1888.] 



of the Vibration of the new Tay Bridge. 



399 



The seismograph was afterwards set up just above the pier at the 

 south end of the span in the middle of which it had previously been 

 standing, and five more records were obtained in the new position. 

 Except that the motion was somewhat less, they had much the same 

 characteristics as before. The following notes refer to the passage of 

 a slow goods-train from Dundee as observed from this position : — 



Mins. Sees. 



30 40 Train entered bridge : minute longitudinal oscilla- 

 tion began. 



32 Train entered straight portion of bridge. 



33 Lateral oscillation began. 

 36 Train passed seismograph. 



38 10 Tail van of train off bridge : oscillation ceased. 



In all seismometric work, whether it be the measurement of earth- 

 quakes proper, or of such shakings as these, the trustworthiness of 

 the record depends on the degree to which the presumed " steady- 

 point "of the instrument remains at rest during a protracted dis- 

 turbance of the base. The accuracy of a seismograph admits of 

 easy experimental test in the manner which the author described 

 and illustrated when communicating to the Royal Society an account 

 •of his Horizontal Pendulum Seismograph, for recording separate 

 components of motion upon a moving plate.* The test consists in 

 placing the instrument upon a stand which may be shaken by hand, 

 and causing a true autograph of the motion of the stand to be drawn 

 by an independently supported index, side by side with the record 

 that is drawn by the seismograph itself. Fig. 3 shows how this test 

 was applied to the instrument with which the Tay Bridge observa- 

 tions were made. The seismograph was mounted on a stand which 

 was constructed to give it two degrees of freedom of horizontal 

 translation, without freedom to rotate. This was done by laying a 

 pair of turned steel rollers parallel to each other on the top of a steady 

 level table ; a small drawing-board rested on them ; on the top of it 

 a second pair of steel rollers were laid at right angles to the pair 

 below; a second small drawing-board lay on them, and the 

 instrument stood upon it. The upper board was then free for 

 translation in all azimuths, and was shaken by hand so that it 

 imitated the motion in an actual earthquake. A record of this 

 motion was drawn by the seismograph index, and beside it a second 

 record was drawn by the lever and index g (fig. 3) which was held 

 by a gimbal joint in a stiff bracket h secured to the upper board, and 

 took its motion from a true steady-point i obtained by making the 

 bottom end of the lever in the form of a small ball socketed in a 



* "Ona new Seismograph," £ Roy. Soc. Proc.,' vol. 31, 18S1, p. 410. 



