222 ASTRONOiMY. 



of granite, or about five times that of water. Therefore it 

 cannot be a hollow shell, as some have formerly supposed ; 

 nor can its internal parts be occupied by central fire, or by 

 water. The solid parts must greatly exceed the fluid parts ; 

 and the probability is, that it is a solid mass throughout, 

 composed of substances, more ponderous the deeper we go. 

 Nevertheless, we may consider the present face of the earth 

 to have originated from the revolution of a sphere, covered 

 with a surface of a compound mixture ; the fluid and solid 

 parts separating, as the surface became quiescent. Here 

 then comes in the moderating hand of the Creator. If the 

 water had exceeded its present proportion, even but by a 

 trifling quantity compared with the whole globe, all the 

 land would have been covered ; had there been much less 

 than there is, there would not have been enough to fertilize 

 the continent.* Had the exsiccation been progressive, such 

 as we may suppose to have been produced by an envaporat- 

 ing heat, how came it to stop at the point at which we see 

 it? Why did it not stop sooner; why at all? The man- 

 date of the Deity will account for this : nothing else will. 



VI. Of centripetal forces. By virtue of the simplest 

 law that can be imagined, viz. that a body continues in the 

 state in which it is, whether of motion or rest : and, if in 

 motion, goes on in the line in which it was proceeding, 

 and with the same velocity, unless there be some cause for 

 change ; by virtue, I say, of this law, it comes to pass 

 (what may appear to be a strange consequence) that cases 

 arise, in which attraction, incessantly drawing a body to- 

 wards a centre, never brings, nor ever will bring, the l3ody 

 to that centre, but keep it in eternal circulation round it. 

 If it were possible to fire off" a cannon ball with a velocity 

 of five miles in a second, and the resistance of the air could 

 be taken away, the cannon ball would for ever wheel round 

 the earth, instead of falling down upon it.t This is the 



* Nearly three quarters of tlic earth's suiface are covered by the 

 sea. Now evaporation is proportionate to the surface of the fluid, and 

 consequently a less expanse of waters would not have afforded a suffi- 

 cient f-upply of rain, which does not now fall upon the whole, in greater 

 quantities than are required " to fertilize the earth." Paxton, 



t If a body be projected horizontally from a station A, Fig. 6, Piute 

 XXXIX, which is at a certain height, its weight or the force of gravity 

 will draw it towards the earth. It may be supposed to come down, for 

 example, at B. But from the tendency which the body has to continue 

 in the state of motion which is communicated to it, it will be carried 

 further before it falls, if it is projected with a greater force. Hence, 

 if this force be increased, it may be made to reach C ; by a greater in- 

 crease, it may be carried to D; or even round to A, from whence it 

 originally set out. Ibid. 



