70 ANIMA.L CHEMISTRY LECTURE IT. 



which that object receives is as surely struck by the man as if he 

 had delivered it directly with his fist. Let us now take an illus- 

 tration in which motion is transmitted through the intervention 

 of some simple instrument, as in the case of a bow and arrow, 

 for example. Here, also, the motion of the arrow through the 

 air does not take place in virtue of any force originating in the 

 bow, but is consequent on the muscular force exerted in drawing 

 the bow and cord asunder ; and, limited only by the suscep- 

 tibility of the bow, the momentum of the arrow depends en- 

 tirely upon the amount of force exerted by the archer in 

 pulling the bow-string. 



(73.) Again, in discharging a cross-bow, the cord is first 

 drawn over the catch by a muscular effort, and the arrow after- 

 wards projected by the release of the cord. Now, although the 

 susceptibility of this kind of bow is far more limited than that of 

 a long bow, still it is evident that the force with which the arrow 

 is eventually projected does not originate in the bow, but is only 

 a transformation of the muscular force exerted in separating the 

 bow and cord from one another. Moreover, if instead of pulling 

 the trigger of the cross-bow immediately after the cord is 

 drawn over the catch, we allow the bow to remain in its bent 

 state for weeks, or months, or years, and then release the cord ; 

 still the force with which the arrow is at last projected will 

 be the original muscular force exerted at the moment of 

 pulling the bow and cord asunder. Suppose, for instance, that I 

 were to draw the cord of a cross-bow over the catch to-day, and 

 a century hence some one were to release it by touching the 

 * trigger, still the force of the projected arrow would not be his 

 force, but my force, exerted by me to-day, lodged in the bow for a 

 hundred years, then manifested in the motion of the arrow, and 

 finally transformed into heat upon the cessation of its flight. 



(74.) Similarly, the force with which a cannon-ball falling from 

 the top of the Monument would strike the pavement beneath, would 

 be the exact expression of the force exerted in lifting it to the 

 top that is to say, in separating the cannon-ball and the pave- 

 ment a certain distance from each other, no matter how many 



