198 POPULAR LECTURES AND ADDRESSES. 



proportions, because without that it would be 

 impossible in a diagram to show the three 

 bodies. This illustrates the tendency of the 

 tide-generating forces. 



(3) Elastic Tides. But another question arises. 

 This great force of gravity operating in different 

 directions, pulling at one place, pressing in at 

 another, will it not squeeze the earth out of 

 shape ? I perceive signs of incredulity ; you think 

 it impossible it can produce any sensible effect. 

 Well, I will just tell you that instead of being im- 

 possible, instead of it not producing any such 

 effect, we have to suppose the earth to be of 

 exceedingly rigid material, in order that the effect 

 of these distorting influences on it may not mask 

 the phenomenon of the tides altogether. 



There is a very favourite geological hypothesis 

 which I have no doubt many here present have 

 heard, which perhaps till this moment many here 

 present have believed, but which I hope no one 

 w r ill go out of this room believing, and that is that 

 the earth is a mere crust, a solid shell thirty, or 

 forty, or fifty miles thick at the most, and that it is 

 filled with molten liquid lava. This is not a sup- 

 position to be dismissed as absurd, as ludicrous, as 

 absolutely unfounded and unreasonable. It is a 

 theory based on hypothesis which requires most 

 careful weighing. But it has been carefully weighed 

 and found wanting in conformity to the truth. On 

 a great many different essential points it has been 



