268 OUTLINES OF NATURAL PHILOSOPHY. 



In the acceleration of a ball by the fluid thus genera- 

 ted, three circumstances must be attended to. 



362. 1. The elasticity is inversely as the space 

 which the fluid occupies, and, therefore, as it forces 

 the ball out of the gun, it continually dimini- 

 shes. 



2. The elasticity would diminish in this ratio, 

 even if the temperature remained the same ; but 

 it must diminish in a much greater ratio, both 

 from the dispersion of heat, and the absorption of 

 it by the fluid itself, during its rarefaction. 



3. The air propels the ball by following it, and 

 acts with a force that is, cceteris paribus, propor- 

 tional to the excess of its velocity above the velo- 

 city of the ball. The greater the velocity that the 

 ball has acquired, the less, therefore, is its momen- 

 tary acceleration. 



The effect of the elastic fluid must, for these reasons, 

 decrease much faster than the space it occupies in- 

 creases ; and a formula expressing the law of acce- 

 leration, as depending on all these causes, more espe- 

 cially on the latter, might be expected to be very 

 complex. Nevertheless, it appears from Dr HUT- 

 TON'S experiments, that the velocity with which a ball 

 actually issues from the mouth of a cannon, other 



things 



