130 NOMOb. 



which the engineer has to make in laying down the 

 individual pieces of a railway; and carrying out 

 this idea, it is possible to see a reason why the earth 

 should be principally water or sandy waste where 

 the sun 's rays fall the hottest. 



Considered in this point of view, then, the wonder 

 is, not that the earth should be expanded by the 

 solar heat, but that it should be expanded to so tri- 

 fling an extent. How little or how much it is ex- 

 panded we shall have occasion to consider presently. 

 In the meantime, we must be content to understand 

 that the side of the earth which is exposed to the 

 sun must expand to a greater extent than the side 

 which is away from the sun. We must be content 

 to understand that one effect of the solar heat will 

 be to cause the earth to bulge out towards the sun. 



But there is another effect of solar heat which is 

 not less probable or important than the one which has 

 just been considered, an effect which is consequent 

 upon the form of the earth ; and in order to under- 

 stand this we must also have recourse to an imagi- 

 nary case. Let us suppose, then, that the earth is 

 replaced by a ball of water of equal size and form, 

 and let us ask what will be the effect of the action 

 of solar heat upon it. The effect will be that on the 

 side exposed to the sun a large quantity of water 

 will be converted into vapour, and the probability 

 will be that some of the rays will pass through the 

 water, and, being refracted in their passage, will be 

 condensed into a focus at some little distance beyond 

 the surface on the side away from the sun. This 



