180 ELEMENTARY PRINCIPLES OF GUNNERY. 



the usual estimation of the weight of air, is 131 grains. If 

 the ratio of the bulk of the gunpowder to the bulk of this 

 fluid is wanted, it will be determined by knowing that 17 

 drachms of powder fill 2 cubic inches; so that, proportioning 

 the one to the other, it will be found that 472 cubic inches 

 of elastic fluid are obtained from 2 cubic inches of powder. 



But the amount of gas given off from a fixed quantity of 

 powder is not always a test of its utility in throwing pro- 

 jectiles. Thus many explosive powders, such as fulminating 

 silver and mercury, generate a much greater proportion of 

 gas and far more rapidly than ordinary gunpowder; but the 

 explosion being too sudden, they have not so good an effect 

 upon the projectile in proportion to the power which they 

 exert, and which, from its great suddenness, is apt to burst 

 the tube in which it is contained. Up to the present time 

 two substances only have been found which combine the 

 exact properties required, and these are gunpowder and gun- 

 cotton. In experimenting on different kinds of gunpowder, 

 with a view to determine the relative powers of each, it is 

 found that the density of the air has no effect whatever, but 

 in proportion to its dryness will the elastic force of the powder 

 be exerted. This is very important to know, for it very often 

 happens that powder which has been used on a damp day 

 with less effect than other powder tried in dry weather, loses 

 credit, although, perhaps, really equal, or even superior to the 

 antagonistic material. So also powder which is rendered 

 damp by being poured down a foul barrel is weakened greatly 

 in its effects; and here again is another element which must 

 be taken into the account. In all trials of gunpowder, there- 

 fore, dry days should be selected. 



ACTION OF THE POWDER ON THE PROJECTILE. 



The action of the powder on the projectile ceases as soon as 

 the latter escapes from the barrel. This fact is capable of 

 demonstration mathematically; but as it is not disputed, I 

 shall not go into the calculation. But there is another theory 

 propounded by Robins on this subject which demands a little 

 consideration, because it is contrary to the opinions of most 

 other writers, and was subsequently modified by himself. He 

 asserts, in his earlier writings, that all the powder in tJie 



