364 Prof. Ewing on Earthquakes and hoiv to Measure them. [June 1, 



were quite perceptible in wliicli the greatest extent of motion was no 

 more than y^^ of an inch. In one case, on the other hand, Professor 

 Sekiya had obtained a record in which the motion was as much as an 

 inch and three-quarters. Even that was in an earthqnalce which did 

 comparatively little damage, and there was therefore reason to expect 

 that in a severely destructive shock (such as had not occurred since 

 the present system of seismometry was developed) the motion might 

 be considerably greater. 



Professor Ewing concluded his lecture by pointing out that the 

 seismographs he had described might find practical application in 

 measuring the stitfness of engineering structures. He exhibited, by 

 the lantern, seismographic records he had recently taken on the new 

 Tay Bridge, to examine the shaking of the bridge during the passage 

 of trains. The instrument had been placed on the southernmost of 

 the greater girders, where there was reason to expect the vibration 

 would be a maximum. The extent of motion was remarkably small. 

 It was less than an eighth of an inch, even while the train was passing 

 the seismograph — a fact which spoke well for stiffness of the structure. 

 Nevertheless, by watchiug the index of the seismograph he had been 

 able to tell whenever a train came on at the Dundee end of the 

 bridge, a distance of I/5 mile from the place where the instrument 

 was standing. One could then detect a vibratory motion, the extent 

 of which was probably not more than ^-^^ of an inch. This began in 

 the longitudinal direction, and for some time longitudinal vilu-ation 

 only could be seen. As the train came nearer lateral vibration also 

 began, and the amj^litude of course increased. It reached a maximum 

 when the train was close to the seismograjdi, and continued visible 

 until the train had passed off the bridge at the other end.* 



[J. A. E.] 



* Particulars of tliese experiments have been communicated to the Royal 

 Society, and will be found in the ' Proceedings' for June 21, 1888. 



GENERAL MONTHLY MEETING, 



Monday, June 4, 1888. 



His Grace The Duke of Northumberland, E.G. D.C.L. LL.D. 

 President, in the Chair. 



F. W. Bayley, Esq. F.C.S. 

 Jacob Feis, Esq. 

 Charles Albert Flint, Esq. 

 Arthur Holland, Esq. 

 Mrs. John Mackinlay. 

 Thomas Woolner, Esq. P. A. 



were elected Members of the Eoyal Institution. 



