Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/866

 GUN

GUN

iained either In the powder or between the intervals of the grains. They have fuppofed that air to exift in its natural elaflic ftate, and to receive all its addition of force from the heat of the explofion ; but from what was faid before, relat- ing to the incrcafe of the elafticity of the air by heat, we may conclude, that the heat of the explofion cannot aug- ment the elafticity to five times its common quantity ; con- iequcntly, the force arifing from this caufe only, cannot amount to more than the 20Cth part of the real foi'ce exerted on this occafion. Ibid. p. 43.

We muft here obferve, that the experiment of firing gun- powder, in the top of a tube, by means of a burning-glafs, and thereby making the water defcend, and hence deducing that there is a permanent air or eiaftic fluid contained in gun- powder, and that thofe who pretend to account for the effects of gun-powder, from the rarefaction of the natural air, al- Jedge an infufficient caufe ; this, I fay, was fo long ago as the year 160,0, fliewn by Mr. John Bernoullitn his difierta- tion De effervefeenijei et fermentatione. But as the experiment of this ingenious author was made with fo very fmall a quantity of powder as four grains or corns (granules) and that the fpace the water fubfided was not exactly meafured ; it is no wonder that he fhould conclude this factitious air to be only fomething more than 100 times (ccnties et ampliui) denier than the natural air. See Jo. Bernoulli, Oper. vol. i. P-35. 3f

JSut if this gentleman has fuppofed the force of gun-powder too little, his fon Mr. Daniel Bernoulli, in his Uydrodyna- miea, has on the other hand fuppofed it a great deal too much, making its elafticity not lefs; than 10000 times greater than that of the air. His reafons are founded, chiefly, on the great quantity of powder that efcapes unfired from guns. But that this quantity of unfired poziedcr is not very confider- able, feems to be fully proved under the head Gunnery. The variations of the denfity of the atmofphere does not any way alter the action of powder. By comparing feveral trials, made at noon in the hotteft fummer fun, with thofe made in the frefhnefs of the morning and the evening, no certain dif- ference could be perceived ; and it was the fame with thofe made in the night and in winter. Indeed, confidering that the fame quantity of that eiaftic fluid in which the force of powder confifts, is generated in a vacuum and in common air, it is difficult to conceive how this force can be affected by the greater or lefs denfity of the atmofphere. But the moifture of the air has a very great influence on the prce of potvder, for that quantity which in a dry feafon Atould communicate to a bullet a velocity of j 700 feet in one fecond, will' not in damp weather coaimunicatc a velocity of more than 12 or 1300 fe&t in a fecond, or even lefs, if the powder be bad and negligently kept. New Princip. of Gunnery, p. 43, 44.

This agrees with an experiment made before a committee of the Royal Society, where powder having been dried by be- ing put into a phial placed in boiling water, threw a ball out of a mortar twice as far as the fame quantity of powder taken out of the fame barrel before it was dried. Now the ranges under the fame circumftances of charge, elevation, C«V. being as the fquarcs of the velocities of the ball, thefc velocities, in this experiment, will be to each other nearly as 17 to 12, which gives ranges as 280 to 144. Phil. Tranf. N. 465, p. ,82, 183.

If powder be damp, fhot made with equal quantities of it out of the fame parcel, will differ confiderably from each other, perhaps ten times more than if the powder was in good order. A fmall charge feems to lofe a greater part of its force than a larger, each being equally damp. Another circumftance attending damp powder is, a remarkable foul- nefs in the piece, after firing, much beyond what arifes from an equal quantity of dry powder. New Princip. of Gunnery, p. 44.

That powder will imbibe moifture from the air, and thereby increafe in weight, is certain. A parcel of very good^ciu- der being placed on a white paper, pierced with a great num- ber of fine holes, and held over the fteam of hot water • the powder in half a minute was increafed about T ' T in weight. Another parcel continuing longer in the fteam was increafed by T ' T part. That the moifture of the atmofphere has a like effect, appears from this ; that an ounce of powder kept for fome time in a room having a fire in it every day, bein<* dried before the fire, loir above t-Jtt part of its weight ; one third of which it regained in lefs than two hours, °by bcin« removed to a part of the room diftant from the fire. And as tire air is often much moifter than when this experiment was tried, and that the open air is more moift than a room with a fire ; it cannot be doubted but that the twentieth or thir- tieth part of the fuHtance of the beft powder is water. Now as a certain quantity of water mixed with powder will pre- vent Its fir|ng at all, it cannot be doubted but every decree or moifture muft abate the violence of the explofion ; °and hence the eiiefls of damp powder are not hard to account for. It is to be obferved, that the moifture imbibed by powder

does not render it lefs active when dried again. Indeed, if powder be expofed to the greateft damps wilhout any caution, or if common fait abounds in it, as often happens thro' neg- ligence in refining the nitre, the moifture it imbibes may, in fuch cafes, be perhaps fufneient to dill"oK r c fome part of the nitre, which is a lafting damage that no drying can re- trieve. But when tolerable care is taken in preferring pow- der, and the nitre it is compofed of has been well purged from common fait, it will retain its force much longer than is ufually fuppofed ; and it is faid that powder has been known to have been preferved for 50 years, without any apparent damage from its age.

Some care is necefiary in the drying of damp gun-powder ; for there is a degree of heat, which tho' not furiicieiit to fire the powder, will yet melt the brimftone, and deftroy the tex- ture of the grains. Nay more, there is a heat with which the brimftone will flame and burn away gradually, and yet the powder will not explode ; of this any one may fatisfy himfelf by heating a piece of iron red-hot, and then throw- ing a {tw grains of powder on it at different intervals,' during the time of its cooling, for by this means he will find, that at a certain time the feparate grains that fall on the iron will not explode, but will burn with a fmall blue flame for fome fpace of time, the grain ftin remaining unconfirmed. In- deed, when it has begun to burn in this manner, it fome- times ends with exploding, but this more commonly happens when a number of grains lie near together, for then tho' each feparate flame is not fufficient to explode its respective grain, yet the whole fire made by them all together grows ftrong enough at laft to end in a general explofion ; however, by attending to the proper temperature of the iron, and fpreading the grains, two or three inches fquare may be co- vered with a blue lambent flame, which will laft a confider- able time without any explofion, and the, grains afterwards will not apparently have loft either their colour or their fhape. Now fince thefe grains, when the brimftone is thus burnt or even melted out of them, will no longer act as powder, it is evident that polvder may be fpoiled°by beintr dried with too violent a heat. Ibid. Schol. to Prop, x, °

The velocity of expanfion of the flame of gun-powder, when fired in a piece of artillery without either bullet or other body before it, is prodigious. By the experiments of the author fo often quoted, it feems this velocity cannot be much lefs than 7000 feet in a fecond. This however muft be under-, flood of the moft aftive part of the flame. For, as was ob- ferved before, the eiaftic fluid in which the activity of gun- powder confifts, is only ,4 of the fubftance of the powder, the remaining T -J will in the explofion be mixed with the eiaftic part, and will by its weight retard the activity of the explofion ; and yet they will be fo completely united as to move with one common motion, but the unelaftic part will be lefs accelerated than the reft, and fome of it will not even be carried out of the barrel, as appears by the confiderable quantity of unctuous matter, which adheres to the infide of all fire-arms after they have been ufed. Thefe inequalities in the expanfive motion of the flame, render it impracticable to determine its velocity otherwife tiian from experiments. The foundation of which determination is, that a barrel be- ing fixed in a proper fituation on a pendulum, fuch as defcrib- ed under the head Gunnery, and being charged with I2dwr. of powder, without either ball or wad, the powder beinJ- only put together by the rammer; on the difcharge the pen* dulum afcended through an arch whole chord was 10 or I0f 8 inches. Now if the piece be again loaded with the famu quantity of powder, rammed down 'by a wad of tow weigh- ing idwt. it may be fuppofed, that this wad being very light, will prefently acquire that velocity, with which the eiaftic part of the fluid will expand itfelf when uncompreffed ; and it was accordingly found, that the chord of the amend- ing arch was by this means augmented to 12 inches -Jo that by the additional weight of idwt. of matter moviW with the velocity of the fwifteft part of the vapour, the pen- dulum afcended through an arch whofe chord was two inches longer than before. And by calculating upon thefe facts and the principles laid down in his book, Mr. Robins determines that the velocity with which this 1 dwt. of matter moved, muft be about 7000 feet in one fecond. Ibid. prop. xi. It is this prodigious celerity of expanfion ■ of the flame of fired gun-powdir, which is its peculiar excellence, and the circumftance in which it fo eminently furpalTes all other in- ventions, either antient or modern, for the purpofe of mili- tary projefiions: for as to the quantity of motion of thefe projectiles only, many of the warlike machines of the antients produced this in a degree far furpaffing that of our heavicft cannon- fhot or fhclls ; but the great celerity given to thefe bodies cannot be in the lead approached bv any other means than by the flame of powder. The reafori of 'this difference is, that the antients could by weights, or the elafticity of fprmgs and ftretched cords, augment their powers to any de- gree defircd ; but then each addition of power brought with it a proportional addition of matter to be moved : fo that as the power increafed, thofe parts of the machine which were to communicate motion to the projectile and were confe-quently 1 »