As to the nature of the forces which operate to produce this astounding result, Herschel puts forward a theory of singular simplicity and directness.

“The origin,” he says, “of such an enormous power thus occasionally exerting itself, will no doubt seem very marvellous—­little short, indeed, of miraculous intervention; but the mystery, after all, is not quite so great as at first it seems.  We are permitted to look a little way into these great secrets of Nature; not far enough, indeed, to clear up every difficulty, but quite enough to penetrate us with admiration of that wonderful system of counterbalances and compensations, that adjustment of causes and consequences, by which, throughout all nature, evils are made to work their own cure, life to spring out of death, and renovation to tread in the steps and efface the vestiges of decay.”  And he finds the clew to the secret, the key of the whole matter, in the earth’s vast central heat.  This it is which produces the convulsions that change the terrestrial configuration, and fill the minds of men with fear and awe.  Conceive of “a sea of fire, on which we are all floating, land and sea,”—­a boiling, seething, incandescent reservoir in the centre of our planet; and the solution of the problem will seem to you not difficult.  Such a sea would necessarily roll its liquid matter to and fro; and the removal of ever so small a portion from one point to another on the earth’s surface would tend to disturb the equilibrium of the floating mass; just as, when a ship is launched into the river, the water it displaces is carried to the opposite bank with greater or less violence, according to the amount of displacement.

It is impossible, adds Herschel, but that this increase of pressure in some places and relief in others must be very unequal in their bearings.  So that at some point or another our planet’s floating crust must be brought into a state of strain, and if there be a weak or a soft part a crack will at last take place.  This is exactly what happened in the earthquake which originated the Allah Bund, or God’s Wall, in Cutch.

Volcanic eruptions are easily explicable on this principle,—­the volcano being simply a vent for the passage of heated and molten matter, which the elevating pressure of the liquid below tends to eject.  It is a well-known fact that volcanoes and earthquake-centres are nearly all situated on the borders or in the immediate neighbourhood of seas and oceans; and the reason would seem to be, that at such positions the accumulation of transported matter would necessarily attain its maximum, to whatever cause it might be due.  Then again, as Herschel points out, the eruption of scorite and lava from the mouths of volcanoes, the result of the upward movement of the fiery liquid below, compensates in some degree for the downward transfer of material by detritus and alluvial deposits.  Hence it may be inferred that, on the whole, the quantity of solid matter above the ocean-level probably remains nearly always at the same amount.