Much speculation has been spent upon the fact of an earthquake being very severe in two or three different localities, but being imperceptible or very mild in intervening places. In South America it has become so common a peculiarity that the natives speak of such localities as “bridging” the earthquakes. Not improbably the reason is the same that produces calm when two waves interfere, crest to trough; the motions destroy each other. It may be also that the character of the underlying rocks has much to do with such cases. Experiments with explosions in mines show that vibrations of the soil travel over three hundred yards per second through sand beds, or about as rapidly as in the air; over five hundred yards in granite; while through iron they travel over three thousand eight hundred and fifty yards per second. So a vibration extremely destructive to a region underlaid by massive rocks might be comparatively harmless to a town on a sand-bed or mud-bank. Observations on earthquakes themselves have shown great variation in the rate of speed. The earthquake of Germany of 1846 moved four hundred and ninety-two yards per second; while the earthquake of Viege in 1855 traveled nine hundred and sixty yards a second toward Strasburg, but only half that speed towards Turin. So, also, the Lisbon earthquake traveled three times as rapidly around the coast as down the Rhine valley. So it must be that certain regions owe their comparative immunity from earthquakes to the nature of the ground beneath. One or two ingenious savants have suggested that the earth is a vast thermo-electric pile, and that disturbances in the electrical equilibrium of the earth are the cause of earthquakes and volcanic eruptions. But as already seen, the electrical phenomena of volcanic eruptions are fairly considered an effect, and not a cause, of the eruptions, as the hydro-electric machine illustrates. In the theory of these men, the molten veins of the interior represent conductors which are too small or imperfect to allow the electricity to pass freely, and are fused in consequence. One of these men, Steffens, alleges that such phenomena can only occur where large veins of coal exist, because large masses of carbon would be necessary to keep up a strong electric tension in the interior. Herr Steffens must account for the fact that the great coal regions of the world have been peculiarly favored in their comparative immunity from shocks. Still others have advocated the idea that atmospheric whirlwinds and cyclones produce earthquakes. While not a few shocks have been accompanied by violent storms, the exception seems to be the rule. And in the case of storms, we have seen that the outpour of heat and vapor in a volcanic eruption would necessarily produce one. As concerns the winds that have accompanied earthquakes, they have as often come after the shock as before.