VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. IOQ 



sensibly moved from its place ;" such reasonings and conclusions may lead to 

 very dangerous errors. 



It is undoubtedly true, that if the principles assumed by Mr. Robins with 

 respect to the manner in which gunpowder takes fire, and the relation of the 

 elasticity of the generated fluid to its density, or the intensity of its pressure on 

 the bullet as it expands in the barrel, were just ; and if the loss of force by the 

 vent and windage was in all cases inconsiderable, or if it was prevented, the de- 

 ductions from the theory respecting the velocities of bullets of different weights 

 would always hold good. But if, on the contrary, it should be found, on 

 making the experiments carefully, and in such a manner as entirely to prevent 

 inaccuracies arising from adventitious circumstances, that the velocities observe a 

 law different from that which the theory supposes, we may fairly conclude that 

 the principles, on which the theory is founded, are erroneous. 



Mr. T. now makes a comparison of this relation, by means of the experiments 

 from N° 84 to U2 inclusive, and finds that the ratio above-mentioned does not 

 generally hold good. But it may be justly objected to this comparison, that 

 those very experiments were very bad ones, being condemned as such by Mr. T. 

 himself in several parts of his paper ; and that besides, he makes the calculation 

 from the velocities as deduced from the recoil of the gun, instead of those from 

 the pendulum, which have in this instance a very different relation. 



Mr. T. proceeds. There are many reasons to suppose, that the diminution 

 of the action of the powder on the bullet, when it is lighter, is not so much 

 owing to the smallness of the quantity of powder that takes fire in that case, as 

 to the vis inertias of the generated fluid. It is true, that a greater portion of 

 the charge takes fire when the bullet is heavy than when it is light ; but then the 

 quantity of unfired powder in any case was much too small to account for the 

 apparent diminution of the force when light bullets were used. If the elastic 

 fluid, in the action of which the force of powder consists, were infinitely fine, 

 or if its weight bore no proportion to that of the powder that generated it ; and 

 if the gross matter, or caput mortuum, of the powder remained in the bottom 

 of the bore after the explosion, then, and on no other supposition, would the 

 pressure on the bullet be inversely as the space occupied by the fluid : but it is 

 evident that this can never be the case. 



A curious subject for speculation here occurs : how far would it be advan- 

 tageous, were it possible, to diminish the specific gravity of gunpowder, and 

 the fluid generated from it, without lessening its elastic force ? It would cer- 

 tainly act on very light bullets with greater force ; but when heavy ones came to 

 be used, there is reason to think that, except extraordinary precaution was taken 

 to prevent it, the greatest part of the force would be lost by the vent and by 

 windage. The velocity with which elastic fluids rush into a void space, is as the 



