THE CAUSE OF EARTIiaUAKES. 285 



motions of precession and nutation would be subject to 

 interference. 



M. Delauney objected to the views of Hopkins, on 

 the supposition that the fluid interior of the earth had a 

 certain viscosity. 



Sir William Thomson arrives at the conclusion that 

 the earth on the whole must be more rigid than a con- 

 tinuous solid globe of glass. Mr, Greorge H. Darwin's 

 investigations on the bodily tides of viscous or semi- 

 elastic spheroids tend to strengthen the arguments of 

 Sir William Thomson. 



Some philosophers hold the view that the central 

 portion of the earth, although intensely hot, is solid by 

 pressure, whilst the outer crust is solid by cooling. 

 Between the two there is a shell of liquid or viscous 

 molten matter. 



Another argument is, that although the interior of 

 the globe may be solid, it is only retained in that condi- 

 tion by an immense pressure, on the relief of which it is 

 liquefied — it is potentially liquid. 



As these views, and the arguments for and against 

 them, are to be found in all modern text-books of 

 geology, we will at once proceed to consider the effect of 

 solar and lunar attractive influences in producing earth- 

 quakes upon a globe which is either solid, partially solid, 

 or which has an interior wholly liquid. 



Effect of the attractive influences of the sun and 

 moon, —In 1854 M. F. Zantedeschi put forward the view 

 — that it is probable there is a continual tendency of 

 the earth to protuberance in the direction of the radii 

 vectores of the two luminaries which attract it. Incon- 

 sequence of these protuberances, pendulums ought at one 

 time to swing more slowly than at others, Zantedeschi 

 remarks that the periods of earthquakes appear to confirm 



