16 ATTRACTION. 



As all bodies gravitate towards the earth, so does? 

 the earth gravitate towards all bodies, as well as 

 bodies to particular parts of the earth, as has been 

 proved by the attraction which mountains exhibit 

 on a plumb-line, drawing it towards them so that 

 it does not tend exactly to the centre of the earth. 



The cause of gravity is totally unknown. 



It is observed that bodies fall to the earth with 

 a velocity constantly increasing, which is an in- 

 stance of accelerated motion caused by the con- 

 stant action of gravity. 



To illustrate this, let us suppose the time of de- 

 scent of a falling body to be divided into a number 

 of very small equal parts; the impression of gravity 

 in the first instant would make the body descend 

 with a proportionate and uniform velocity; but in 

 the second instant, the body, receiving a new im- 

 pulse from gravity in addition to the first, would 

 move with twice the velocity as before ; in the 

 third instant, it would have three times the velocity, 

 and so on. 



To illustrate the doctrine of accelerated motion, 

 let us suppose that, in the triangle ABC, (Plate 1. 

 fig 2.) A B expresses the time which a body 

 takes to fall, and B C the velocity acquired at the 

 end of the fall. Let A B be divided into a number 

 of equal parts indefinitely small, and from each of 

 these divisions suppose lines, as D E, drawn parallel 

 to B C; it is evident from what has been said, that 

 those lines will express the velocities of the falling 

 body in the several respective points of time, each 

 being greater than the other, by a certain quantity 

 of increase, which follows from the nature of the 

 triangle. Now, the spaces described in the same 

 time are in proportion to the velocities; and the 

 sum of the spaces described in all the small portions 



