THE BAROMETER.

[595] SIR,-A method of showing the variation of the barometer, when such fluctuation is too minute to be easily recognized by the usual scale will, I opine, be acceptable. I propose to place inside the tube, floating on the mercury, a small steel magnet, whose attraction shall reach through the glass, and influence the motions of a magnetic index placed conveniently before it. I think also it would be possible to surround a portion of the tube with coils of wire, and make the motion of the magnet sensible to a delicate magno

meter.

The changes of a magnet placed as I have suggested would, of course, be impeded slightly by friction against the sides of the tube; however, this fault might in great measure be remedied by tapering the magnet somewhat like an obelisk, the thick end down, and tapping the instrument before reading.

At least by those whose business is much out-doors, and who consult the barometer frequently, it will be conceded that a device for quickly showing its tendencies and changes will be an estimable boon. I send you a sketch. It is not quite as I would make the im

[597] SIR,-A short time ago I wrote a letter to you containing an account of my first experiences in speculum working, but thinking it too long for what it was worth, I did not post it, but I will, with your permission, give an account of my more successful and latest attempt to make a 6" silver on glass reflecting telescope. My apparatus was as follows:

1. A circular disc of cast-iron 6" diam., 1" thick. A piece of board " thick, 12in. +8, with a wooden rod nailed to one end as a handle, and three wooden blocks to hold the speculum screwed on to one side, and a cwt.

answer the purpose of most gas consumers, large or
small, and would not be very expensive. Anybody
making them at a reasonable price, showing the differ-
ence in light and economy, would find a ready sale
for them. No gas consumer ought to be without one.
Can be manufactured of tin fittings, and all complete
for 58.; that sum can be saved in a month by its use
by most shopkeepers. It can be made to hold half a
pint or pint of the material in the lower chamber; will
last some time; but of course that depends upon the
number of burners.
JACK OF ALL TRADES.

LUNAR COSMOLOGY.

[598] SIR,-I would submit a few critical remarks on this subject to the notice of your readers. Ap

pended is a rough section of the lunar crater Theo-provement, but is designed to illustrate the principle.

-R

philus, plotted to scale from the main dimensions as given in Webb's "Celestial Objects." Outside height of ring, 3,200ft.; width, 337,920ft., or 64 miles; depth of cavity, including ring, about 16,000ft.; height of central hill, 5,200ft. It will be observed on reference to the figure how insignificant the mass of the ring is compared with the volume of the enclosed cavity, whereas in the case of the terrestrial crater mountain or volcano the cavity is small compared with the mountain. I therefore would object to the term "crater mountain" as tending from terrestrial analogy to mislead. The lunar formation partakes, in fact, more of the nature of a pit or valley than of a mountain; the term "walled pit" would be perhaps more suitable. As there is so great a contrast in the two cases one would suppose that the originating causes must also be dissimilar.

WINDMILL.

S. T. PRESTON.

B, tube of barometer.

M, magnet floating on column of mercury.
P, index hand; also a magnet delicately balanced
at A.
EDWARD B. FENNESSY.

THERMO-BAROMETER.

[596] SIR, I have been much too tardy in acknowledging "Cornubia's" explanatory letter and diagram. But I have been very busy for the past two months, and have had illness at home, which kept my hands full, or I would have acknowledged his courteous communication at once. I have no difficulty in understanding his second explanation, and managed to construct an instrument like his, in a temporary way, which convinced me of its practical usefulness. It is just a pity it is not portable.

I venture to enclose a sketch of the instrument I mentioned in my first letter. If you think it worth the space, I would be glad to see it inserted. It is the same in principle as "Cornubia's," and so far as that goes, his description will answer mine.

B, a glass tube, with the bulb and reservoir in one piece and is filled with mercury.

C, a wood bar, sliding between the scales, by means of the screw D, to which the tube is fixed.

to put on the top of this board. I obtained my glass disc, 6" diam., cut from lin. plate glass, for 4s. 6d. With the help of a friend's lathe I turned my iron tool to the required curvature, and filed grooves in its face at about " apart at right angles to one another. I then fastened the tool to a post, and the glass disc to the board, and having pat some common moist emery on the tool began to grind, with the cwt. on the top of the board, secured to it by a few nails driven in around it. The grinding seemed to progress very fast at first, but afterwards became slower. During the process I frequently shifted the position of the glass, by twisting it round between the three blocks of wood; I also walked round the post all the time. The glass was completely ground down in 15 hours, and with only two scratches, which were so deep that I could not get them out.

During the grinding, I only used common emery un washed, and finished the grinding with rouge added to the emery after it had been well ground down. At the commencement of the grinding, I added emery moistened with water about every eight minutes, but for the last half-hour I merely added rouge and clean

A, is a mahogany case, with the front off exposing all the parts of the A CHEAP POLARISCOPE.-TO "VULPECULA " instrument. AND OTHERS INTERESTED. [598] SIR,-As is well known to all who have the least knowledge of optics, the phenomena of polarization are easily produced, and in one of my papers on the microscope I referred to several means by which polarized light might be made available by the microscopist. This evening I thought I might profitably spend un mauvaise quart d'heure in devising some form of polariscope suitable for those who, having limited means, desire to economise their resources. It is with much pleasure that I ask the editor to allow me to briefly describe an efficient instrument which may be made, by any person possessing the least ingenuity, at the large cost of twopence.

A pointer is attached to this bar, which marks the variation of the thermometer E. There is another on the other side, attached to a sliding rod, worked by a cord and pulley, which indicates the barometric pressure.

It will be necessary to explain, before taking an observation, that the tube B, or rather the mercury in it, is set in motion by two different forces, the one is atmospheric pressure, which is its proper function to indicate; and the other is atmospheric temperature, which is the peculiar function of this instrument to correct. To accomplish this I ascertained, after repeated trial, to what extent the mercury in the tube B was influenced by temperature. I then got the thermometer E made of exactly the same range. Suppose now that E stands at 60' and B at 29-50. After a while E rises a degree, the barometric pressure being the same will yet not read the same on the tube B,but a thermometric degree lower. Now turn the screw D till its pointer is on a level with the risen thermometer, and then the two pointers indicate the real atmospheric conditions, an increased temperature, and an unaltered density. I think an instrument constructed on this

To each of the two sides of the frame fix a wire with a milled head (or stick a piece of cork on the wire). Next fit into the "well" of the microscope stage a pasteboard tube, into which the polarizing frame just described is to be fitted, the wires passing through the tube, that the frame may be placed at any angle desired with relation to the axis of the tube. That is the polarizer, and may be used either by reflection or refraction. The analyzer may be constructed as described by "Bridgeton," on p. 61 of this journal. I have tested this little instrument severely, and have found its performance quite equal to any other. It has, of course, one great drawback: it does not afford facility for rotation of either the polarizer or the analyzer.

To use the instrument you have merely to fit the polarizer beneath the stage, and having fixed the analyser above the eyepiece, to adjust the polarizing frame to the needful angle of polarization. I shall be glad to hear of the results any reader may obtain through the use hereof.

[599] SIR,-In reply to Henry Hammond (5045), I send the following papers on rocket making, which I have no doubt will be interesting to other readers besides. The three prime materials of the art of Pyrotechny are nitre, sulphur, and charcoal, along with filings of iron, steel, copper, zinc, and resin, camphor, lycopodium, &c. Gunpowder is used either in grain, half crushed or finely ground, for different purposes. The longer the iron filings, the brighter red and white sparks they give; those being preferred, which are made with a coarse file, and quite free from rust. Steel filings and cast-iron borings contain carbon, and afford a very brilliant fire, with wavy radiations. Copper filings give a greenish tint to flame; those of zinc, a fine blue colour; the sulphuret of antimony gives a less greenish blue than zinc, but with much smoke; amber affords a yellow fire as well as colophony and common salt, but the last must be very dry. Lamp-black produces a very red colour with gunpowder, and a pink with nitre in excess. It serves for making golden showers. The yellow sand, or glistening mica, communicates to fireworks golden radiations. Verdigris imparts a pale green; sulphate of copper and sal-ammoniac, a palmtree green. Camphor yields a very white flame and aromatic fumes, which mask the bad smell of other substances. Benzoin and storax are used also on account of their agreeable odour. Lycopodium burns with a rose colour and a magnificent flame.

Of all fireworks, rockets are among the most noble and effective. The ingredients for these, the apparatus employed, and the detail of the manufacture of them may be considered the foundation of all fireworks, and to make them well involves the same principles, and requires the same caution, as in making all others. SIZE OF ROCKETS. The size of rockets is indicated by ounces or pounds, thus we say, an eight ounce rocket, a pound rocket, and so on; by this expression it is not meant that the rockets weigh so much as their name indicates, but that the bore or cavity of them will just suffer a leaden bullet of that weight to pass down them. For example, a pound rocket will admit a leaden bullet that weighs a pound. Rockets may be made of any size from one ounce up to fifty or more pounds.

FORMING THE CASES OR CARTRIDGES.-These may be made of any kind of stiff thick paper, either cartridge paper, or what is equally as good and much cheaper, namely, common bag-cap paper. To roll up

the cases, you must have a smooth round ruler, or as
it is called a former, exactly the size of the cavity of the
rocket, and 10 or 12 times as long. Then lay a sheet of
the paper upon a table or board and paste 4 or 5in.
along the end of it, leaving the rest of the sheet of
paper without paste; then roll it smoothly over the
former, dry end first, until the whole is rolled up,
when, of course, the paste will stick and a thin case be
formed. Keep rolling it along the table with the
hands, in the same way as a rolling pin is used, for
two or three minutes, until the various folds of the
paper set close and tight to each other; then put on
another sheet in the same way, and so on, till the case
is thick enough, and this is known by the measurement
across it. If the former without the case measures 5
parts, when the case is upon it they must measure
together 8 parts. That is, the paper must be rolled on
till it forms a case, the thickness of the sides of which
are a trifle more than one-third of the thickness of the
former. The length of the rocket case, and conse-
quently the width that the sheets of brown paper are
to be cut before pasting, varies with the size of the
rockets; in small rockets the length of the case may be
six times the diameter, in larger rockets 4 or 5 times is
sufficient. When the case has proceeded thus far, it is
to be choked while yet damp, that is, to be contracted
in diameter near one end, and for this purpose, a
simple contrivance is requisite, called a choking cord, be closed at one end. It only, requires to be primed at
and also the former is made with a hole drilled at one the other end, and that it will be observed is the end
end, and a second joint made to fit on by means of a
wire projecting at one end of it, and which fits into the which was choked, and which is still open, and has a hole
hole of the former (Fig. 4). To choke the case, draw passing up it which the piercer occupied. To prime it
the former partly out, until you can see about an inch fill up the hole with loose gunpowder and paste a piece
of the inner cavity of the case, then put on the second
of touchpaper over it.
joint (the wire of it going into the hole of the former),
and pass this on until its end is about half an inch
within the case, and consequently leaving a space of
about half an inch between the two joints occupied by
the wire alone. Then going to an apparatus similar to
that figured in No. 5, turn the cord once round the case
where the cavity is, put the foot upon the treadle,
which tightens the cord and squeezes the paper case
at the point required, and that it may squeeze it
equally and neatly on all sides the case should be held
in the hands and moved up and down upon the cord,
until the operator sees that it is sufficiently and properly
compressed. Let it be observed, that although the
choking apparatus used by the firework maker is repre-
sented and above alluded to, yet to the amateur it is
by no means necessary. What will do quite as well,
is a thin cord fastened at one end to a staple in the
wall, and by the other, tied round the waist of the
operator; as he may lean back, of course the cord would
be tightened, and the desired purpose accomplished.
When the case is sufficiently compressed it is to be tied
with two or three turns of strong string. The case is
now complete, except that the part of it where it is
choked is perhaps rather rough and uneven inside;

GLYCERINE AS A PRESERVATIVE OF

MICROSCOPICAL OBJECTS.

[600] SIR,-Next to Canada balsam, glycerine (Price's, of sp. g. 1240) occupies a prominent position amongst media in general use. Professor Beale, than whom no higher authority could be quoted, speaks in the highest terms of this medium, and says, "from the use of it I have learned more than from any other preservative medium." Indeed, were it not that almost insuperable difficulties attend its use with respect to its great action on nearly all cements, there is little doubt that it would be far more generally used than any other medium soever. We cannot perhaps spend our Thursday to better purpose than in learning to mount in glycerine. We will choose muscular fibre for our first attempt. From the prime leg of lamb our butcher has sent in for dinner we will cut a small piece of lean. Removing this to our table, we cut a very thin slice, and place it on a glass slip and apply a drop of glycerine. After half an hour has elapsed, we proceed to tear the fragment to pieces under our lens; then, having placed a thin cover over it, we carefully press it with a cycloidal motion until we have reduced the meat to the thinnest possible condition. Carefully raising the cover, we with a tine camel's-hair brush wash away the glycerine, great care being used lest we wash away the object also, and replace that removed by a new drop and carefully place on the object one of our thinnest covers. If properly done, no air-bubbles will be included, and the thin cover will be almost perfectly close to the slip and will adhere with sufficient tenacity to enable us to wash the glyce rine from its surface and edges. This done, and the cover dry, we paint carefully round the cover with Bell's cement or gold size, preferably the former, and lay it aside to dry. In a few hours we repeat this process, and again next day, and at intervals of four days for a fortnight. A slide thus prepared will under a good in. show the fibrille of the muscles and their striæ admirably, and by care in giving the cover a ring of varnish once or twice a year will be serviceable for a

lifetime.

Glycerine must not be used with any structure containing lime.

Glycerine and water, equal parts, with the addition of camphor, is most valuable for the preservation of desmids, fungi and the like, and also of many vegetable structures. Desmids, by the way, may now be easily obtained in almost any pool, more especially the very common species, of semi-lunar form, the closterium. The collector may easily obtain these from the stems or leaves of aquatic plants, by drawing these between the centre fingers, with the hand so held as to receive them as they are rubbed off. It is convenient to be provided with some means of examining the collection in the field, that a great quantity of useless material may not be carried home. For this purpose the Coddington lens is specially adapted, or the student may use the much cheaper, but very efficient," Bristol Microscope," of Messrs. Swan & Co., possessing a considerable power, with facilites of The collected diatoms and desmids have merely to be placed in the centre of a slip, and the medium placed on them; the whole allowed to remain for an hour under cover, and then (no air-bubbles being

use.

H. P.

GAUZE NEGATIVE BATTERY PLATES.
[601] SIR,-Wishing to ascertain the most eco-
nomical size of wire for exposing its greatest surface in
a battery plate, I found the theoretical consideration
of the subject presented me with highly interesting
results.

Let a piece of gauze be considered as consisting of
two sets of equidistant parallel wires, in two parallel
planes.
Let a area of plate of same dimensions as gauze,
d = diameter of wires,

Care

and frequently exposed to light, are still beautifully the key, and with a flat or half-round smooth file, file
fresh. Other media, such as gum and glycerine, it from top to bottom all round, till it is perfectly
Goadby's solution (bay salt, 4oz.; alum, 2oz.; corrosive clean. Take a round smooth file and file the barrel
sublimate, 4 grains; boiling water, 4 pints), an clean, holding it in the one hand, and the file in the
aqueous solution of chloride of lime, and carbolic other; catch the key in the vice, and twist the barrel
acid and water, are all useful for special purposes.
round on the key, till any parts of the key are bright
and bearing hard, and continue rubbing and filing
alternately till the parts are bearing all round from top
to bottom. Next procure a piece of common freestone;
pound it to a powder, sieve it through a piece of coarse
packsheet; mix it up with water; add a third of brown
soap; boil them together till the soap liquefies; catch
the key at the end of the vice horizontally; put on the
sand, and continue to turn the barrel backwards and
forwards, frequently withdrawing it, adding a little
sand where it may be dry or bearing hard; continue
this tall the file marks are all out, which will be in about
15 minutes; continue another ten, adding no sand, but
keep the key wet with spittle from the mouth.
must be taken in keeping the barrel well pressed
against the key, and when withdrawing it do so in a
straight line, so that it may not rub on the key. Dry
the key and barrel perfectly clean; moisten the key
with grease; put on the barrel, and turn it direct
round three or four times; take it off; if the key and
barrel bears from top to bottom regular it is right;
if hard in any part add a little sand, and continue as
before; the key will be about an eighth further through,
which, if required, must be taken off, to get the nut and
washer to bind as before. This, I hope, is plain
enough, and in the hands of "Inductorium" will, I
think, be accomplished. I should have been glad to
have done those he has for him, but I fear we live too
far apart. Should he ultimately fail, by sending sketch
of size and address to the MECHANIC, I will make him
one, and describe its alloy, as this has something to do
in the matter; he can then report after trial, and thus

and

D = distance between adjacent wires,
= area sought,

A

d
D+ d'

which is the ratio between the

area of the projection of one set of wires upon a plate
of same dimension parallel to them, and the area of
the plate itself;

d

=

1, consequently
the surface of the guaze is to the surface of the plate
as 3.14159.... is to 2. The practical deductions from
the above are, that large wires, while being stronger
and better conductors, allow the distance between them
to bear a small ratio to their diameters, without being
so near together as to obstruct the evolution of gas,
and the polarization and circulation of the fluid.
The facts of the points of contact not being mathe-
matical points, and the wires not being straight and
longer, somewhat compensate each other in the cor-
rection of the above estimate.

W. H. COFFIN.

[604] SIR,-In answer to query, No. 4932, "H. P." writes:-"The plant possesses the power of assimilating the carbon, hydrogen, &c., in its vicinity, and of transTo the animal mitting them into organizable matter. this power is wanting." "E. L. G." says:-"Plants alone can convert inorganic or non-living matter into living." And "Osa" follows in the same strain by saying:-"The most tangible difference lies in the nature of the food, that of plants being inorganic, and that of animals organic."

I conclude, therefore, that these three gentlemen concur in believing that animals cannot derive nourishment except from matter which has been already organized either as a vegetable or as another animal. Now we know that a very great part of an animal's body is composed of oxygen taken from the organic, is only a mechanical mixture. This is proved surrounding atmosphere, which, so far from being by the fact that human beings are heavier in the morning before they have touched food than they are after meals; they can receive that accession of weight only by assimilating the inorganic air.

Moreover, although men can live for weeks without food that has been organized, they can by no means last longer than about three days without water, of which

included) a thin cover cemented over them. This ELECTRICAL QUERIES. THE SULPHATE OF inorganic liquid an animal's body is principally com

plan, of course, only applies when the natural ap-
pearance of the diatom is desired. Objects of less
tenuity require to be mounted in a cell, which may be
of Bell's cement, of glass or metal. Care must be
taken to guard against the inclusion of air-bubbles,
otherwise there is no special difficulty to be met.
Spirit and Water. Very useful for molluscan
palates, for wood sections, and a variety of objects. May
be easily prepared by an admixture of about 1 part of
absolute alcohol to 5 or 7 of distilled water; methy-
lated alcohol answers equally well with
alcohol, provided it be of equal strength. For des-
midi, Thwaites' fluid may be used instead of glycerine
and water. This is composed of water, 16oz.; spirits
of wine, loz.; creosote, enough to saturate the spirit;
chalk, as much as needful. To be used in the same

way as the other fluid media.

4oz. Joz. 6 drops. Soak the gelatine in water until soft, and to it add the honey which has previously been raised to the boiling point in another vessel. Now let the mixture be boiled, and when it has somewhat cooled, add the creosote dissolved in the spirits of wine, and filter the whole through thick flannel. Glycerine jelly consists of equal parts of glycerine and gelatine, and is prepared in the same way.

We have a delicate scale-moss. We will soak it a

little in glycerine, and transfer it to our slip of glass. We slightly warm a little of the medium, and place it on the slip which we have on our warm mounting plate so that the object shall be immersed in it. Having removed such air-bubbles as may be visible, we breathe on a thin cover and carefully place it over the object, and allow the whole to cool. A ring of varnish completes the whole. Certain slides of mosses in our collection mounted some years since,

The makers say the power is 20,000 times; this, of course, is in superficies. Reduced to a common-sense statement it is 140 diameters.

MERCURY BATTERY.

[602] SIR,-I shall feel obliged if you will allow me to place the following questions before the readers of the ENGLISH MECHANIC:

1. Is there any cell more suitable than this for ringing bells, or working a domestic telegraph, requiring only occasional use?

2. Does the mercury which forms at the bottom of the cell contain zine, or is it pure?

3. Which is the most advantageous distance for the
zinc and carbon to be apart? I suppose the further
they are apart the more the resistance and the feebler

the current, and (I presume), consequently, the longer
the cell will work without requiring recharging?
4. I suppose I can increase the power of this form of
battery by making the plates larger? Will increasing
the size of either the zinc or the carbon produce the
same result? but in the former case would a greater
quantity of the sulphate of zinc be formed than if I
enlarged the surface of the carbons only, or would in-
creasing the size of the carbons (and leaving the zinc)
create increased action on the zinc, and bring about

posed.

If either of the above-mentioned contributors would be kind enough to explain this contrdiction, they would oblige a great number of your readers. In "Osa's" letter it is implied that the animality of the coral is doubtful. I have always understood that it belongs to the group of Zoophytes; the woodcuts I have of it would certainly mark it as a Radiate. A. F. JONES.

COTTON-SPINNING.

able journal a long time. Some months ago I was
very much delighted to see articles on the staple trade
of Lancashire-viz., cotton-spinning; but I am very
sorry to say that I have seen or heard very little of our
cotton friends lately. Now, I think, if we could only
now and then have papers on cotton-spinning from our
friends, it would very much enhance the value of
our journal. Will Mr. Slater, or any of our friends,
kindly explain how the spindles of a mule are driven
and governed during the winding or taking-up of the
Which are best for making level yarn-1 or
yarn?
2-thread rollers, dead weights, or 3 threads weighted
as I find
with levers-and the why and wherefore?
in Oldham nothing but the latter will take, and, in
Bolton and districts, the former are adopted. What
are crackers in yarn, and how caused?

[605] SIR,-I have been a subscriber to your valu

If any of your clever correspondents will kindly answer the above, they will confer a great favour on A FACTORY HAND.

THE AGE OF MAN. [606] SIR,-Glancing through Sir J. Lubbock's new work on "Civilization," Part I., my attention was closely fixed upon his print of a worked bone from La Madelina, Dordogne. Subsequently I met with a cast of it at South Kensington, and have since inspected a duplicate at the Christy Collection, No. 103, Victoriastreet, Westminster. It stands in Room I., central desk case; and is described as "part of a mammoth's tusk, engraved with an outline of that animal."

[603] SIR,-Our worthy contributor, "Induc- My object in writing is to protest against any repetitorium," after replying to a number of questions, puts tion of this statement as a distinct assertion; for it two himself, and asks if any one will answer them. appears to me that the finished prints do not correI now beg to answer the latter of the two questions-spond with the figure rudely outlined upon the bone. grinding taps to stand a vacuum pressure. Take out Admitting that the bone, which is in Paris, may be

fossil mammoth ivory, I am unable to satisfy myself
that the mammoth is anything but an elephant. Con-
sequently we have no proof that the pre-historic artist,
who inscribed this bone, really lived in the mammoth
period. Individuals may hold to what opinions they
please; but a mere vague resemblance should not be
trumpeted forth as an undeniable reality in our educa-
tional museums.
A. H.

and money in inventing a new index. The old-fashioned as the roads are good or bad, without much fatigue,
Perhaps, among your numerous readers, some one
index with its peg is certainly a very insecure fastening and I prefer it greatly to walking.
for any work that is the least heavy. The tangent
screw has not that defect and, in addition to the common could suggest a method of fixing on india-rubber tires
index and dividing plate, is an immense improvement to wooden wheels, tired with iron in the usual way,
to a lathe; for with it the most minute division of the which I am assured would be a boon to many and
DANDY 'Oss.
circle can be obtained, and differently divided heads would enable the generality of bicycles to compete
can be applied to the screw; as this screw is lifted into with the Phantom.
gear by a cam underneath, the pulley can be fastened
in any position with perfect rigidity, which is a very
useful capability for many purposes; and, again, with
the winch handle on the tangent screw, the mandrel
can be revolved with the slowest possible movement,
and for any required distance or part of the circle;
this power is especially useful with segment stops. I
do not fancy the screw would meet every requirement
and so allow the total abandonment of the index, or
that it would suit double counting, of which I share
"J. K. P.'s" horror.

I, with many others, should like to hear more of "W. H. N.'s "planing machine (480), and hope that he will favour us with a drawing and detailed explanaQ. YORKE. tions.

VENTILATION.

[612] SIR,-Your correspondent, D. Clarke (letter 487), has not explained how he gets a supply of fresh air in place of that driven out by steam. Some one acting on his idea might find a draught worse than the smell, and in default of fresh air would find the action of his steam pipe not quite so satisfactory. HY. FLETCHER, Ventilating Engineer.

MR. GOFFIN AND MR. DAVIS. [607] SIR,-I cannot imagine how Mr. Davis could for a moment suppose that I was jesting in my remarks on his chapters on chemistry. Allow me to assure him that I was in sober earnest in all I said, and that he is right in concluding that I know better than to jest in such valuable space. Fun is all very well in its place, but I seldom indulge in it. Now, of course I was not How aware of Mr. Davis's arrangements with you. could I be ? With reference to the question of atomic weights, &c., the paragraph headed "Atomic and Molecular weights" (eminently unsatisfactory in Acids, Bases, itself) being followed by another headed" and Salts," led me to conclude that the question was disposed of. I am, however, glad to see it was not so; CARBONATE OF SODA, IRON AND LEAD ORE, Preventing accidents from foul air, &c. Now the prinand for the chapter specially treating on it, I have little but praise; but as I could not tell that it was coming, I only drew attention to what evidently appeared an omission. It would be of no use pointing it out when the series was ended. Then with regard to the typical examples, I do not object to all the examples adduced, Nitric, sulphuric, and phosphoric acids are simple enongh, and very appropriate; but Mr. Davis does not hydrogen pyro repeat hydrogen ortho-silicate, phosphate, disodium hydrogen phosphate, hydrogen sodium, potassium phosphate, and microcosmic salt as examples given. Surely he might have found some simpler.

Now,sir, it generally happens that persons having no definite object in view, either in speaking or writing, fall into errors that betray their want of method and carefulness, and I am afraid that Mr. Davis is no exception to the rule. I will only use his own words as his whip. First, he makes me say, "why did I not give the atomic weights as found from the specific heat of the elements." Pray where did I say this? He had given them. I think you will find that I said "why not have given the method of finding them ?"

Mr. Davis says "a moment's thought would have convinced Mr. Goffin that the articles were not intended for the readers of other publications; if they were it would have been a folly to draw their attention to articles in their own papers." How is it, then, that Mr. Davis is guilty of the mistake? for in Chapter I. he refers his readers to other articles in the ENGLISH MECHANIC no less than three times. This is perfectly legitimate, and a very good practice; I did not object to it. Mr. Davis asks me whether I thought he intended to introduce qualitative and quantitative analysis to the elementary student. Well, I confess I had my doubts about it. There appeared internal evidence of such an

the class of readers the chapters are intended for we

THE SAMPLE POST, ETC.
some other
[609] SIR,-Perhaps Mr. Davis or
of your chemical readers would kindly inform me what
change occurs when carbonate of soda is fused in an
iron crucible-that is to say, are its chemical con-
I would also feel obliged if
stituents altered, and does this preliminary fusion de-
crease its power as a flux?
any of your readers would furnish me with an easy
method of estimating the iron, sulphur, and arsenic, if
any, in a sample of silver or lead ore.
"Semper Paratus" will find Hood's "Eugene Aram"
I do not
in Mrs. Bell's "Class Book of Poetry."
know the publisher, but it can be ordered from the
compiler, Kildare-place, Dublin.
"Achromatic" deserves great praise for his able
letter on the sample post question. It is absurd that
when the State can afford to drive the stationer out of
the market it cannot afford to act a little more
generously in its own proper sphere.

Thanks to "F.R.G.S.I." for his kind promise in re
blow-pipe and mineralogy." If "our" editor
the "
thinks that a few papers on assaying would not be un-
welcome to "our" readers I shall be happy to con-
tribute, if not why I must only look about to try and
of mind,
make myself useful, in some other way, to the great
joint-stock
MECHANIC.

association

the ENGLISH
UN IRLANDAIS.
[The papers on assaying would be welcome.-ED.]

[613] SIR,-In answer to your correspondent, Mr. D. Clarke, p. 60, No. 487, on ventilation, I see that he proposes to use high pressure steam for the ventilation of coal and other mines as a very effective means of ciple Mr. Clarke proposes was tried by Mr. Buddle at Hebburn Colliery, in the year 1811, where so great a discharge of gas had taken place (during a creep) that the very inefficient. We seem to hear nothing more of it ordinary furnace could not be used, but was found until the year 1835, when Mr. Goldsworthy Gurney an improved plan of applying high pressure steam by proposed to the Parliamentary committee then sitting, means of a number of small jets, from 20 to 30, accordan inch in diameter, having a good pressure of steam. ing to the requirements of the mine, and about gths of Also in the year 1848, Mr. Forster, viewer of the Seaton Delaval Colliery, applied it and published some accounts of its superiority over the furnace. This induced several other trials to be made, but I believe that they have all been give up as inferior to the furnace. Mechanical ventilation seems to be the thing that mining Now if my brother readers, who are mechanically inengineers are discussing at present as the best mode. clined, would put their heads together, they would be able to give us a good machine for exhausting the foul What we want is a machine that will air from mines. produce a large current of air with very little engine I am aware that we have at present some first power, and one that is not likely to get out of order soon. they are far from being perfect and cost more to work COAL. class machines for this purpose, but still, I believe, them than there is need for.

[614] SIR,-D. Clarke appears to labour under the erroneous impression that his idea of using the steam jet for ventilating coal mines is an original suggestion. But such is not the case, for this mode of ventilation has been in operation in the north of England for a great number of years, and a series of very

HERAPATHITE AND POLARIZING. [610] SIR,-It is certain that many of the readers of the MECHANIC would be glad to effect their purposes intention, but now that Mr. Davis has told us distinctly at as small a cost as possible; and yet, unfortunately, extensive and exhaustive experiments were made about in many cases, they are taught to do this, that, or the shall be better able to judge of their fitness for such other in a more expensive and troublesome way than other collieries to test its efficiency and economy, as And so, for instance, many a is really necessary. persons. But why, if the chapters are intended for such as have passed through Buckmaster and Roscoe, does person would like a good powerful electrical machine -one which will really perform experiments in good this sentence occur in Chapter II. ?- Probably the work without trouble in warming, and coaxing, and beginner will be puzzled to know the signification of style,-one ready at all times and in all weathers to the above letters or symbols." Puzzled indeed!

But

The result of

twenty years ago at Delaval, Belmont, Hetton, and
compared with furnace ventilation.
these experiments was to prove by practical demonstra-
use of the ordinary furnace, possesses the advantages
of greater cheapness, regularity, and efficiency over
tion that ventilation by rarefaction, produced by the
every conceivable mode of applying the steam jet

would those who have passed through Buckmaster and Roscoe be puzzled with such symbols (or rather for. / into a man's pocket who has not much to spare. Now It was shown that the "current created by the steam"

mula, for no symbols are given)?
Again, in his letter Mr. Davis says, "It is no trouble
to teach the tyro the elements of crystallography (at
least I have found it so) in the earliest stage of his
study," ergo, he intended the chapter on crystallo-
graphy for the beginner in his earliest stage. I do not
dispute its value to such as have passed through "Buck-
master and Roscoe," but to introduce it to the tyro I
consider mere folly, and I think most practical teachers
will bear me ont in this. My former letter assumed
that "Introduction to Chemistry" was intended as a
real introduction, i.e, for beginners
Finally, with regard to the word radical, I was aware
that it was written radicle by Miller, and although I
very greatly respected Dr. Miller, having the pleasure
of his personal acquaintance, and am deeply grieved
to read of his decease, still I cannot agree with him
as to the use of the word; and although I have not yet
read Harcourt and Madan's "Practical Chemistry," I
am sorry to hear that they have introduced the word.
Why does Mr. Davis follow the minority instead of the
majority in this as in some other things?
Radical" is a class desiguation, adjective abstract,
takes the form of radicalis, the abstract to which radix
is concrete. Radicle is a proper designation, substan-
tive concrete, and our best lexicographers, as well as
most of our chemists, assign the former to chemistry

and the latter to botany.

Mr. Davis may think me harsh perhaps. I assure him I have no intention to be so; I only wish to show that his logic is weak, and to give it as my opinion that a series of chapters ought to aim at something definite, and strictly to the purpose, for whatever class of readers they may be intended. I am sure that Mr. Davis is a hard worker, and desires to do good, and he has my hearty good wishes in everything.

ROBERT E. H. GOFFIN.

DIVIDING APPARATUS. [608] SIR,-I should like "W. H. N." (480) and "J. K. P." (576) to try, if they have not already done so, a tangent screw to their pulleys before they spend time

amalgamizing and one which won't dip too deeply
to effect this with the usual run of cylinders, plates,
&c., and to buy apparatus would cost a good bit of
money. I often see them advertised, but after the ex-
perience I have had I should be sorry to give the prices
asked; and so it was that I recommended what for
many years I found most effectual, and neither the
muddle nor the ill-looking affairs "Sigma," in the last

was almost entirely due to its heating properties,

little or no effect being produced by the mechanical
As a reader of the MECHANIC I consider it my duty to
correct any little mistake which comes within the
propulsion of the air by the jets.
of
range
Every other leader, I trust,
my experience.
will do the same, and endeavour to make our journal
W. P. SHIELD.
and useful.

ON

OF CARRIAGE
THE CONSTRUCTION
WHEELS, CHAIRS, AND OTHER ARTICLES OF
FURNITURE, INTERIOR HOUSE FITTINGS,
ETC., WITHOUT JOINTS.

part (who, I presume, has not tried them) pronounces the promulgator of nothing but what is unimpeachable
them to be. I can only say that by using a green glass
carboy, it may be fitted up with conductor and stand
for £1; will look well, be no trouble to get to work at
any time, and be more powerful than four or five times
the money would buy, as machines are generally sold.
And so, again, I see in the present part that some per-
sons, "Vulpecula," for instance, would like a polari-
cost, and so he is trying to get a crystal of herapathite.
scope at a cheaper rate than Nicol's prisms usually
Well, by the time he has got a crystal which will do
well, what with materials and trouble, and one thing
or another, it will have cost him more than a most
efficient polariscope for the microscope need cost.
The one I have used for many years is a small slice of
tourmaline, cost 28. or 3s., let into a piece of boxwood,
so as to fit over the cap of the eyepiece, and a reflector
made of sixteen pieces of thin glass. All that has to be
done is to tilt the microscope to the proper angle and
The angle for
then mark it, so as to know in future.
sixteen plates of glass is, I believe, about 69°; a
few trials will show the best position.

BICYCLE RIDING.

E. T. S.

[611] SIR,-I am sure all bicycle riders would esteem it a favour, if some person, who having ridden the several descriptions of bicycles, would candidly give us his opinion on them, and point out to us the best machine, stating his reasons for doing so through the medium of your columns.

The machine I have used for the last six months is an ordinary bicycle, having a 36in. driving wheel, 6in. cranks, double steel spring, wooden wheels with iron tires, and was made by a firm at Bridgewater. I find that I can travel from 5 to 9 miles an hour, according

[615] SIR,-I have long thought that if we could persuade a hickory, rosewood, or walnut tree to grow in the form of a carriage wheel or a chair, we should not only save all the cost of construction, but, from the absence of any joints and cross-grained portions, they would be far stronger and more durable than they are at present; for it is a fact that both chairs and wheels ordinarily yield and go to the bad at their joints; but as my faith in culture, however great, is hardly suthalthough I am cient to cause me to hope for trees being trained to grow in the required forms"free to confess" that it is hard to say what changes culture may not produce in vegetable and animal life, especially that very puzzling specimen of the latter yclept human life-I am induced to suggest that it is possible artificially to construct such articles as those mentioned in the title of this paper, so far resembling vegetable growths and metallic castings that we may (for all practical purposes) regard them as having no joints to give way to the strains to which they are ordinarily subjected.

It has been said that the bitter satire of one age sometimes becomes the practical science of a succeeding one, in illustration of which frequent fact, I may adduce the relations of that veracious traveller, Capt. of the members of the then new Royal Society-by the Lemuel Gulliver, who when satirizing the proceedings way some of their "transactions" well deserve to be

[620] SIR,-The remains of the coal-measure fish Ctenodus are remarkably abundant in some portions of the Northumberland coal-measures. The seam in which they occur is known as the Low Main Seam, and the colliery in which they are found most abundantly is Newsham Colliery, near Blyth.

I have many reasons for believing that the Ctenodi are not uncommon in the Yorkshire and Staffordshire coal-measures, as I have seen very excellent and new species in the York and Derby measures, and in the excellent cabinet of Mr. Ward, Longton, Staffordshire. Several of the species collected by Mr. Ward are identical with those that have been collected in Northumberland; their numbers are not quite so abundant, but there is in Mr. Ward's possession the largest specimen of Ctenodus tuberculatus that has yet been discovered.

The portions of the Ctenodi generally found are ridged teeth of a very peculiar form, strong round curved ribs, strong circular opercular plates, various superficial head bones and sphenoid bones of very elegant form, and in most perfect preservation. I annex a sketch of a sphenoid bone of one of the larger

READINGS FROM THE GLOBES.-XIV. [621] SIR,-The two great globes which chiefly attract our attention in the sky are the sun and the moon of these the former has generally been regarded as the finer, and hence in most languages it occurs in the masculine gender; the Germans, however, have thought proper to put the moon in the masculine gender (and have accordingly designated her as "Der Mond ") and the sun in the feminine gender. In this respect, perhaps, they are more philosophical than the ancients, who looked upon the cold moon with peculiar veneration, paying her the adoration dne only to the Creator, and discharging religious rites and ceremonies in her honour. Amongst these, the Phonicians, Sidonians, and even the Israelites made them