156 COUNT RUMFORD. 



important light. The velocity was determined in two 

 ways : first, by the ballistic pendulum, invented by 

 his predecessor and namesake, Benjamin Robins ; and 

 secondly, from the recoil of the gun itself. The ballistic 

 pendulum is a heavy mass, so suspended as to be capable 

 of free oscillation. Against it the bullet is projected, 

 and from the weight of the bullet, the weight of the 

 pendulum, and the arc, or distance, through which it is 

 urged by the bullet, the velocity of the latter may be 

 calculated. 



To determine the recoil of the gun, he had it sus- 

 pended by a bifilar arrangement, which permitted it to 

 swing back when it was fired. Action and reaction being 

 equal, the momentum of the gun was the momentum of 

 the bullet on leaving the gun, and from the weight of 

 the piece, and the arc of recoil, the velocity of the bullet 

 was computed. The agreement bstween the results 

 obtained by these two methods was in many cases re- 

 markable. Until quite recently, Rumford's experiments 

 on the force of gunpowder were considered to be the 

 best extant. A mind so observant could not fail to 

 notice the heating effects produced by the percussion of 

 the bullet against its target, and by the jar of the gun 

 at the moment of its discharge. By such facts he was 

 naturally led to reflect on that connection between 

 mechanical power and heat which he afterwards did so 

 much to illustrate and develop. 



The phenomena both of light and heat fascinated 

 him ; and we accordingly find him from time to time 

 abandoning practical aims, and seeking for knowledge 

 which had no apparent practical outcome. Thus we see 

 him experimenting on the action of green vegetables 

 and other matters upon light, or rather the action of 

 light on the green leaves of plants. From this inquiry 

 he turned to estimate the quantities of moisture taken 



