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, 81, Tavistockstreet, Covent Garden, W.C.

section of the moon, and mark the limits of libration,
he will find it difficult to draw the outline of an atmo-
sphere (though he might draw that of an ocean) which
would be unrecognizable by terrestrial astronomers.
Sir John Herschel has intimated to me his acceptance
of this objection to the theory as demonstrative.

66

I may add here, with reference to "F.R.A.S.'s" view that Saturn and Jupiter probably have atmospheres of enormous depth, that in the Intellectual Observer for June, 1866, I adopted this very view as a means of explaining Saturn's occasional assumption of the square-shouldered" aspect. I notice that Mr. Webb All Cheques and Post Office Orders to be made pay in his charming work "Celestial Objects for Common able to J. PASSMORE EDWARDS. Telescopes" ascribes to me an explanation of the above phenomenon founded on a singular optical illusion which I described in the same paper. But I only described the illusion (a very striking one) to reject at once the theory that Saturn's square-shouldered aspect is illusory. Combining the square-shouldered aspect with other very remarkable changes of figure, and with the amazing forms assumed by the shadow of the planet on the ring as described by Dawes, I have no hesitation in saying that it appears to me demonstrated that (as "F.R.A.S." suggests) Saturn has an atmosphere deep enough to modify his apparent dimensions, and (as I suggest) his apparent shape (at times) also. Further (and I believe demonstrative) evidence as to the inherent nature of the forces disturbing Saturn's belts appears in the second edition of my "Other Worlds." RICHARD A. PROCTOR.

"I would have every one write what he knows, and as much as he knows, but no more; and that not in 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 does, and yet to keep a clutter with this little pittance of his, will undertake to write the whole body of physicks: a vice from whence great inconveniences derive their original."-Montaigne's Essays.

In order to facilitate reference, correspondents when speaking of any Letter previously inserted will oblige by mentioning the number of the Letter, as well as the page on which it appears.

ILLUMINATING POWER OF TELESCOPES. [811] SIR,-"F.R.A.S." has misapprehended me. It was his general remark as to the illuminating power of telescopes, not his opinion as to Solomon's 3in. telescopes, that I had in my thoughts. To advertise these telescopes as "warranted to show the fifth star in the trapezium "is absurd, and for a practised observer to deny that they can do such work needs no apology. Nor can I by naming any instance in which a trust worthy person has seen the fifth star with a 3in. glass afford "F.R.A.S." the luxury of apologizing in leaded type.

But on the general question I think I am right; and there is one reason why, supposing me to be right, it was very desirable that I should speak. We might begin to doubt the veracity of those who said they had seen such and such stars with such and such telescopes, if a general and I think incorrect principle of judging were laid down.

I will quote one instance, showing that the differences of eye-power I have referred to may be much greater than "F.R.A.S." supposes. He knows that the largest telescope possessed by the late M. Goldschmidt was a Steinheil somewhat under 4in. in aperture; and he will not think I exaggerate in assigning the 10ft. refractor of the Hartwell Observatory an illuminating power twice as great. Nor, again, will he regard Mr. Pogson, the discoverer of several asteroids, as deficient in the special visual power we are considering. Now this is what Mr. Pogson says of M. Goldschmidt having seen the star 7 (Winnecke's b) near U Geminorum :

"Well might M. Goldschmidt say that the star was painfully seen'; for I estimate 1 130, Mr. Baxendell 13.3...He must possess an exquisite eye and telescope to peer so far beyond the limit of his aperture as to have glimpsed it at all. But some of his exceedingly faint planetary discoveries have proved this long since. I can only say that, ordinarily, the star 1 is the limit of the Hartwell equatorial, and that with a larger but inferior telescope át Oxford I never saw it at all!"Monthly Notices of Royal Astronomical Society, vol. xx., p. 38. It needs no very remarkable powers of vision to see the fifth star of the trapezium with a 4in. aperture. M. Goldschmidt (we may infer from the above instance) would glimpse it with half that aperture (superficial) or with an aperture of 2.828in.

But as I have said I lay no stress on this special instance. What I have really desired is to prevent a rule being laid down which might lead to questioning the veracity of observers.

RICHARD A. PROCTOR.

"E. L. G." ON THE MOON. [812] SIR,-I venture to point out a slight error in "E. L. G.'s" letter on the moon (let. 771, p. 202). He says that "supposing her further side inhabited, the inhabitants could not, even with our astrolabic means and refinements, quite detect that their monthly motion is anything more than an axial rotation." They would be but poor astronomers if they failed; for we with our relatively minute motion of revolution around the common centre of gravity of the earth and moon have found in the effects of this motion a means of measuring the sun's distance (I refer to Leverrier's method). In other words, the effect of this minute motion of revolution is clearly apparent in the records of solar observations at Greenwich, Paris, and other leading observatories. The corresponding effects in the case of the lunarians would be eighty times as great. I may mention here that in Guillemin's "Heavens" (which has been placed in my hands for revision and annotation) Mr. Lockyer had added a note to the effect that Leverrier's mode of determining the sun's distance is founded on the moon's motions. This is a mistake which will be found corrected in the new and cheaper edition now passing through the press.

Mr. Symons was not right, I take it, in his opinion about the possibility of a lunar atmosphere on the farther side of the moon. If "E. L. G." will draw a

TODHUNTER'S MATHEMATICS.

dissemination of knowledge by keeping it down to its present size."

As I can't call a mass meeting at which readers from what some geographers have oddly termed the fri quarters of the globe ought to be present, perhaps thos who are now present in the spirit, and with the writer desire to make our journal a yet more efficient engine of civilization by enabling its proprietors to enlarge it will" signify the same," not by holding up their hands but by expressing their approval under their sign manual-i. e., in their hand-writing addressed to on editor-especially those who have something else of importance to write about, for if all wrote he would require many subs indeed to read for him even if only on postage cards, which Punch terms letters patest, because they don't need opening. THE HARMONIOUS BLACKSMITH,

COAL-MEASURE REPTILES OR MAMMALS. [816] SIR,-Among the many objects of interest that have been found in our Northumberland coalmeasures there may be enumerated specimens of humeri that occasionally, and only very occasion. ally, occur.

Reptile teeth and fragmentary remains of reptiles are not uncommon, but portions of reptile limbs are very rare indeed. I have in my possession about half a dozen humeri, and about half that number of scapuls. There are two or three facts in relation to the humen and scapula that are worthy of notice. The humer are stout, strong, short, well-rounded bones, and very [813] SIR,-Not to know Todhunter argues oneself closely resemble the corresponding bones of Cetacea unknown, or at least unknowing. Todhunter a recent viz., whales, dolphins, porpoises, &c.-so far as conwriter! Why, fourteen years ago his books were text-figuration is concerned. The scapula also closely resemble the scapula of Cetacea. With the exception books in the hands of every Cambridge student of mathematics. His excellent treatise on the differential of the scapulae of whales, dolphins, and porpoises, I am calculus (I am not by the way forgetting my engage- such an entire absence of ridge or keel. So far as I not aware of any scapula in respect to which there is ment in this matter) was published in 1852. mammalia and in all reptilia, but those scapula that I am aware the scapular ridges exist in all quadrupedal have obtained do not possess the ridge and are in that respect like the prescapulae of lizards and the scapul

For me to praise Todhunter would be an impertinence. But I may say that every competent judge regards his works on mathematics as unequalled for lucidity and soundness. He is one of the most successful mathematical lecturers and tutors at Cambridge, and was senior wrangler in 1844 or 1845.

As to the point referred to by Mr. Tonkes (let. 803, page 209), I can only express my wonder how he works without using the period at times for a sign of multiplication. I should particularly like to see his method of indicating factorials, or the coefficients in binomial or multinomial expansion. RICHARD A. PROCTOR.

MISCELLANEOUS.

[814] SIR,-I should be perplexed to say why I wrote
"amongst " (and not "among") "the many surprising
discoveries," &c. I am often surprised at the decided
way in which my mind rejects certain expressions
while I am penning a sentence. Now I find that I
should write "among the many beauties of nature,"
not "amongst ;" while I should not write (I feel almost
as if I could not write) "among the many surprising
discoveries," &c. I am inclined to look for the reason
in the S's in the word "surprising:" they seem to
want balancing. Is this fanciful? I have no doubt it
is. But I am a fanciful writer.

I think "except" and "excepting" " you are an
honest man," have the disadvantage of being both un-
grammatical, though familiar, modes of expression.
I should write "while we were anxiously waiting,"
not" whilst; " but "whilst we were anxiously staying,"
not "while." I cannot say why.

"I do not like thee, Dr. Fell,

The reason why I cannot tell."

I do not, by the way, at all like compositors' rules for
cases of this sort, nor for punctuation, nor in some
instances for spelling. Different "houses" however
have different rules. The printers at one "house"
invariably change my "inquiries" into "enquiries,"
to which on philological grounds I entirely object. I
would as soon have "intirely."

If "T. A." will draw a section of the earth and show
where his zodiacal light comes from and whether it is
wholly or partially self-luminous, I shall be able to
discuss it.
R. A. PROCTOR.

THE SIZE AND PRICE OF THE "ENGLISH
MECHANIC."

you.

of cetaceous mammals.

Have we in this fact another piece of evidence supporting the theory that higher animals than reptiles mals inhabiting water deposited themselves in the carlived during the carboniferous era, and that mamboniferous seas as Cetaceans do in our present oceans?

cumulative. I have now in my possession from the The evidence of mammalia during the coal period is carboniferous strata a jaw which I believe is mammalian, several footprints of what appears to me to have been a small mammal, and now we have scapul and humeri with marked mammalian characteristics. I enclose a sketch of a humerus, front and side views,

MI

VOLTAIC PILE (5236, No. 290). [817] SIR,-A dry voltaic pile, or the closest approximation to one yet produced, was constructed and used with excellent results by De Luc, an eminent French electrician, and supposed to constitute an apparatus liable to no derangement, unaffected by either chemical action or evaporation, and with an activity [815] SIR,-When (in No. 295) I urged my fellow perpetual and invariable. It is entirely composed of readers to bestir themselves for their own benefit and solid substances. Its great defect is its very weak increase the circulation of our journal so that you power. The following is recommended as an easy and could afford to increase its size in accordance with your efficacious method of construction. "Lay silver leaf on very liberal promise, I refrained from again suggest-writing-paper, that is, silver the paper, and then with ing any increase of its price because, on a former occasion, you objected thereto; I am glad to see the receipt of more money is now less distasteful to Personally, I should much prefer paying 3d. for 42 pages to receiving eight pages for nothing, and I strongly suspect that if I could call a mass meeting of my fellow-readers-the mass would be "pretty considerable I guess"-and were to put it to the said meeting, "All you who are in favour of the ENGLISH MECHANIC being extended to 48 pages, please signify the same by holding up one hand," every "hand" present would hold his hand up; and when I came to putting the amendment only its mover and seconder would vote, supposing (which I cannot) two such unwise persons could be found among the readers of the ENGLISH MECHANIC-for the said amendment would, however worded, really mean "that we take this opportunity of obstructing intellectual progress by preventing the ENGLISH MECHANIC from becoming a yet more powerful medium for the

a common punch of rather more than fin. diameter, cut out a series of discs; a corresponding number of discs of common paper and thin sheet zinc should be provided in a similar way. These discs are now to be arranged within a tube of glass, carefully varnished within and without, in the order of zinc, paper, silverpaper, with the silver upwards; then again zinc, paper, silver-paper, silver upwards, and so on; by which as is evident we obtain a succession in the order of silver, zinc, paper; silver, zinc, paper, &c., the silver and zinc being always in contact. The glass tube in which these discs are placed is capped at each end with brass caps; through each of these passes a wire rod; being screwed through the cap, it compresses the series from each end, and serves at the same time as a conductor from each extremity of the column."

When this apparatus is insulated and a delicate electroscope attached to its opposite extremities, the electroscope at the zinc extremity diverges with posi

tive electricity, and that at the copper extremity with
negative electricity.
The annexed figure is illustrative of the whole ar-

rangement, supposed to contain from 1,000 to 1,200 allowing it to revolve as it would, but I pressed much

THE LUNAR CRATER GASSENDI. [818] SIR,-"A Foreigner" (let. 633), a few numbers back, sent you a sketch of the lunar crater Gassendi. I have been examining it with a 4in. Gregorian, by Short, and send you a sketch of what I saw in the early morning of the 16th of this month. I believe "A Foreigner" was using an instrument of more than twice the aperture of mine, but that does not account for the fact that I see a row of bright mounds running in a north-easterly direction, nearly across the floor of the crater, while in his sketch the principal row of hills seems to run in a south-western direction, i.e., nearly at right angles.

THE LATENT PHOTOGRAPHIC IMAGE. [821] SIR,-Whether "Urban" (let. 746, p. 179) differs or not from the editor of the Photographic News in his idea of the nature of the latent photographic image, I am unable to say, it being, as before observed, an undecided question. But pending a final decision, will "Urban" try the following easy experiment ? Before developing, place the exposed plate in a saturated solution of hyposulphite of soda, clearing the film thoroughly of the iodide. On examination he will not find the slightest visible trace of a latent image; but that it is present will be proved on proceeding to develop, as, with care, it can be brought to fair printing intensity. Now I think there can be no doubt this image consists entirely of silver precipitated by the developer.

With regard to the facts (?), allow me to inform "Urban" of my perfect acquaintance with the appearances presented by stains and fogs, due to dirty glasses and slides, impure chemicals, and the int uin of white light; also that the recent experiments of M. de Constant are proving the accelerating action of red light. Has "Urban" heard or read of the analogy between tones and colour? Because there I fancy he will find the reason why; the red ray vibrating in harmony with the actinic.

[822] SIR,-Mr. Harding (let. 784), is I think, a little too sweeping in his refutation of Achromatic's" remarks upon the possibility of obtaining good wood sections in a carpenter's shop. Certainly the best radial sections of wood in my cabinet at this time were thus obtained, and I have repeatedly been struck with the extreme delicacy of the shavings a good workman throws off. At the same time wood sections to be valuable for the purposes of study do require careful treatment, and

a half years' working through the same thing. The
real remedy for the above is lots of boiler power with
good construction, and I think if boiler makers and
engine builders would be a little more guided by expe-
rienced sea-going engineers in the construction of
boilers there would be less boiler priming and de-
struction of engines than at present.

The best formed boiler would be one having large
steam space, so that pure steam could be used, and, in
technical terms, as clean and dry as possible. A better
vacuum is the result, and increased power for the same
amount of coals expended, which all shipowners know
the cost of. I have improved priming boilers by re-
ducing the furnace area, fitting large domes to the top
of the boilers, and merely perforating the shell plate,
which has acted, as a dash plate, and introducing
the feed through pipes inside the boilers, and fit-
ting a series of small pipes to the main feed, and
outlet of each being placed under the ashpit, the
feed is injected at the coolest part of the boiler.
This prevents concussion caused by the cold and
hot water meeting, and disturbing the regular
circulation; and the feed passing through the small
pipes inside the boiler enters at the temperature
of boiling point, and as steam passes off the surface
the cooler water from the bottom gradually rises to
replace it, and creates a regular circulation. I have
found the latter little arrangement do more good than
anything else, and as I trust others have found some-
thing better, I hope they will kindly communicate it.
M. E.

of "The British Association in Jeopardy, and Pr fessor De Morgan in the Pillory without hope of escape. He will then probably be enabled to judge how for is worth his while to be entrapped into a distastef because useless and unnecessary, discussion, with e who has boldly avowed that he used such means before to inveigle an editor into a wrangle with him, and finding that neither the editor nor the professor could or would agree with his follies, he issued the above named pamphlet as a vent to his spleen.

I am much obliged to Mr. A. J. Jarman for his courtesy in giving such full and explicit details the manganese cell, with which, as I before stated, I was unacquainted. His letter gives me great pleasure, a find, he for one, admits that platinum contacts are dispensable between the carbon element and its o ducting wire; at the same time I hope he will do s the justice of referring once more to letter 526, p. xl, a closer examination of which will, I think, convin him that the confusion originated not with me, but wat the letter as it appeared.

I would pity the microscopist who only obtained his sections by means of the jack plane. To be of real use a section should be very thin. The section instrument should be capable of accurate adjustment to at least the 1-5,000 of an inch, and its screw should have a micrometer head, that the thickness of the sections cnt may be read off, and if needs be registered. Very few amateurs turn out wood sections that are worth looking at. They are nearly always too thick, the gum and resin are rarely soaked out by water and spirit respectively, and the use of turpentine and balsam is too often made to subserve the place of careful manipulation. This is to be regretted, for there are few branches of microscopic research more interesting, and certainly none more easily accessible. We will suppose that our tyro has resolved to commence the study of woods. If he have not great delicacy of manipulative power he must possess a good section instrument with a screw having 50 to 100 threads per inch. Let him cut a twig of an oak tree, one of three or four years' growth if possible; three inches of this will serve his purpose. He must place it in strong spirits for some days, and then transfer it to pure water, where it should remain for ten days or a fortnight longer. A few hours in a warm oven will do it no harm during this stage. He will now fix it into the tube of his instrument, and having made all tight with slips of fir or of cork will cut off as thin sections as possible with a sharp razor. Each section will be floated off into a saucer of spirits and water. He will now take a section and mount it in that medium, a second he will mount in glycerinejelly, a third he will dry between two slips of glass and mount in balsam, and a fourth he will soak in magenta dye a few seconds and mount in glycerine jelly. These are all transverse sections. The same process will be gone through with as regards longitudinal and radial as well as diagonal sections, that 16 or 20 good slides may make his commencement of the study of his woods. Now these slides will all present different features; not the least beautiful, and by no means the addition to this, I have refreshed my memory by con- tensity of current is required, it is of no service to

least valuable, will be the slides stained with magenta.
I have in my cabinet slides which display the advan-
tages derivable from this treatment most markedly.
Such a slide when viewed by polarized light against
the violet ground produced by a proper mica film
exhibits most beautifully its very minutest structure.
The most interesting point probably being the young
and isolated wood cells just outside the existing
wood. These are stained of a slightly lighter tint than
the older wood, the space between the bark and wood
being unstained.
H. P., H.

SINGULAR.

[829] SIR, -Being struck with the remark in "E. L. G.'s" letter (765) that in Shanks's determination of the value of , the Scottish Astronomer Royal showed that there were three 3s to two 7s, it occurred to me to divide the sums of each by the other viz., 3 x 3 = 9,7 x 2 = 14. 9 14= 061428571 14285711 ad inf. Now if we strike off the first decimal we have 6 out of the 9 figures, the three eliminated being 3 and its multiples 6 and 9, and which add to the

=

cube of 3, 27. 149 = 1·555 ad inf., and 1.555 ad
inf. 7 or 2 = 2222 ad inf., or 7777 ad inf. The pe-
culiarity seems singular.
S. B.

BOILER PRIMING.

[8241 SIR,-As an engineer I have taken great in terest in the replies in your ENGLISH MECHANIC on boiler priming to "A Working Engineer," particularly the one by "R. D.," who appears to know as much about the cause of priming as any of your correspondents. I have anxiously looked forward to seeing a little more on the subject, and knowing that the ENGLISH MECHANIC is read by an immense number of marine engineers, I think we should be greatly benefited, as well as all owners of steam-ships, if a few of our fraternity-marine engineers-would write through the ENGLISH MECHANIC their experience with priming boilers, and what means they have taken to cure or improve them.

I can speak, as an engine builder and marine engineer, from twenty-five years' experience, that there are more accidents and destruction of machinery at sea from priming boilers than from all other causes put together, and now that high pressure boilers and surface condensers are coming into such general use, it will be more than ever the case, if priming cannot be prevented. As there are no works written on the subject that are of any use to practical or working marine engineers, we can only communicate to each other our experience through your very valuable journal, and any friends of a superior position, such as "R. D.," may help us very much.

In

INSULATION, RECIPROCAL GEOMETRY, ETC.
[825]SIR,-In obedience to the respectful request
of Mr. S. T. Preston (let. 787), I have once more con-
sulted Montaigne, but fail to find anything in the
quotation at all bearing upon the question of insu-
lation, as to where or how it should be applied.
sulting not only the opinions, but the practical appli-
cation of the theory held by all modern electricians
(within my reach) who kave either made, or ordered
an induction coil" to be made, and I find as a result
they invariably use, or order to be used, an ebonite tube
for enclosing the fasciculum. And why is this? not be-
cause it is cheaper than other material, but simply be-
cause it is the best insulating material that we know of
at present that can be formed into a thin tube, and yet
allow of the primary being wound round it.
Over-insulation, like every other thing in excess,
would be an evil, but nature has deprived us of the
means of obtaining or applying such excess, inasmuch
as she has denied us the material which would give us
an approximation to sufficiency. No word of mine in
the letter referred to by Mr. P., can be construed to
imply excessive thickness of insulating material, there-
by necessarily removing the primary further than
necessary from the core, and I apprehend there is a
vast difference between insisting upon the best insu-
lation, and recommending the thickest insulator. If
the outer coils of an electro-magnet are as efficient as
Schaffner is any authority, he has stated,
"that
vet in the magnets used for telegraphic purposes (such
those in immediate proximity to the iron core," and
as he alluded to) the outer coils are removed much
further from the core than the necessary thickness of
any good insulating material would require. On the
other hand I have insisted quite as strenuously as
Mr. P. upon the absolute necessity for insulation of
the highest order being placed between the primary and
secondary coils, a necessity thoroughly understood and
appreciated by me at the time I wrote the papers upon
the "Induction Coil," in your fourth volume, some
years before Montaigne was introduced to
columns; and as the rod sometimes is suspended in
terrorem above refractory schoolboys, apropos
this, I have found through life that the one most
recently flogged, or who feels that he deserves to be so,
is generally the first to point out the rod to his school
mates.

our

to

Reciprocal Geometry (let. 744).-As it is an invariable rule in practice to obtain a given end by the smallest possible means, allow me to suggest to Mr. W. Gillespie that for mnemonic purposes it would be better to adopt the 5236 for retaining in the memory, than it does to subtract, which he will easily perceive as it requires much fewer figures to add of a result by applying this suggestion to the example he gives in full.

The necessity for this remark will become evident by reference to the six solutions given in answer to query 5390, on page 190.

Of these there is only one practically correct answer, viz., that given by "E. L. G.," which, carried out to completion, gives the diameter of the globe sought

= 27-44478in. nearly.

The heading is, in clear bold type, mangone battery," and from first to last there is no referer whatever in it, either to sulphate of lead or a cell wherein that compound is used as an electroly What, then, could be the inference drawn by thongani of readers equally ignorant as I acknowledge my to be, yet desirous of obtaining information, but tha the engraving represented the battery (cell) to wh the heading of the letter gave a name? Seeing so maj applicants for information upon the nature and con struction of this cell, and finding it patented like t erewhile ozokerit, I was under the impression that sme new combination had been discovered, whereby the between zinc and carbon could be increased me electromotive force due to the difference of tensi cheaply and without the offensive fames of the B it; but since it turns out to be fit for nothing when sen arrangement: hence my desire to know more d

at present.

(Let. 759.) My natural instinet would lead me intes very elaborate disquisition upon the nonsense c tained in this short letter, but knowing from experies that unwelcome truths cannot be plainly spoken w out danger of being Montaigneized at every ste I leave the able suggestor of the "Rainwater Tap Bedrooms to try a few experiments on the sa necessary, and will humbly await the publication

"

his results.

curred the displeasure of Sigma" by hinting (let. -
(5350.) I sincerely hope, sir, that I have not in
P. 134) at the extent of his "ability and will" to ca
fer a small favour, since almost every issue of ta
ENGLISH MECHANIC bears witness that he posses
the first, and I have not found any one yet who han
made a fair call upon the second point has been self
empty away. We have had our tilt in tournament in
the wide Electrical Field in time gone by, but wal I
have been rusting, he has steadily kept on in the path
of progress, and I respect him (for his steadfastues of
purpose alone) too much to allow any stray lop
calami of mine to either wound his feelings or stand in
each other. My present purpose is to say tha
the way of that better feeling which will one day carst
between us, when we become better acquainted with
answer (5950) line 16, the word "increases" is safet
tuted for "decreases," marring the accuracy of the să-
tence; and to thank him for the qualified mania:
which he uses the word quantity (as it is calledứa
great step towards removing a difficulty from tas
student's path.

riority of the Cornish engine, as a whole, lay p
(5470.) I have always understood that the spe-
pally in the superior steam-producing power resid
in the form of boiler used, and not from the utilisan
of the steam in the engine proper, the experimets
upon which this superiority was based having
made long since the principle of the expansive act
known and duly apprecised
steam had become
"Punch" will do well in addition to studying the de
scription of the cataract given vol. XI. p. 455, to pir-
chase and carefully read Armstrong's Rudimentary
Treatise on Steam Boilers," Weale's series.

(5551.) S. Shirtcliff can use the carpenter's sliderule for all the purposes, and in the same manness given, each independently of the other, in the ca described by me and Mr. J. Baskerville, for the g neer's slide-rule, in the respective treatises we hare volumes of the ENGLISH MECHANIC, the only differe being that he must multiply his results by 0625. hand, as I venture in the purest sympathy for a (5559.) Stand thou, Montaigne, firmly by my r ing one to offer advice, lest my identity with Mr. La may be lost once more in that of Mr. Pulvermekti. To" Lame One "I most unhesitatingly say once t fetter thy limbs with Pulvermacher's chains. par I have had trouble enough in my time with priming In the other solutions given, the diameters are used chasing those of his last new form of construction, in boilers, but with the old plan of jet condensers, regu- instead of the radii, thus involving an unnecessary out element (zinc) can be as readily renewed as a which, as he justly states in his prospectus, "any lar firing and a little grease injected in the boilers number of figures. Threo introduce the unnecessary along with the feed water would stop the priming at multiplication by 5236 in four operations, and a division pen can be placed in its holder," and these element once. But grease with boilers and surface condensers by the same number in the fifth operation. "Comus" can always be obtained from the maker. The bare is most destructive, the water being continually used over and Frank Chessell have given practically correct fact that, after wearing these chains for five again from the condensers to boilers, causes the acids of answers, and that of C. Henfrey, Jun., would have been months, the sufferer has grown no worse, while the grease to combine with the copper absorbed in so if he had not taken the radius for the diameter of viously he was rapidly declining, is sufficient to s friction through the steam pipes; this coming in con- the fourth globe, but where Mr. Wm. Airy got the that the remedy applied was at least competent to ar tact with the boiler plates forms a thin solution of number 25-546in. from I must leave him to explain. rest the progress of the disease, whatever it here oxide of copper and iron, the priming increases, steam He has chosen the right track in using logarith, but may have been; once so certainly arrested it her th carries it through the slides to cylinders, chokes up the is off the line at some of the points (Montaigne him to retain the same means that has done is to condenser tubes, cuts the forces of valves and cylinders, again, Mr. Tonkes). good service, and it is the duty of the medical at and friction increases and vacuum decreases; the Curiosities of Mathematics.-Before Mr. J. Jones do his small share, thus left clearly open to his the boilers are forced to keep up speed, and prime more ventures to touch the tempting bait held out to him rapeutic skill, to restore the disordered members to than ever, so that I have known ships go for days and (let. 764), I hope, if he has not already done so, he the water has never been seen in the gauge glasses ex- will read the modest and retiring pamphlet, published cept when the flue doors have been opened. I have in 1866, by the author of the simple query put in the

their normal condition.

WM. TONKES.

P.S.-Allow me to suggest the substitution of the

known both engines and boilers destroyed with two and letter (764). This pamphlet bears the unassuming title in his

1

reply to "Chemical" (p. 179),-"As far as possible
every statement made authoritatively in the ENGLISH
MECHANIC should be perfectly reliable, and so no error
should be allowed to pass unquestioned,"-for the anti- A cubic metre will weigh 7,788 kilogrammes.
quated and never-to-be-obtained philosophical yearn-
7,788 grammes, or 7 kil. 788.
ings of Montaigne, who, however perfect himself,
7 grammes, 788.
will never get human nature to reach his ideal. This A square metre x 1 millim. weigh 7 kilog. 788.
would be a truly valuable exordium to nine-tenths of
centim. x 10 metres, weigh 7 kilog. 788, &c.
"our" contributors.
J. N. D. C.

The ordinary metal balance is not affected in any appreciable manner, it is the pendulum-spring, there- and Thompson's "deep-sea" dredgings had upset all fore "Economy's" introduction of another material for the balance would be useless; added to which, metal can be fashioned in a more useful manner than wood

could be, inasmuch as a plain disc to form the wooden
balance would load too heavily the small pivots form-
ing the spindle of the balance, whereas, by the use of a
metal balance almost all the matter of it can be ap-
plied at its circumference, that being the most
economical and advantageous application for using a
quainted with watchmaking) when referring to a watch
fly-wheel," a term familiar to most persons (unac-
balance. It must be understood the springs referred
their tension, so is their power."
to have the following rule concerning them: "as is

SECONDS PRACTICAL WATCHMAKER.
P.S.-I may add, there would be no compensation When the curve is less:
quality in the wood-balances referred to by your corre-
spondent "Economy."

[830] SIR, In your impression of the 11th "Urban" would make it appear that I have made a very erroneous statement regarding the third or platinic The following formula is an average mean one: sulphide. This has brought to mind an error which exists uncorrected in your pages, made by all who V = 0.08731 [d + 2D]2; answered Red Heat's " "Urban "not excepted. the ullage query, 1-767 h2, h being the height above the It was stated that when iron pyrites (Fe S2) was heated liquid; of course these are all but approximative fordecomposition took place into ferrous sulphide and mula. The last formule, if more than half the cak sulphur; such is, however, not the case when heated be out, represents the volume of the remaining in close vessels, about one-third of the sulphur is ex- liquid. Another French formula (I think of English pelled and magnetic pyrites (Feg 04) remains. That this origin) gives: is correct and all previous answers wrong may be seen by consulting Miller's Elements," the article on the extraction of sulphur. Now to the erroneous answer to query 4938.

CAMERA FOR DRAWING SMALL OBJECTS. [833] SIR,-I am in the habit of drawing outlines of wings of small insects, &c., by means of the microscope and neutral-tint reflector, but even with a 3in. object-glass is only the smallest wings that can be so drawn all at once. Now can any of your correspondents assist me with the idea of a form of camera by From the analysis it is evident that the compound means of which I could draw the image of an object, obtained by the above process is the persulphide, and say in. circumference, magnified only two or three why should it be heated to redness to "drive off every-diameters, and with the whole of the figure in the field? thing volatile" if at that temperature it would be This could not be done by means of the compound decomposed ? I say again, as I said before, when microscope, but I fancy some contrivance might be heated to low redness it remains unaltered, but I might arranged, the magnifying power of which should conhave added that a bright redness, approaching whiteness, sist of a large simple lens of low power. Possibly some decomposes it into platinous sulphide and sulphur. form of "camera obscura" might answer the purpose, GEORGE E. DAVIS. but I fail to catch the idea. ENTOMON.

it could be well compressed; the hole being larger at THE LECLANCHE BATTERY AND ELECTRICAL the bottom would hold it firmly, while the platinizing APPARATUS. would in a great measure prevent local action being set up, which is the object of this suggestion, and not with any view of providing a metallic surface for the solder to unite to. The union can then be covered with melted paraffine or shellac varnish as a protec