Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/865

 GUN

GUN

be produced by the continued preflure of the powder, but by the percufiion of the flame. From whence it follows, that the velocity of a bullet, laid a confiderable diftance be- fore the charge, ought to be greater than what would be com- municated to it by the preflure of the powder acting as be- fore-mentioned in this article. And this deduction from theory is confirmed from experience, by which it was found that a ball laid in a barrel, at the diftance of 1 1 i inches from the breech, and impelled by 12 penny- weight of powder, acquired in its difebarge a velocity of about 1400 feet in 1" ; whereas if it had been acted on by the preflure of the flame only, it would not have acquired a velocity of 1200 feet in 1". The fame was found to hold true in all other greater diftances (and alfo in lefler, tho' not in the fame degree) and in all quantities of powder. And from hence arifes a con- federation of confequence in practice, which is, that no bul- let fhoukl at any time be placed at any confiderable diftance before the charge, unlcfs the piece be extremely well fortified : for a moderate charge of powder, when it has expanded it- felf thro' the vacant fpace, and reaches the ball, will, by the velocity each part has acquired, accumulate itielf behind the ball, and be thereby prodigioufly condenfed ; whence if the barrel be not extraordinary ftrong, it murt burft. This is confirmed by the experience of an exceeding good tower mufket of very tough iron, which being charged with 12 dwt. of powder, and the ball being placed 16 inches from the breech ; upon firing the piece, part of the barrel, juft behind the bullet, was fwelled out to double its diameter, and two large pieces burft out of it. If the powder be not placed together at the breech, but fcattered uniformly thro' the whole cavity left behind the bullet, the progreflive motion of the flame may hence be fuppofed to be prevented by the ex- panfion of the neighbouring parts. And it was found that the ball being laid as before, n| inches from the breech, its velocity, in this cafe, inftead of 14.00 feet in 1", was on'y 1 100 feet. See New Princip. of Gunnery, Part I. Prop, xii.

It appears from experience, that bullets of the fame diameter and denfity impinging on the fame folic! fubftance with dif- ferent velocities, will penetrate that fubftance to different depths, which will be in the duplicate ratio of thofe velo- cities nearly. Thus a leaden bullet of I of an inch in dia- meter, being fired againft a folid block of elm with different velocities, as of 1700 feet, 730, and 400 feet in 1" ; the cavities were found to be as 55, 10 and 3 reflectively ; which are nearly in the duplicate proportion of thofe velocities : perfect regularity in cafes of this nature cannot be expected, when the unequal texture of the fame piece of wood, and the change of the form, and the bullet by the ftroke, are conftdercd.

From the penetration being in the duplicate proportion of the velocity of the impinging body, it follows, that the re- fiftance of the wood, like that of gravity, is uniform. See New Princip. of Gunnery, p. 94, 95. GUN-POWL?ER (Cyd.) — Gun-powder, 'fired either in a va- cuum or in air, produces by its explofion, a permanent elaftic fluid 8.

If a red-hot iron be included in a receiver, and the receiver Ije exhaufted, and gun-powder be then let fall on the iron, the powder will take fire, and the mercurial ga^e will fud- denly defcend upon the explofion ; and, tho' it immediately alcends again, yet it will never rife to the height it firft ftood at, but will continue depreficd by a fpace proportioned to the quantity of gun-powder which was let fall on the iron. By this means (firing fmall quantities at a time) the mercu- rial gage may be reduced from 29^ inches to i*j|. Now this experiment, which has been often repeated, proves the pro- pofition with refpect to the production of a permanent elaftic

fluid in a vacuum ; for the defcent of the

uld

be effected by the preflure of fome new generated fluid in the receiver, ballancing in part the preflure of the external air. That this fluid, or fome part of it at leaft was perma- nent, appears from thence, that tho' in thefe experiments the quickfilvcr afcended after the operation, yet next day it had afcended no higher than to 22^, at which place it feem- ed to continue fixed. And, that this fluid is elaftic, is prov- ed from the defcent of the mercurial gage : fince the quan- tity of matter contained in this fluid, could not by its gra- vity alone have funk the quickfdver by the leaft fenfible quan- tity ; alfo from its extending itfelf through any fpace, how- ever great, the experiment fucceeding in either a large or fmall receiver, only the larger the receiver the lefs will be the defcent of the mercurial gage to the fame quantity of powder j. the preflure of the generated fluid diminishing as its denfity dimintfhes. See Phil. Tranf. N°. 295. The fame production likewife takes place, when gun-powder is fired in the air a ; for if a fmall quantity of powder be placed in the upper part of a glafs tube, and the lower part of the tube be immerged in water, and the water be made to rife fo near the top that only a fmall portion of air is left in that part where the gun-powder is placed j if in this fituation the communication of the upper part of the tube with the external air be clofed, and the gun-powder be fired, Append.

(which may eafily be done by a burning-ghifs) the writer will, in this experiment, defcc'nd on the^exp'aiion, -Jit b> quickfdver did in the laft, and will always continue d. prl e i below the place, at which" it ftood before the cxplolio.'i, and the quantity of this depreflion will be greater, if the tr-..*ui- tlty of powder be increafed, or the diameter of the tube be diminifhed. From whence it is proved, that as well iir air as in a vacuum, the explofion of fired p&utfer produces a permanent elaftic fluid b . — [ a Raukfbee, Phyf. Median. Exp, p. 81. b Robins's New Prin. of Gunnery, "Prop. L] it alfo appears from experiment, that the clafiieitv or preflure of the fluid produced by the firing of gun-powder, is ceteris paribus, directly as its denfity. This fellows from hence, that, if in the fame receiver a double- quantity of powder be let fall, the mercury will fubfide twice as much as in the firing of a fingle quantitv. For the va- pour produced from the double quantity, being contained in the fame receiver, will be of double the denfity of that pro- duced from the fingle quantity ; whence the erafticity or prefilire, eftimated by the defcent of the mercury beinp- like- wife double, the preflure is directly as its denfity. Alfo the defecnts of the mercury, when equal quantities of poivder are fired in different receivers, are reciprocally as the capacities of thofe receivers ; and confequently as the denfity of the produced fluid in each.

To determine the elafticky and quantity of this elaftic fluid, produced from the explofion of a given quantity of gunpow- der, Mr. Robins premifes, that the elasticity of this fluid increafes by heat, and diminifhes by cold, in the fame man- ner as that of the air; and that the denfity of this fluid, and confequently its weight, is the fame with the weight of an equal bulk of air, having the fame elaftrcity and the fame temperature. From theie principles, ami From his experi- ments, for a detail of which we muft refer to the book it- felf, he concludes, that the fluid produced by the firing of gun-powder will be -r 3 -^ of the weightof the gun-paivder ; and the ratio of the refpectrve bulks of the powder, and the fluict produced from it, will be, in round numbers, 1 to 244. See New Princip. of Gunnery, Scholium to Prop. II. Hence we are certain, that any quantity of powder fired in any confined fpace, which it adequately fills, exerts at the inftant of its explofion againft the fide's of the veflcl con- taining it, and the bodies"it impells before it, a force at leaft 244 times greater than the elafticity of common air, or, which is the fame thing, than the preflure of the atmofpherc; and this without confidering the great addition which this force will receive from the violent degree of heat with which it is endued at that time ; the quantity of which augmenta- tion is the next head of Mr. Robins's enquiry. He deter- mines that the elafticity of the air is augmented, when heat- ed to the extremeft heat of red-hot iron, in the proportion of 796 to 194.} ; and fuppofing that the flame of fired gun- powder is not lefs hot than red-hot iron, and the elafticity of the air, and confequently of the fluid generated by the ex- plofion, being augmented by the extremity of this heat .in the ratio of 796 to 194^, it follows, that if 244 be aug- mented in this ratio, the refulting number, which is 999", will determine how many times the elafticity of the flame of fired powder exceeds the elafticity of commoit'air, fup- pofing it to be confined in the fame (pace which the powder filled before it was fired.

Hence then the abfolute quantity of the preflure exerted by gun-powder at the moment of its explofion, may be afiigned ; for fince the fluid then generated, has an elafticity 999^, of in round numbers, 1000 times greater than common air ; and fince common air, by its elafticity, exerts a preflure on any given furface equal to the weight of the incumbent at- mofpherc, with which it is in equilibrio, the preflure exert-. ed by fired powder, before it has dilated itfelf, is 1000 times greater than the preflure of the atmofphere, and confequent- ly the quantity of this force, on a furface of an inch fquare, amounts to above fix tun weight ; which force, however, di- minifhes, as the fluid dilates Itfelf.

Tho' it has here been fuppofed, that the heat of gun-powder, when fired in any confiderable quantity, is the° fame with iron heated to the extremity of a red heat, or to the begin- ning of a white heat, yet it cannot be doubted but that the fire produced in the explofion is fomewhat varied (like all other fires) by a greater or lefs quantity of fuel ; and it may be prefumed, that according to the quantity of povjeler fired together, the flame may have all the different degrees from that of a languid red heat to the heat fufficient for the vitri- fication of metals ; but as the quantity of poivder requifite for the production of this laft mentioned heat, is certainly greater than what is ever fired together' for any military pur- pofe, we fhall not be far from our fcope, if we fuppofc the heat of fuch quantities as come more frequently in ufe to be when fired, nearly the fame with the ftrongeft heat of red- hot iron ; allowing a gradual augmentation to this heat in larger quantities, and diminifhing it when the quantities are very fmall.

Some authors have attributed the force of gkn-fowder, or at

leaft a confiderable part of it, to the action of' the z\x con-

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