Experimental Philosophy* [Lecture 15. 



practical mechanics : and it holds universally 

 true, that when two bodies are suspended on 

 any machine, so as to act contrary to each other ; 

 if the machine is put into motion, and the per- 

 pendicular ascent of one body multiplied into 

 its weight is equal to the perpendicular descent 

 of the other body multiplied into its weight, 

 those bodies, how unequal soever in their weights, 

 will balance "one another in all situations : for, as 

 the whole ascent of one is performed in the same 

 time with the whole descent of the other, their 

 respective velocities must be directly as the 

 spaces they move through ; and the excess of 

 weight in one body is compensated by the excess 

 of velocity in the other. Upon this principle it 

 is easy to compute the power of any mechanical 

 engine, whether simple or compound; for it is 

 but only finding how much swifter the power 

 moves than the weight does (i. e. how much 

 further in the same time), and just so much is 

 the power increased by the help of the engine. 



The second law of motion laid down by Sir 

 Isaac Newton is " That the alteration of the 

 state of any body from rest to motion, or from 

 one motion to another, is always in proportion 

 to the force which is impressed, and in the direc- 

 tion of that force." 



All motion is naturally rectilinear. A bullet 

 projected by the hand, or shot from a cannon, 

 would for ever continue to move in the same 

 direction it received at first, if no other power 



