220 Experimental Philosophy. [Lecture 1 5, 



which the rods are usually formed, to lengthen of 

 contract. 



It is from that sluggishness of motion, which 

 is called the tis inertice of bodies, that there 

 proceeds something like an endeavour in all bo- 

 dies to preserve the state in which they are ; 

 when at rest to continue in a state of rest, and 

 when in motion to continue in motion. This 

 position may seem abstruse, but it will admit of 

 illustration by the most common facts. If I push 

 a bowl of water with my hand, the water flies 

 backwards over the edge upon my hand, for it 

 endeavours to continue in the state of rest in 

 which it was. But if I take the bowl in my 

 hand, and run along with it, and suddenly stop 

 short, the water flies forward the way I was run- 

 ning, from its vis inertice, or tendency to continue 

 in the same state of motion. In the same man- 

 ner, if I am sitting in the front of a carriage, 

 which, after going very fast, stops suddenly, I 

 am jolted from my seat, and my head will, with- 

 out care, drive through the front glass of the 

 carriage. 



It is a plain and obvious principle, that the 

 greater the quantity of matter is which any body 

 contains, the greater will be its vis inertias. The 

 heavier any body is, the greater is the power 

 which is required, either to set it in motion or 

 to stop it. So again, the swifter any body moves, 

 the greater is its force ; as was sufficiently exem- 

 plified in the case of a bullet, which was supposed 



