EARTHQUAKE MOTION DEDUCED FROM OBSERVATION. 75 



at a station situated about one and a half mile dis- 

 tant, but on flat ground, appears to have lasted four and 

 a half minutes. The largest wave had a period of 0*7 

 second. 



In the earthquake of March 8, 1881, there were on 

 an average 1*4 vibrations per second. These vibrations 

 were executed in a direction transverse to the line joining 

 the observing station and the locality from which the 

 disturbance must have originated as determined by time 

 observations. It can, therefore, be assumed that these 

 vibrations, having so slow a period, were transverse 

 motions, this slowness or sluggishness being due to the 

 fact that the modulus for distortion is less than the 

 modulus which governs the propagation of normal vibra- 

 tions. 



The ATYiplitude of Earth Movements. — In making esti- 

 mates of the distances through which we are moved back- 

 ward and forward at the time of an earthquake, if we judge 

 by our feelings, we may often be misled. If a person is 

 out of doors and walking, an earthquake may take place 

 sufficiently strong to cause chimneys to fall and unroof 

 houses, which, so far as the actual shaking of the ground 

 is concerned, will be passed by unnoticed. On the other 

 hand, to persons indoors, especially on an upper story, it is 

 impossible even for a tremor to pass by without creating 

 considerable alarm by the angular movement that has 

 been taken up by the building. 



Many observers have endeavoured to make actual 

 measurements of the maximum extent through which the 

 earth moves at the time of an earthquake. Among the 

 reports of the British Association for 1841 is the report 

 of a committee which had been appointed 'for obtaining 

 instruments and registers to record shocks of earth- 

 quakes in Scotland and Ireland. We read that in one 



