Page:Popular Science Monthly Volume 29.djvu/329

Rh beneath. A smaller number of earthquakes originate at actual volcanoes. Some earthquakes are produced by the sudden fracture of rocky-strata or the production of faults. This may be attributable to stresses brought about by elevatory pressure. Lastly, we have earthquakes due to the collapse of underground excavations; and these may have been produced by evisceration caused by volcanic eruptions, by the washing away or solution of the earth by chemically charged waters or hot springs, or by other causes.

Considerable attention has been drawn lately toward the study of small vibratory motions of the ground which, to the unaided senses, are usually passed by without recognition. They are called earth-tremors, and were only discovered when difficulties caused by them were encountered in the adjustment of extremely delicate astronomical and other instruments. These movements have been most carefully studied in Italy by Father Bertelli, of Florence; le Conte Malvasia, at Bologna; M. di Rossi, at Rome; and le Baron Puet, at Nice. Delicate instruments have been devised for detecting and recording them, the most important of which is the normal tromometer of Bertelli and Rossi. It consists of a pendulum (Fig. 9) one and a half metre long, carrying, by means of a very fine wire, a weight of one hundred grammes. To the base of the bob a vertical stile is attached, and the whole is inclosed in a tube, terminated at its base by a glass prism of such a form that, when looked through horizontally, the motion of the stile can be seen in all azimuths. In front of this prism a microscope is placed. Inside the microscope is a micromatic scale, so arranged that it can be turned to coincide with the apparent direction of oscillation of the point of the stile. In this way not only can the amplitude of the motion of the stile be measured, but also its azimuth. The extent of vertical motion is measured by the up-and-down motion of the stile due to the elasticity of the supporting wire.

Another instrument, the microseismograph of Professor Rossi, gives automatic records of slight motions. It consists of four pendulums, each about three feet long, suspended so that they form the corners of a square platform. In the center of this platform a fifth but rather