1888.] Prof. Eicing on EartJiqiiaJces and how to Measure them, 361 



WEEKLY EVENING MEETING, 



Friday, June 1, 1888. 



William Crookes, Esq. F.E.S. Yice-President, in the Cliair. 



Professor J. A, Ewing, F.E.S, University College, Dundee. 



Earthquakes and how to Measure them. 



The lecturer pointed out that seismology was a science with two 

 sides, one geological, the other mechanical. The geologist 

 attacked the subject by at once attempting to refer earthquakes to 

 their source in the crumpling, tearing, or slipping of strata, in 

 volcanic eruption, in the collapse or explosion of subterranean cavities. 

 The mechanical student of earthquakes, on the other hand, concerned 

 himself with the character of the motion that was experienced, and 

 with the means by which an earthquake spread from point to point 

 by the elastic vibration of rock and soil. His first business was to 

 find out exactly how the ground moved during an earthquake, to 

 determine by direct measurement the amount and direction of every 

 successive displacement, and the velocity and rate of acceleration at 

 every instant while the shaking went on. This was the problem of 

 seismometry, and the lecture would deal with the solution of this 

 problem, and with some of the results which had been obtained in the 

 measurement of earthquakes in Japan. Earthquakes happened there 

 with a frequency sufiicient to satisfy the most exacting seismologist. 

 It had been estimated that one or another part of the empire was 

 shaken every day, and in Tokio, where the measurements had been 

 made, there was an earthquake, on the average, about once a week. 



Most early attempts to reduce the observing of earthquakes to an 

 exact science had failed, because they were based on a wrong notion 

 of what an earthquake was. It had been imagined that an earthquake 

 consisted of a single isolated jerk, or of a few jerks, easily dis- 

 tinguishable from any minor oscillations that might accomj^any them. 

 The old column seismometer, for instance, the use of which was 

 recommended in the ' Admiralty Manual of Scientific Enquiry,' at- 

 tempted to measure what was called the intensity of the shock by 

 means of a number of columns of various diameters which stood like 

 ninepins on a level base. It was expected that the shock would 

 overthrow the narrower columns u}) to a breadth which would gauge 

 the intensity of the disturbance, and also that the line in which they 

 fell would show the horizontal directicm of transit of the earthquake 

 wave. In fact, however, such columns fell most capriciously when 

 they fell at all. The reason was that in an earthquake there was no 

 single outstanding impulse. There was a confused jumble of oscilJa- 

 tions, very numerous, and very irregular, which shifted their direction 



Vol. XII. (No. 82.) ' 2 b 



