EARTHLY GRAVITY. '6 



latter in its path round the former, and both in 

 their yearly course around the sun. It maintains 

 the equilibrium of our own solar system, and also 

 that of the whole universe. 



Earthly Gravity — the mutual attracting power 

 of the atoms of the earth — tends to arrange them 

 about a common central point. If the earth be 

 supposed to be at rest, and covered with water, the 

 attractions exerted upon any one particle near the 

 surface by all the other particles, must together 

 make up one compound force, acting perpendicu- 

 larly to the motionless surface ; for with any other 

 direction of the force, the liquid would yield to it 

 and flow over the surface, which would thus cease 

 to be motionless. The sum of the attractions of 

 all the other particles for any single particle, or 

 for any body upon the earth, is what is called the 

 weight of that particle or body. You will thus see 

 why the direction of the pressure, which a body 

 exerts by its weight, must of necessity be perpen- 

 dicular to the surface of still water, quite indepen- 

 dently of the general form of the earth. This 

 form, however, can be no other than a sphere, 

 supposing for the present that the earth is covered 

 with water, and at rest ; for it is the only form in 

 which all the parts of the surface keep a like 

 position with regard to each other, and therefore 

 suffer an equally strong attraction. Hence it is 

 the only form in which a state of rest is pos- 



b 2 



