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 continuous solid globe of glass. Mr. George 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 condition 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 earthquakes 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. In consequence 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 such a view, insomuch as they occur more often at the syzygies, or epoch of the spring tides, than at neap tides—an observation found in the works of Georges Baglivi (1703) and Joseph Toaldo (1770).[130]
Prof. Perrey, of Dijon, who did so much for seismology, held the view that the preponderance in the number of earthquakes felt at particular seasons was possibly due to the attractive influence of the sun and moon producing a tide in the fluid interior of the earth, which, acting on the solid crust, produced fractures.
Rudolf Falb, whose writings have of late years attracted considerable attention, brings forward views which may be regarded as amplifications of those suggested by Perrey.
According to Falb, the inner portion of the earth must be regarded as fluid. In the crust above this fluid reservoir are cracks and channels, into which, by the attraction of the moon and sun, the fluid is drawn. On entering these cracks cooling takes place, together with explosions of gas and subterranean volcanic disturbances. The attractions producing the internal tides required by Falb are chiefly dependent upon the following factors:—
1. The nearness and distance of the sun from the earth (January 1 and July 1).