Page:The American Cyclopædia (1879) Volume VI.djvu/372

 364 EARTHQUAKE the horizontal component Palmier! employs four open U-shaped tubes partly filled with mercury ; they are so set up that the planes of the tubes are directed toward different points of the compass ; when the earthquake shock dis- turbs the quiescent mercury, a galvanic current is made or broken, by which the time record is made on a moving fillet of paper ; light floats are also disturbed, and show by the extent of their movements the force of the shock. 13. The preceding instrument has some points of resemblance with that first proposed in 1846 by Mallet ; the latter is however calculated to resolve with greater ease and accuracy all the problems connected with terrestrial waves of shock. Mallet employs one vertical and four horizontal L-shaped glass tubes, each separated from the other and filled for a short distance with mercury ; the mercury being originally in a quiescent state, the linear motion that is com- municated to it relative to the sides of the tubes by the wave of shock breaks or makes a galvanic circuit by which the time is recorded on a revolving cylinder ; the extent of the vi- brations of the mercurial column is also re- corded by appropriate dots or lines, and the ap- paratus is so placed that we are able to deduce the movements of the earth in three directions, vertically, meridionally, and at right angles thereto, whence the angle of emergence and all the other local phenomena may be calcu- lated. 14. Kreil of Vienna proposed in 1855 to employ a pendulum suspended by two inde- pendent springs, of which the lower for in- stance allowed of motion in the meridian, but the upper in a plane at right angles thereto ; the movement of the pendulum is recorded by its lower end marking on a revolving disk of paper or metal. Connection between Earth- quake* and Volcanoes. The intimate relation of these phenomena is apparent at once to even the most careless observer, but correct and exact ideas on the subject have been at- tained only within the present age. On the one hand, earthquakes are most numerous in the volcanic regions of the globe ; on the other, they are not at all confined to these centres ; they are sometimes attended by the growth or formation of volcanoes, and in gen- eral no volcanic activity is manifested without being attended by more or less notable earth- quakes, the quakings being however to a great degree confined to the volcano in eruption ; the only exception to the latter statement can indeed be shown to be explicable in such a way as to give further confirmation of the gen- eral statement that the strains, pressures, and explosions occurring within the depths of the solid earth give rise at the surface of the globe, if feeble, to slight earthquakes, and if stronger, to more severe shocks accompanied by upheavals and depressions ; if yet stronger, though not paroxysmal, to the formation of fissures, volcanic vents, and eruptions of steam, gases, hot water, mud, scoria?, ashes, lava, and living stones. A slight earthquake central in a non- volcanic region is "an uncompleted ef- fort to establish a volcano." The presence of a volcano is demonstrative of a previous earth- quake, and the volcanic vent, offering as it does a way of easy escape for gases, &c., is, unless it becomes clogged up, to some extent an assurance that subsequent earthquakes in its immediate region will be of less severity. The area thus protected may extend to a dis- tance of 100 miles from the volcano, but is usually much less, showing that within the earth's surface the centres of the origin of earthquake disturbances are often quite in- sulated. The exceedingly great variety in the strata of the earth and the nature of the vol- canic eruptions necessitates a special study of the individual cases, in order to perceive how intimately and yet how diversely these phe- nomena are associated with every variety of earthquake. Atmospheric Relations. The con- nection between the atmosphere and the earth- quake, though probably incidental, is an impor- tant one. It is found that there is a decided increase in the frequency and severity of shocks during the rainy season, at least in cer- tain localities, and especially, as Mallet says (" First Principles of Seismology," 1862), in very dry countries such as Asia Minor and Syria, and in the volcanic regions where erup- tions of steam prevail, and where the melted snows and heavy rains are rapidly drained off into deep fissures. There seems to be no reasonable doubt that in these cases the pressure of the drainage water directly af- fects the liquid in the interior of the earth, and increases the supply of both steam and lava for the volcanoes, as well as the lia- bility to such internal explosions of steam as may originate the earthquake shocks. That the diminution of atmospheric pressure such as prevails over the central regions of some severe storms, or during certain seasons, cannot di- rectly be of importance in comparison with the above mentioned more powerful hydrostatic pressure, seems quite evident; yet there are volcanoes, such as that of Stromboli, where the conditions of equilibrium are so delicately adjusted that, according to the testimony and traditions of those living in their neighborhood, their activity increases perceptibly on the ap- proach of every storm. Geographical Distri- bution of Earthquakes. Besides the preceding general relations between earthquakes and vol- canoes, it is important to consider the distribu- tion of the former, or rather their segregation in certain well marked regions. It is not neces- sary in this place to enumerate the regions of volcanic activity, although these are peculiarly also subject to earthquakes. (See VOLCANO.) The regions that are far removed from active volcanoes, yet specially subject to earthquake shocks, are, in the order of the severity of the phenomena, the Himalaya mountains and In- dia, Syria, Algeria, the "W. coast of North Ame- rica, the Mississippi valley, Scotland, New Eng- land, and the Saint Lawrence valley. The re-