146 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q7. 



Mr. Robins has shown, that if all the permanently elastic fluid generated in the 

 combustion of gunpowder be compressed in the space originally occupied by the 

 powder, and if this fluid so compressed be supposed to be heated to the intense 

 heat of red-hot iron, its elastic force in that case will be 1000 times greater than 

 the mean pressure of the atmosphere; and this, according to his theory, is the 

 real measure of the force of gunpowder, fired in a cavity which it exactly fills. 

 But what will become of this theory, and of all the suppositions on which it is 

 founded, if I shall be able to prove, as I hope to do in the most satisfactory 

 manner, that the force of fired gunpowder, instead of being 1000 times, is at 

 least 50,000 greater than the mean pressure of the atmosphere ?* 



For my part, I know of no way of accounting for this enormous force, but by 

 supposing it to arise principally from the elasticity of the aqueous vapour-^ gene- 

 rated from the powder in its combustion. The brilliant discoveries of modern 

 chemists have taught us, that both the constituent parts of which water is com- 

 posed, and even water itself, exist in the materials which are combined to make 

 gunpowder ; and there is much reason to believe that water is actually formed, as 

 well as disengaged, in its combustion. M.Lavoisier, I know, imagined that the 

 force of fired gunpowder depends in a great measure on the expansive force of 

 uncombined caloric, supposed to be let loose in great abundance during the com- 

 bustion or deflagration of the powder: but it is not only dangerous to admit the 

 action of an agent whose existence is not yet clearly demonstrated, but it appears 

 to me that this supposition is quite unnecessary; the elastic force of the heated 

 aqueous vapour, whose existence can hardly be doubted, being quite sufficient to 

 account for all the phenomena. It is well known that the elasticity of aqueous 

 vapour is incomparably more augmented by any given augmentation of tempera- 

 ture, than that of any permanently elastic fluid whatever; and those who 

 are acquainted with the amazing force of steam, when heated only to a few 

 degrees above the boiling point, can easily perceive that its elasticity must 

 be almost infinite when greatly condensed and heated to the temperature 

 of red-hot iron; and this heat it must certainly acquire in the explosion of 

 gunpowder. But if the force of fired gunpowder arises principally from the 

 elastic force of heated aqueous vapour, a cannon is nothing more than a steam- 

 engine on a peculiar construction ; and on determining the ratio of the elasticity 

 of this vapour to its density, and to its temperature, a law will be found to obtain, 

 very different from that assumed by Mr. Robins, in his Treatise on Gunnery. 

 What this law really is, I do not pretend to have determined with that degree of 

 precision that I wished; but the experiments of which I am about to give an ac- 

 count will, I think, demonstrate in the most satisfactory manner, not only that 

 the force of fired gunpowder is in fact much greater than has been imagined, but 



* We shall examine afterwards the fallacy of this assertion. t On the contrary, the less 



quantity of moisture that is in the powder, or the drier it is made, the stronger it is always found 

 to be. 



