SEISMOLOGY

tho scientific study of earthquakes, which may be earth tremors so slight as only to be detected by delicate seismographs. Earthquakes so recorded may last but a few seconds or persist for hours, or the earth may be shaken by succe ive shocks for days. Large earthquakes are recorded by seismographs, however distant they may be from the center of disturbance. Such earthquakes commence by tremors (Preliminary tremors), giving place to large waves which work up to a Maximum, The simplest instruments are the seismoscope and seismometer. The seismoscope may be a column so balanced that it falls when earthquake occurs and shows direction of earth-waves. Tne setsmometer Measures intensity of shock. The seismograph is the form of instrument mostly used at present day. The Milne seismograph consists of an upright column supporting without friction a horizontal boom, which being therefore free to move, sways according to the intensity of earth movement communicated to it. A needle at the end of the boom is slit to allow a spot of light to fall on sensitized paper, which therefore shows a straight line when the instrument is at rest. The column is built into concrete sunk into the ground. The newest seismograph, the Galitzin, involves electro-magnetic principles and gives both distance and direction of the shock. Earthquakes are thought to be due to the shrinkage of the earth’s crust and the nece ary adjustments to the inner nucleus. About sixty great earthquakes are recorded annually. Great earthquakes of recent years include those of Val para i so, following one off Alaska a few minutes earlier, and San Francisco, 1906, Jamaica, 1907, Me ina, 1908 and Japan, 1923. region in S.W. Asia, deriving its name from Lake S., or Hamun, a large swamp in W. of Afghanistan, near Khor as san, province of Persia; total area, 7000 sq. miles, of which considerably more than half belongs to Afghanistan, the remainder to Persia; surface is chiefly steppe land, but large area is rendered fertile by periodic floodings of Lake Hamun; produces cereals, pulse, cotton, fruits; inhabited by various native tribes, some of which are nomadic. Per sian 8. is divided into the two districts of S. proper and Outer S., and has a total pop. of e. 250,000.