1888.] of the Vibration of the new Tay Bridge. 39,5 



of the vibrations to be measured is neither very much greater nor 

 very much less than is usual in earthquakes, and provided, of course, 

 the amplitude of vibration does not exceed the capacity of the in- 

 strument. For vibrations of high frequency a greater rigidity in the 

 multiplying and recording apparatus would be necessary ; in vibra- 

 tions of very long period, on the other hand, the* mass whose inertia 

 furnishes the steady-point of reference will not remain at rest. 

 Between these extremes, however, there is a wide range within 

 which such seismographs as are now used to measure earthquakes 

 may be trusted to give a record that is correct in all substantial 

 particulars, and the vibrations to be referred to below fall within this 

 range. 



The writer has recently employed his Duplex Pendulum Seismo- 

 graph to examine the vibration of the new Tay Bridge while railway 

 trains are passing over it, facilities for this examination having been 

 kindly given by Mr. Fletcher F. S. Kelsey, resident representative of 

 Messrs. Barlow, the engineers of the bridge. The results are per- 

 haps worth publishing, not so much for any interest they have in 

 themselves, as because they exemplify a novel method of inquiry 

 which may prove of use in other cases to engineers. The duplex 

 pendulum seismograph, which was designed for and applied to the 

 measurement of earthquakes in Japan in 1882,* consists essentially of 

 a pair of masses which are supported and connected in such a manner 

 that they form an astatic combination with freedom to move in any 

 horizontal direction. One of the two is hung from above and is 

 stable ; the other is supported from below and is unstable ; and the 

 two are constrained to move together by a ball-and-tube coupling. 

 Their equilibrium is adjusted to be very nearly neutral, and this fits 

 them to furnish a steady-point with respect to which motion of the 

 ground in any azimuth may be recorded and measured. The motion 

 is recorded by a lever, the marking point of which draws a magnified 

 copy of the horizontal motion of the ground upon a smoked-glass 

 plate. Fig. 1 shows the construction of the duplex pendulum seis- 

 mograph as used in these experiments, and as now made by the 

 Cambridge Scientific Instrument Company for earthquake observa- 

 tories. The stable mass is a disk of lead a cased in brass (shown in 

 section in fig. 1) hung by three parallel wires from the top of the contain- 

 ing box. This trifilar suspension has several advantages over the usual 

 suspension of a pendulum from a single point ; in particular it prevents 

 twisting about a vertical axis. The unstable or inverted pendulum b is 

 also a disk of lead below the other, and is held up by a tubular stmt 

 which ends in a hard steel point resting in an agate socket in the 



* See 'Transactions of the Seismological Society of Jopan,' toI. 5, p. 89, or the 

 author's memoir on " Earthquake Measurement " (' Memoirs of the Science 

 Department of the University of Tokio,' No. 9, 1883). 



vol. xrjv. 2 g 



