ASTRONOMY. 253 



ges of ancient lluidUy. But this fluidity must have gone 

 down many hundred times further than we can penetrate, to 

 enable the earth to take its present oblate form ; and whether 

 any traces of this kind exist to that depth, we are ignorant. 

 Calculations were made a few years ago, of the mean density 

 of the earth, by comparing the force of its attraction with the 

 force of attraction of a rock of granite^ the bulk of w^hich 

 could be ascertained ; and the upshot of the calculation was, 

 that the earth upon an average, through its whole sphere, 

 has twice the density of granite, or above five times that ol 

 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 sohd parts must greatly 

 exceed the fluid parts ; and the probability is, that it is a 

 solid mass throughout, composed of substances more ponder- 

 ous the deeper we go. Nevertheless, w^e may conceive the 

 present face of the earth to have originated from the revolu- 

 tion of a sphere covered by a surface of a compound mixture ; 

 the fluid and solid parts separating, as the surface becomes 

 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 evaporating heat, how came it to stop at the point at 

 which we see it ? Why did it not stop sooner ; why at all ? 

 The mandate of the Deity will account for this ; nothing 

 else wdll. 



IV. Of centripetal forces. " By virtue of the simphst 

 law that can be imagined, namely, that a body conli7iue& in 

 the state in w^hich it is, whether of motion or rest ; and, if 

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

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

 change : by virtue, I say, of this law, it comes to pass — what 



