228 Experimental Philosophy. [Lecture 15. 



perfectly safe. If the anvil were very small, 

 only the weight of a pound or two, the first 

 stroke would kill the man. 



A pretty experiment of Mr. Walker's will 

 serve also to illustrate this part of the subject. 

 " Let a be a little cannon, (PI. XX. fig. 91.) 

 and b a hollow piece of iron or brass, to slip on 

 pretty tight upon c c, and of the same weight as 

 a. Now if half a thimbleful of gunpowder be 

 put in a, and b shut upon it, both being sus- 

 pended by two strings ; if the powder is fired, 

 the parts a and b will be thrown equally distant 

 from r, the center where they hung; showing 

 the re-action to be equal to the action. Hence a 

 heavy gun seems to recoil less than a light one, 

 on account of its greater vis inertice ; otherwise 

 its re-action is the same, with the same charge." 

 Hence it is evident, that when a load is drawn 

 by a horse, the load acts against the motion of 

 the horse, and the action of the animal is as much 

 impeded by the load, as the motion of the load 

 is promoted by his efforts. Many other illustra- 

 tions of these laws may be seen in the larger 

 treatises of mechanics. 



Before I proceed to the consideration of the 

 six mechanic powers, it is necessary to say a few 

 words on what is called the centre of gravity. 



The centre of gravity is that point of a body 

 in which the whole force of its gravity or weight 

 is united, and to which its action may usually 

 be referred. Whatever, therefore, supports that 



