Page:Popular Science Monthly Volume 27.djvu/373

Rh of concentric rings, all points on the circumference of each ring receiving the shock at the same moment, even though they may be hundreds of miles apart. In other words, all points at equal distances from the center of the earthquake receive the shock at the same moment. Although this is theoretically the case, according to well-known physical laws, still, in practice, the facts are somewhat different; for the shock is retarded or accelerated according as the rock opposes or favors the passage of the wave. The seventy of the shock in a given place is dependent upon a variety of causes. These are: 1. The strength of the original shock; 2. The distance from the earthquake center; and, 3. The kind of rock on which one is standing, loose gravels greatly diminishing: the force of the shock. The destructiveness of earthquakes depends rather upon the suddenness of application than the amount of motion. In that at Rio Bomba, it is reported for a fact that a man was hurled across a stream a distance of one hundred feet, and landed on an elevation fifty feet higher than his original position. It is an undoubted fact that objects are frequently thrown great distances. In the Mississippi Valley, during the earthquakes of 1811 to 1814, the tops of trees were twisted and entangled, and strong log cabins were thrown to the ground. Rivers are sometimes checked in their flow, and, in past geological ages, some have been completely turned from their course by earthquakes.

At least four theories have been seriously advocated by scientists to explain these phenomena. The first, which is now abandoned, is based upon the supposition that the earth's interior is in a fluid condition. This being the case, the combined action of the sun and moon upon this molten mass beneath the surface causes it to surge and swing in tides of liquid fire. It is the shock from this that we feel on the surface. (I dismiss this theory, as one having no value whatever).

Another theory is, that earthquakes are due to volcanic action. The passage of the immense quantities of gas which escape from volcanoes must necessarily cause shock after shock. This gas, under pressure of thousands of pounds, is contained in a subterranean boiler, from which it is continually trying to escape. The moment the pressure becomes sufficient, the walls in some part give way, and the transmission of this shock reaches us as an earthquake. When the pent-up gases have broken through successive strata, always coming nearer to the surface, they finally reach the uppermost stratum, and this, which has been weakened by previous eruptions, gives way before the pressure, and lets the steam out as a volcanic eruption. It is thus, then, that the continual succession of earthquakes in volcanic regions is produced. When, however, we study the country about many earthquakes, and find no evidence of volcanoes, we are forced to abandon this theory for those regions, and look further. This is the case with all New England earthquakes, and with those in Spain.