OF THE LAWS OP MOTION. 159 



tension and motion may be referred to force as their cause, and have 

 no other cause. 



We come therefore to this principle with regard to the actions of 

 bodies upon each other, that so long as the tension or pressure is the 

 same, the force, as shewn by its effect in producing motion, must 

 also be the same. 



10. This force or action of bodies upon one another, is that which 

 is meant in the Third Axiom, and we now proceed to consider the 

 application of this axiom in mechanics. 



Pressures or forces such as I have spoken of, may be employed in 

 producing tension only, and not motion ; in this case, each force prevents 

 the motion which would be produced by the others, and the forces 

 are said to balance each other, or to be in equilibrium. The science 

 which treats of such cases is called Statics, and it depends entirely 

 upon the above third axiom, applied to pressures producing rest. It 

 follows from that axiom, that pressures, which acting in opposite di- 

 rections thus destroy each other's effects, must be equal, each measuring 

 the other. Thus if a man supports a stone in his hand, the force or 

 effort exerted by the man upwards is equal to the weight or force 

 of the stone downwards. And if a second stone, just equal to the 'first, 

 were supported at the same time in the same hand, the force or effort 

 must be twice as great ; for the two stones may be considered as 

 one body of twice the magnitude, and of twice the weight; and 

 therefore the effort which supports it must also be twice as great. 

 And thus we see in what manner statical forces are to be measured 

 in virtue of this third axiom ; and no further principle is requisite to 

 enable us to establish the whole doctrine of statics. 



11. The third axiom, when applied to the actions of bodies in 

 motion, gives rise to the third law of motion, which Ave must now con- 

 sider. Here, as in the cases of the other axioms, we must inquire 

 how we are to measure the quantities to which the axiom applies. What 

 is the measure of the action which takes place when a body is put 

 in motion by pressure or force? In order to answer this question, we 



X 2 



