THE CAUSES OF EARTHQUAKES. 519 



tensive an earthquake as that of Lisbon on the Ist of November, 1755, 

 was produced in this way ? John Mitchell (Royal Society, 1760, vol. 

 X, p. 751) drew from this memorable example the conclusion that the 

 vapor of water intervenes in these shocks as well as in the eruptions 

 of volcanoes. Manifest effects of a class of internal explosions, un- 

 doubtedly due to the production or sudden moving of a great quantity 

 of superheated vapor, are exhibited at the present epoch, and are not 

 rare. Such explosions, for instance, are exceptionally formidable in 

 the region of Java, and the mind is naturally led to the one which has 

 just convulsed the zone between that island and Sumatra, which has 

 caused the disappearance of the island of Krakatoa and its mountains, 

 has raised other mountains, and has claimed more than forty thousand 

 victims. 



At a period more remote from us, the explosive force of interior 

 gases gave rise to very remarkable circular cavities, which have been 

 called " craters of explosion," and are well known. Examples of them 

 are found in Auvergne (Lake Pavin) and in the district of the Eifel, 

 where the stratified beds have been sharply cut as if with a punch. 

 What gases thus put in motion are capable of, as a mechanical power, 

 could hardly have been suspected till since the explosive effects of gun- 

 cotton, nitroglycerine, and dynamite, have been known. The effects of 

 compressed air in the air-gun and of the powder-gases in fire-arms have 

 been wonderfully surpassed, for we now measure explosive pressures 

 of six thousand atmospheres and more. In the experiments in which 

 I have had occasion to observe gases at high pressure in order to ex- 

 plain the action that a meteor coming with planetary speed is sub- 

 jected to on the part of the atmosphere into which it plunges, I have 

 been surprised at witnessing the great energy of gaseous masses. They 

 engrave themselves deeply, as if with a burner, into the pieces of steel 

 that are opposed to them, and of themselves reduce a part of it to an 

 impalpable dust shot into the atmosphere as if it were volcanic ashes. 

 It is no less surprising — and this observation is of much importance in 

 explaining the problem that occupies us — ^to remark the tenuity of the 

 gaseous mass that produces such results. Yet its force causes rup- 

 tures which the pressure of a weight six hundred thousand times 

 heavier than the gas could not effect ! 



In short, gaseous movements under high pressure, put in operation 

 from time to time by a simple mechanism like what Nature can and 

 does present, will account for all the essential features of earthquakes. 

 Much better than the hypothesis of interior collisions of solid bodies, 

 they explain the effect of the shock, resembling the blows of a ram, 

 their violence, their frequent succession, and their recurrence in the 

 same regions after many centuries ; they explain also the production 

 of earthquakes in regions of dislocation, especially in those in which 

 the disturbance is recent, and their subordination to deep fractures of 

 the crust of the earth. 



