CHAP. I.] MATTER AND ENEKGY. 1 7 



from self-evident. When wood or coal, for instance, is 

 burned, it appears to be destroyed ; but we know that the 

 carbon which disappears combines with the oxygen of the 

 air to form carbonic acid gas. 



II. The Conservation of Momentum. — The total mo- Conserva- 

 mentum of any tocJy or system of bodies is unchangeable by ^°^^°,j. 

 any mutual action of its jMrts ; or, in other words, the turn. 

 motion of the centre of gravity of any body or system of 

 bodies is unchangeable by any mutual action of its parts. 



That is to say, if the centre of gravity of any system of 

 bodies — as, for instance, the solar system — is at rest, no 

 mutual action of the bodies composing it can set it in 

 motion ; and if it is in motion, no mutual action of the 

 bodies composing it can bring it to rest ; nor can any such 

 action alter the motion of the centre of gravity, either in 

 direction or in velocity. 



The case of an animal or a railway engine that moves 

 itself, is no exception to this law ; for it is set in motion, 

 not by the action of its parts on each other, but by the 

 action of the whole on the ground which serves it as a 

 fulcrum. 



III. The Conservation of Eotation. — The total rota- Conserva- 

 tion of any body or system of bodies is unchangeable by any ^^^J^^^ 

 mutual action of its parts. That is to say, if any system of 

 bodies has no total rotation, no mutual action of the bodies 

 composing it can cause it to rotate ; and if it is rotating, no 

 mutual action of the bodies composing it can bring it to 



rest ; nor can any such action alter the total amount of 

 rotatory motion, either in direction or in velocity. 



The case of a rotatory steam-engine, or a horse in a mill, 

 is no exception to this law ; for they are set in motion, not 

 by the action of their parts on each other, but by the 

 action of the whole on the framework or the ground that 

 serves as a fulcrum. 



The total amount of rotation is thus estimated. Let Amount of 

 a line of indefinite length be drawn as an axis. It how'e^ti- 

 may be drawn anywhere, and in aay direction ; it mated. 



c 



