448 RECAPITULATION. 



force in a different direction, these two forces toge- 

 ther will cause it to describe a curve. 



13. The curve described by a ball projected from 

 a cannon, and brought to the earth by the action 

 of gravity, is that of a parabola. 



14. The momentum of a body is the force with 

 which it moves, and is in proportion to the weight 

 or quantity of matter multiplied into its velocity. 



Attraction and Repulsion. 



1. The causes of these powers are totally un- 

 known. 



2. There are various kinds of attraction — the at- 

 traction of cohesion, of gravitation, of electricity \ 

 magnetism, and chemical attractions. 



3. The attraction of cohesion acts only at very 

 small distances. 



4. The attraction of gravitation is that which 

 masses of matter exert on each other at all dis- 

 tances. 



5. Gravitation decreases from the surface of the 

 earth upwards, as the square of the distance in- 

 creases. 



Central Forces. 



1. The central forces are the centrifugal and 

 the centripetal force. 



2. The centrifugal force is the tendency which 

 bodies that revolve round a centre have to fly off 

 from it in a tangent to the curve they move in — as 

 a stone from a sling. 



3. The centripetal force is that which prevents 

 its flying off, by impelling it towards the centre — 

 as the attraction of gravitation. 



