EARTHQUAKE PHENOMENA. 357 



of concentric rings, all points on the circumference of each ring receiv- 

 ing 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. Al- 

 though 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 earth- 

 quakes 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 dis- 

 tances. 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 condi- 

 tion. 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 press- 

 ure becomes sufficient, the walls in some part give way, and the trans- 

 mission 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 press- 

 ure, 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 earth- 

 quakes, 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. 



