The relief of external pressure and the increase of internal pressure, it will be observed, both tend in the same direction—namely, to an elevation of the earth’s crust.
This explanation of the increased activity of earth tremors, which has also been suggested by M. S. di Rossi, is here only advanced as a speculation, more probable perhaps than many others.
We know how a mass of sulphur which has been fused in the presence of water in a closed boiler gives up in the form of steam the occluded moisture upon the relief of pressure. In a similar manner we see steam escaping from volcanic vents and cooling streams of lava. We also know how gas escapes from the pores and cavities in a seam of coal on the fall of the barometrical column. We also know that certain wells increase the height of their column under like conditions. The latter of these phenomena, resulting in an increase in the rate of drainage of an area by its tendency to render such an area of less weight, facilitates its rise. If we follow the views of Mr. Mallet in considering that the pressures exerted on the crust of our earth may in volcanic regions be roughly estimated by the height of a column of lava in the volcanoes of such districts, we see that in the neighbourhood of a volcano like Cotopaxi the upward pressures must be enormously great. Further, the phenomena of earthquakes and volcanoes indicate that these pressures are variable. Before a volcano bursts forth we should expect that there would be in its vicinity an upward bulging of the crust, and after its formation a fall. Further, it is not difficult to conjecture other possible means by which such pressures may obtain relief.
Should these pressures then find relief without rupturing the surface, it is not difficult to imagine them as the originators of vast pulsations which may be recorded on the surface of the earth as wave-like motions of slow period.
As an explanation of the strange movements observed on seas and lakes, Kluge brings forward the following strange and remarkable theory. The oxygen of the air is magnetic, whilst water is diamagnetic and the earth magnetic: we have, therefore, in our seas and lakes a diamagnetic body lying between and being, consequently, repelled from two magnetic bodies. By variation in temperature, the balance of repulsions exerted by the air and the earth is destroyed. Thus, by an elevation of temperature the air expands and flows away from the heated area, where, in consequence, there is less oxygen. The result of this is, that the repulsion of the air upon the waters is less than that of the earth upon the waters, and the waters are in consequence raised up. By a falling of temperature the waters may be depressed, and by either of these actions waves may be produced without the intervention of earthquakes or earth pulsations.