholastic point of view, but I do know that the requirements of a modern electrician involve measurements of very considerable refinement; and it mostly happens that one set of highly paid officials are pitted against another in severe competition. The work to be done consists of measurements from 1 unit (and decimal parts thereof) up to thousands of millions. And the astatic galvanometer is equally available throughout. From short lengths up to a considerable range, the electrical balance or Wheatstone Bridge is generally used, which With a has the great merit of being quite independent of any variation of the galvanometer system. I much prefer that "Sigma "should explain this beautiful gous to the common steelyard in mechanics, and when arrangement, merely premising that it is quite analathis is out of range, although more complicated, yet in practice, the matter is very simple, and necessarily so

The core, consisting of iron wires, need not be above
in. or in. in diameter. It is better not to use a
imagnet for contact-breaker, as the power of the cur-
rent can be sufficiently regulated by a brass draw tube,
thus avoiding the resistance of a magnet. The core
will require to be held in the centre of the reel by being
fixed at the contact-breaker end on to the wood end of
the reel. To do this, wrap the core in fine twine,
waxed silk, or paper, and at one end wrap a strip of
paper about fin. wide, until it is of the same diameter
as the inside of the reel; it may then be cemented in.
Or a much better way is to have the turned end made
extra thick, with a hole in which the core will fit
tight, with a shoulder around this hole to fit into the
The brass tube, which
end of the tube of the reel.
must not be too thick, and should fit well into the reel,
occupying all the space between reel and core, is then
fitted, cut to right length, and fitted with knob.
switches for laying on extra lengths of wire, &c., may
be avoided, as unnecessary complications.
means of controlling battery, the draw tube, and, if
necessary, extra resistance to insert, will enable him to
modify the current to his pleasure. Any good constant
large current-giving battery will do, but if " V. W. X."
is able to construct such a cell, the modification of
Smee's, invented and described by "Sigma" on p.
147, No. 7, Vol. XI., would be the best; but as the coil
I describe offers considerable resistance, I should think
it might be worked with economy by placing the zinc
in a porous jar, and charging with saline solution; if
wanted for emergencies, a few cells of sulphate of lead,
mercury, or even manganese, will be ready at any

minute.

All

I would advise him to purchase the contact-breaker, cost about 7s. From the above he will understand the connections. Any information he or others may wish as to the uses and application of such coils I shall be W. H. COFFIN. happy to give.

TRIALS AND TROUBLES OF A TWO-FEET RULE.
THEORETICAL PASTIME OR PRACTICAL
NONSENSE.

[502] SIR,-I know from past experience that you
do all in your power to assist those who may require
it; perhaps you or some of your readers may assist
me in my present trouble. I have been the pocket
companion and handy guide to an "Old Chip" many
years (I cannot complain of ill treatment, or I should
not have remained with him long); but now that my

for an extensive daily work.

An ordinary resistance box of 10,000 units, may be multiplied by an ordinary "bridge" to represent 1,000,000. Beyond this what other multiplying powers have we ? First then, take a "constant of the galvanometer (a measure of its sensibility for the time), say, 1 cell through box 10,000, with a shunt multiplier of 100 = 1,000,000 units=300; then if a length of core or cable gives 100° with 500 cells, what is the resistance thereof?

It is clear that our "constant" in this case would be 800,000,000 if our deflection were 300°; but being 100 it must necessarily involve a threefold greater resistance, and therefore be 900 millions. This which is extremely simple, is in principle nearly all that is involved. The known resistance of the constant, multiplied by the number of cells, and that by the ratio of deflection (assuming it to be smaller than the constant). The subject is one of great beauty and high practical importance, and there is very much to say on it, but I prefer, with these two amendments, to leave it in the hands of so good a teacher as "Sigma."

Some prefer, knowing the resistance of the galvanometer, to vary the shunts so as to obtain any required