﻿Our Record of Canadian Earthquakes. 325 



and are in general quite misleading. Moreover, the 

 destructive effect of an earthquake depends quite as 

 much on the velocity as on the amplitude of the motion. 



In the Ewing Seismographs the actual movement of the 

 ground in an earthquake is recorded by means of three 

 delicately balanced pendulums, to which light styles are 

 attached, tracing the relative movements of the pendulums 

 on a revolving plate of glass covered with a thin film 

 of smoke black. Two horizontal pendulums are used for 

 recording the movements in the east and west and north 

 and south directions respectively. The traces made by 

 these pendulums magnify the actual movement five times. 

 A third pendulum gives the vertical movement on a scale 

 magnified twice. By combining the three records, the 

 actual movement of a point of the earth's surface at any 

 time may be obtained. In general, the earth movement is 

 extremely complicated, and lasts for a considerable time, 

 consisting of irregular vibrations, which have no relation 

 to the direction of propagation or the origin of the 

 disturbance, and cannot be specified as a single shock 

 of definite direction. 



The accompanying figures represent the record of 

 the earthquake of Tuesday, March 23rd, 1897, 6 hrs. 

 7 min. p.m., at the McDonald Physics Building. The 

 apparatus was started by means of a delicate seismoscope, 

 making an electric contact, three seconds before the main 

 shock which, in this instance, consisted practically of 

 a single movement followed by small subsidiary oscilla- 

 tions. The seismoscope starts a clock, which records the 

 time of the shock, and also marks the time in half 

 seconds on the revolving plate. 



The records obtained on the glass plate have been 

 enlarged to forty times the actual earth movement and 

 are drawn to a uniform scale. The scale of tenths of a 

 millimetre shows the actual extent of the movement of 

 the earth's surface. As compared with the horizontal. 



