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the second chapter of the second memoir, on the Ventilation of Mines, by M. J. Gonot, engineer in chief of mines at Mons. We take this for a specimen, as placing before the English reader information which we are not aware exists any where within the scope of our own language.

The species of ventilation treated of by the author is that denominated "natural," and is that chiefly in use in Eng land, being the production of a continuous current of fresh air-entering the coal-pit by one vertical shaft, traversing its numerous galleries, and ascending by another, or upcast shaft, and then carrying with it the noxious gases from below-motion being produced and maintained simply by maintaining a difference of temperature between the air in the downcast and upcast shafts.

The upcast shaft, in which the ascending current is maintained, is in England usually heated by a furnace in the bottom of the pit; on the Continent often by high-pressure steam, driven down and discharged from pipes low down in it. By this simple arrangement, coupled with suitable brattices, or separations, and air directors below, a continuous current of fresh air, moving at about two feet per second, is maintained, often of many miles in extent, under ground in the coal-mines of Durham and Northumberland.

But to our author:-
:-

"I pass," says he, "at once to the most general case, to that where there exists two apertures, two vertical shafts, for example, of 200 metres in depth, (about the average,) communicating with each other by one or more long and sinuous galleries,-one of these serving for the entrance, and the other for the exit of the air,-having their orifices on the same level, and the ventilation requiring to be artificially maintained. We may compare such a disposition of parts to an immense tube of unequal diameter, having its extremities placed vertically and on the same level. We know, then, that if by any means we disturb the uniform density of the air, filling these tubes-if by any means we diminish the density of that in

one of the vertical tubes, or connecting galleries-the equilibrium will be destroyedthe air or other gases in the whole line of tubes will begin to move, passing out by the tube of diminished density, and being replaced in the mine, or galleries below, by fresh air from the outside, which will enter by the other vertical tube or shaft.

If we reduce by calculation the density of the column of air entering to that of the column of air ascending and passing away, (the equilibrium being supposed to be disturbed by difference in temperature,) and and if we call h=the excess of the height of the first column above the second, we know that, friction omitted, the formula which determines the velocity of exit of the air, supposing the tube to be of equal diameter throughout, is

V = √ 29 h

9 being the velocity impressed by gravity in one second upon a body falling freely in a vacuum, and the value of which is here to be taken at 9.8088 metres, as referred to the latitude and altitude of the observatory of Paris.

"Let, for example, the fluid which fills the tube be throughout the same; let heat be the means of dilating this gas or fluid, and hence diminishing its specific gravity; let the level of both orifices be the same, and their depth =200 metres; let the heating furnace be placed at the bottom of the upcast shaft, and communicate to the whole issuing column of air a mean temperature of 30 centigrade; let the temperature of the air entering the downcast shaft be 10° centigrade; then, if no velocity be lost by friction against the sides of the shafts and galleries, we have the following calculations as to velocity, &c. :

"The density of the entering air-that of air at zero, and at 0.76 metres of barometric pressure being =1 is

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Then multiplying this number by 200 the depth of the shafts, we have 214.46 metres;

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The number, 16.80 metres, thus found, is the theoretic velocity, without deduction for friction, or inequality of area of the tubes; but it is easy to see that in mines the length of the tube is always great in proportion to its section, and that hence we must by no means neglect these causes of retardation. In fact, in a mine of which the workings are as yet by no means large, and with the same data as above, the actual velocity is only 1.15 metres per second, in place of 16.80 metres; that is to say, less than th the velocity with which the air would issue from the mine if it encountered no obstacle from the walls and sides of the galleries and shafts."

M. Combes (in the memoir before alluded to) gives a general formula, by means of which he thinks we can calculate the velocity of a current of air in a mine approximatively, and therefore the expense (or volume of air passed through it) in a given time; but this formula is so complex, and requires such delicate and numerous observations, that it seems scarcely capable of being used in practice; and in addition, it has the disadvantage of not involving the principal elements of resistance to motion, in the way in which they are actually combined in the course of working a mine. may be doubted, also, whether the experiments which have been made on the motion of air in conduit pipes of sheet iron or tin plate, whose sides are smooth or polished, can serve as data whereon to found a theory of the movement of gases in the galleries of a mine or coalpit, whose form and dimensions are constantly changing, and which are always more or less long, contorted, and rough in their interior walls or sides.

It

The author, (M. Gonot,) having studied all the formulæ given by M. Peclet in his "Traité de la Chaleur," and de

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metres, which is equal the velocity per second of the air issuing from the upcast shaft.

'If we represent by h=the height to which the air is rarefied in the upcast shaft, t=the temperature of the air entering the mine, and the mean temperature of the air issuing from it, and by a=t the co-efficient of dilatation of gases and vapours, the preceding calculations may be generalized thus:

1)=

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duced from his own experiments on the
draught of chimneys, and from those of
Daubuisson on the motion of air in con-
duit pipes, has endeavoured to apply
these to the determination of the velo-
city of air in mine workings; and con-
siders that he has arrived at results which
square so well with observed cases, that
he does not hesitate to give them place
in his memoir :-
:-

"Peclet gives the following formula for
determining the velocity of pure air (i. e.,
atmospheric air) in flues or chimneys of
baked clay or pottery :-
:-

v = 9·12/hm (t-t') D

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L+4D

the mean velocity of the air in the chimney.

the vertical height of the column of heated air.

the co-efficients of dilatation of

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L

=

the diameter.

the total length of the flue or chimney."

The author further remarks, that Peclet has suppressed- erroneously he thinks-the denominator of the fraction which is under the radical, the factor (1+mt') which should be restored; but as the temperature of the air descending into a mine approaches always near to 15° centigrade, we can, by considering this temperature as constant, re-establish the factor in the formula, without destroying its simplicity. For this end it is only requisite to divide the co-efficients 9.12 belonging to the radical, by the square root of the factor

√1+0.05625 = 1.05625 = 1.0278

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and we thus fall back upon the formula given by Peclet to express the velocity of burnt air in chimneys; a formula to which we must apply the same correction, in case burnt air may be in action in mines.

The total length of the courses of air in a mine is always very great in proportion to the mean diameter of the galleries and shafts; so that we may neglect the second term (4 D), the denominator of the fraction under the radical. To find the mean diameter D of the shafts, and of the galleries, or drifts, considered as a single canal through which the air is to run, from the mouth of the downcast to that of the upcast shaft, the author multiplies the transverse section of each shaft or gallery by its respective length, adds all the volumes, and divides the sum by the total length of the course = L. (taking care only to reckon as one such portions of the course as are divided into two or more partial or subordinate currents,) as far as the foot of the upcast shaft, or to a cross course, as such are found in well-conducted coal works. He obtains thus a mean section, from which he takes the diameter D, and which, multiplied by the mean velocity V, determined by the formula, gives the volume of warm air due to one second of time at the mouth of the upcast shaft. To render the formula comparable with that already applied, he calls again t= the temperature of the descending current f=that of the ascending or issuing one, and a the co-efficient of dilatation; it becomes then finally

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This formula is capable of giving results rather too high, so that it becomes unnecessary to take account of elbows or sudden inflections in the galleries, or unusually narrow spots-of the state of the surfaces in contact with the current-or of the volume of carbonic acid mixed with the air, especially in mines ventilated by means of furnaces of dilatation. These, moreover, he presumes, cannot in the present state of science be precisely determined.

Introducing now into his formula the data furnished by M. Combes in the memoir before adverted to, the author

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proceeds to show how closely his results will square with those determined by actual experiment.

In the Videtta pit, the currents from two downcast shafts pass into a single upcast, and are maintained by a furnace opening laterally into the latter at 120 metres below the surface; the furnace itself being 141 metres below the same. The fire consumes about 500 kilogrammes of coal in 24 hours. The current descends by one shaft, 152 metres vertically, and travels altogether, through winding and irregular channels, about 1,288 metres.

The surface of worked coal is about 33 304 square metres. The volume of air found to be received by the downcast shaft is 1.15 cubic metres per second.

The volume of air found to be received by the second shaft per second is 0.96 cubic metres per second; its depth is 135 metres, and the length of the horizontal, or inclined currents, 575 metres.

Finally, the volume of air delivered by the upcast shaft has been found to be 3.81 cubic metres per second-a determination, however, which the author considers too high. The air enters the downcast shafts at a temperature of from 15° to 16° cent., and the air issuing from the upcast shaft has a temperature of 24° cent., and is observed to vary (with season, &c., it is to be presumed) from 24 to 30 degrees. Admitting that 3.81 cubic metres of air at 24° cent. issues per second, and under the pressure of 0·7547 of mercury = that on the day observed, we find that the weight of air issuing, assumed to be all atmospheric, is 4:44 kilos. per second, or in twenty-four hours 383 616 kilos. The action of the ventilating furnace heats this mass of air from 16° to 24° or to 30° cent. at most, so that the augmentation is about 8° to 14°.

But, admitting the specific heat of air to be 0.26, and that the combustion of each kilogramme of coal furnishes 7,000 units of heat, we find that the 500 kilos. of coal burnt in 24 hours develope a quantity of heat sufficient to elevate the temperature of the whole mass of issuing air 350 cent.; there is hence a considerable loss of heat, which is to be attributed to the damp surfaces of the shafts and galleries.

M. Combes, by whom the above re

sults have been obtained, does not give the cross section of the shafts; the author, however, from other information, takes them at seven square metres section.

Applying now the dimensions given to the preceding formulæ, he finds in the volume of air in the first case per second, 1.2885386 cubic metres; a result which differs only by 0.14 from that actually obtained by M. Combes experimentally. So by the second shaft; and summing both he obtains for the volume of issuing air, 129 +2.22 3.51 cubic metres

=

per second, which only differs from actual practice or observation by 0:30, M. Combes' determination being 3.81 cubic metres per second.

Space will not permit us to follow the author through similar and much more detailed and precise comparisons of his theory, with the results of observation in other pits; nor can we transfer the copious and valuable tables by which he has illustrated the variations which take place in the activity of the ventilating current, in relation to variable conditions in the structure of the workings. For these, as well as for the modifications of his formulæ, to be applied in cases of sudden diminution of section (étranglement) of the air passages-of changes of forms, bends, &c., we must refer to the original Memoir, which should be in the hands of every coal viewer of sufficient education to comprehend the work in Great Britain; and nothing would be a more acceptable present to British coal owners and coal workers than a good translation of the whole of his report, published in a cheap form, and accompanied with some illustrative notes, comparing our own practices in working coal with those referred to, and showing generally the origin of many of these in the differences of the coal formations themselves.

The author concludes the chapter of his report, of which we have thus given so copious an analysis, by the statement of eight practical maxims or rules to be observed in the ventilation of coal pits by means of rarefaction, and which, in fact, contain, in a condensed form, the whole doctrine; while the next and concluding chapter of his Memoir is devoted to the consideration of its special appli

cations.

Amongst the other Memoirs contained in this report is one by M. Motte, a me

chanical engineer of Marchiennes; M. Pont, on the application of the screw as a machine for producing mechanical ventilation of mines, the method being, in fact, an inversion of the screw propeller, now so much talked of, the screw being 2 fixture in place, and drawing the air through a short tube. Its diameter and velocity are very great, and tables of experiments are given which show its effect in a favourable view.

The volume concludes, as before observed, with a report upon certain improved forms of safety lamps. The presumed improvements consist chiefly in placing strong glass chimneys round the flame, with wire gauze above and below; and in M. Mueseler's lamp, which appears to be the best, accompanied with further precautions against the effects of currents of air, blows, or adhered coal dust, &c. This report contains a useful résumé of the important points in the doctrine of safety lamps, but not much that is further important.

R. M.

NEW FORM OF BATTERY, PARTICULARLY APPLICABLE ΤΟ BLASTING ROCKS, &c., BY GALVANISM. BY MARTYN ROBERTS, ESQ., F.R.S.E., M.E.S. As the process of blasting by voltaic agency has become very general, it may be interesting to our readers to learn the construction of a very simple and effective battery adopted for this purpose by Mr. Roberts, and communicated to the London Electrical Society at their meeting on Tuesday, the 19th inst.

Some four years ago, Mr. Roberts communicated to the Royal Society of Edinburgh, the superior efficacy of combinations of zinc and iron over those of zinc and copper; twenty-six inch bars of which he finds most efficacious for a blasting battery. As the great value of this arrangement depends upon the peculiar manner of connecting the respective metals, we will do our best so to describe it as to enable our readers to construct for themselves. Twenty plates of iron and a like number of zinc are prepared, and are placed parallel and alternate to each other, as in the ordinary arrangement for the acid battery; and they are thus connected:-let the zinc plates be numbered 1, 2, 3, &c., and the iron a, b, c, &c.; and let a be at one end of

the series: then first connect a and b; and afterwards connect 1 to c, 2 to d, and 3 to e, and so on. A rough sketch of such an arrangement will show that all of both sides of each plate is brought into active service, and that there are no counter currents. A box 8 inches long, will contain a series of 20, and may be of wood rendered water tight by white lead. To facilitate the removal and immersion of the plates, a frame is made to contain them. The battery is excited with one of sulphuric acid to thirty of

water.

A MILITARY MACHINE TO PROPEL BULLETS WITHOUT THE USE OF GUNPOWDER.

Take a circular metal plate, not less than six feet in diameter, upon the upper surface of which trace round the centre a circle of 18 inches in diameter. At equal distances from each other fix four straight bars extending from the perimeter of the lesser circle to that of the plate. Let four other bars, touching those at the exterior of the smaller circle, be placed obliquely upon the plate so as to reach the rim at an inch distant from the others in front of them. If the circular plate be now made to revolve upon a vertical axis, and bullets be thrown upon it near the axis, they will be carried to the outward extremity; and will fly off with a force which may be measured by the velocity with which the plate is made to revolve; and they must fly off from the points at which the conducting bars are nearly in contact at the circumference of the greater circle.

It remains to render these points of escape, fixed and stationary points during all the revolutions of the plate. Let an upper plate cover all the above bars outside the smaller circle, leaving a void round the axis for admission of the bullets. Supposing the machine with its axis to be now firmly fixed upon a proper horse carriage, let an iron hoop sur rounding the circular plates be independently fixed upon the same supporting frame, but in such a manner that the plates can revolve freely within the hoop, which shall remain undisturbed. The bullets will have the same tendency to fly off as before from the machine. If a hole is cut in the surrounding hoop, the bullets will all of them fly off at different

instants of time from that fixed open point; and if the velocity of the plates is continued uniformly the same, they will continue to fly off with the same force, and in the same direction, which will be nearly in that of a tangent to the circle, and they will all of them strike any prominent object not too distantly stationed in that line. By a few repeated experiments, the course of the bullets could be accurately ascertained in its degree of variation from the real tangent. Allowing for this ascertained variation from the real tangent direction, and using the diameter of the exterior orifice as that line, the bullets may be made to strike any object within reach, although placed considerably on one side or the other of that line. The hoop, being independant of the revolving plates, might by a mechanical expedient be made to turn a little either way in a moment, so as to give the tangent or line of guidance every useful and efficient direction. This machine would throw off more bullets than a thousand muskets; and if properly managed, would destroy any military column advancing to approach it.

The most powerful agent would be steam, acting as immediately as practicable upon the axis. But a substitute much more convenient in practice might be found; springs might be formed to act by their elastic powers upon the axis; and these springs might be compressed from time to time by the action of a portable steam-engine. This engine, being of small dimensions, might be conveyed upon a separate horse carriage; and would serve to compress the springs of many propelling or propulsive machines. The springs could not often be required to act for more than a quarter of an hour, at the same time or in the same situation; and convenient intervals for fresh compression would frequently occur. Mr. Atwood and other celebrated mathematicians have so ably demonstrated the force with which a rapidly revolving cylinder would propel a ball, that it is sufficient to allude simply to the circumstance, without supporting it by any particular illustration. They say that the force would equal that of a full charge of gunpowder.

These suggestions are offered to the consideration of professional gentlemen who combine a perfect knowledge of military and mechanical science.

E.

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