398 Prof. J. A. Ewing. Seismometric Measurements [June 21, 



source of disturbance (as one learnt later when the train was passing 

 the seismograph) the lateral movement was actually greater than the 

 longitudinal ; it appeared, therefore, that longitudinal disturbance 

 reached the instrument from greater distances than lateral dis- 

 turbance, because it was transmitted along the bridge with less loss. 

 As the train came nearer, lateral movements became superposed on 

 the longitudinal ones, and the index of the seismograph described an 

 immense series of irregular loops, the range of which increased at 

 first slowly and then quickly to a maximum as the train passed the 

 instrument. Along with this progressive increase there was a periodic 

 rise and fall in amplitude, the beat of which apparently agreed with 

 the interval taken by the train to pass from pier to pier over succes- 

 sive spans. The last faint movements terminated abruptly when the 

 train cleared the structure. 



The vibrations were too numerous to allow the diagrams drawn by 

 the seismograph to be at all clear, and a better idea of the motion was 

 to be got by watching the index than by subsequent examination of 

 the record. Fig. 2 reproduces two of the diagrams, and is sufficiently 























B 



A 



Fio-. 2. — Tay Bridge vibrations, recorded by Duplex Pendulum Seismograph. 



representative of the rest. As the figure is printed, the top and 

 bottom are in the longitudinal direction of the bridge. Of the two, 

 the figure marked A was drawn first by a passenger train coming 

 from the south end : after it had passed the seismograph and when 

 the oscillations were again small, it was observed that another- train 

 had entered the bridge from Dundee. The glass plate was accordingly 

 moved by hand to a new position, and the second diagram (B) was 

 obtained. The movements were in general of the form of nearly closed 

 loops resembling ellipses — showing that the periods of lateral and 

 of longitudinal vibration did not differ greatly. In the greatest 

 movements the loops are much wider in the lateral than in the longi- 

 tudinal direction. The greatest lateral movement appears to have 

 been about one-tenth, certainly not more than one-eighth of an inch ; 

 the greatest longitudinal movement about one-fourth of this. There 

 were about three complete vibrations per second. 



