EARTHQUAKES AND OTHER SEISMIC MOVEMENTS. 315 



beneath. A smaller number of earthquakes originate at actual volca- 

 noes. 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 astronomi- 

 cal 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 in- 

 struments have been devised for de- 

 tecting 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 at- 

 tached, 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 mo- 

 tion 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 co- 

 incide 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 



Yia. 9. Nofmai, Tromometer. B. bob of 

 pendulum; P, prism; M, microscope ; S t 

 rim of scale. 



