ON THE ORIGIN OF THE PLANETARY SYSTEM. 145 



In the front rank of all, then, is the law of gravita- 

 tion. The celestial bodies, as you all know, float and 

 move in infinite space. Compared with the enormous 

 distances between them, each of us is but as a grain of 

 dust. The nearest fixed stars, viewed even under the 

 most powerful magnification, have no visible diameter ; 

 .and we may be sure that even our sun, looked at from 

 the nearest fixed stars, would only appear as a single 

 luminous point ; seeing that the masses of those stars, 

 in so far as they have been determined, have not been 

 found to be materially different from that of the sun. 

 But, notwithstanding these enormous distances, there 

 is an invisible tie between them which connects them 

 together, and brings them in mutual interdependence. 

 This is the force of gravitation, with which all heavy 

 masses attract each other. We know this force as 

 gravity, when it is operative between an earthly body 

 and the mass of our earth. The force which causes 

 a body to fall to the ground is none other than that 

 which continually compels the moon to accompany the 

 earth in its path round the sun, and which keeps the 

 earth itself from fleeing off into space, away from the 

 sun. 



You may realise, by means of a simple mechanical 

 model, the course of planetary motion. Fasten to the 

 branch of a tree, at a sufficient height, or to a rigid 

 bar, fixed horizontally in the wall, a silk cord, and at 



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