THE TIDES 319 



towards the moon with more force than an equal mass at 

 the average distance; the most distant parts are drawn 

 towards the moon with less force than corresponds to the 

 average distance. The solid mass of the earth, as a whole, 

 experiences, according to its mass, a force depending on the 

 average distance; while each portion of the water on the 

 surface of the earth experiences an attractive force due 

 to its own distance from the moon. The result clearly is, 

 then, a tendency to protuberance towards the moon and from 

 the moon ; and thus, in a necessarily most imperfect manner, 

 I have explained to you how it is that the waters are not 

 heaped up on the side next the moon, but are drawn up 

 towards the moon and left away from the moon so as to 

 tend to form an oval figure. The diagram (FiG. 125, p. 298) 

 shows the protuberance of water towards and from the 

 moon. It shows also the sun on the far side, I need 

 scarcely say, with an enormous distortion of 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, pull- 

 ing at one place, pressing in at another, will it not squeeze 

 the earth out of shape? I perceive signs of incredu- 

 lity; 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 per- 

 haps till this moment many here present have believed, but 

 which I hope no one will 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 supposition 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 



