402 



The Vibration of the new Tay Bridge. [June 21, 



that is moved uniformly by clockwork. The usual form of thi? 

 instrument comprises two horizontal pendulums, for the two hori- 

 zontal components, and a third piece which is suspended astatically 

 with freedom to move up and down only for the vertical component.* 

 This arrangement employs a distinct mass and a distinct " steady - 

 point " with respect to each component. The duplex pendulum may, 

 however, be modified, or rather supplemented, so that it records two 

 components of horizontal motion separately (on a moving surface) by 

 attaching to one or other of the bobs a pair of slot guides at right 

 angles to the direction of the two components, and pivotting in these 

 the short ends of a pair of recording levers, so that each lever will be 

 moved when the bob moves across the direction of the corresponding 

 slot, but will not be moved when the bob moves along that direction. 

 This makes a compact form of two-component horizontal seismograph, 

 with the advantage that by retaining the ordinary index we have, in 

 addition to the components, a plan drawn of the whole shaking. For 

 the vertical component it is convenient to have a distinct astatically 

 hung mass. But, as a sort of tour de force in astatic suspension, one or 

 other of the bobs of the duplex pendulum may be allowed to have a 

 limited amount of vertical freedom, and may have its equilibrium 

 made nearly neutral for vertical displacements as well as for hori- 

 zontal displacements. Let the upper bob, for instance, be hung from 

 a platform which is free to rise and fall by rotating about a horizontal 

 axis, and which is held up by springs. By applying the pull of the 

 springs in such a manner that the moment of the pull about that axis 

 is always nearly equal to the moment of the weight, we may approach 

 vertical astaticism as closely as may be wished, and, provided the 

 movements up and down are not too great to interfere with the 

 proper gearing of the bobs, the mass will then possess universal 

 freedom of translation, with nearly neutral equilibrium for all 

 directions of displacement. In practical seismometry, however, it is no 

 doubt advisable to restrict the freedom of the suspended mass to (at 

 most) two degrees. 



The Society adjourned over the Long Vacation to Thursday, 

 November 15 th. 



* See 'Transactions of the Seismological Society of Japan,' vol. 3 (1881), p. 140, 

 or the author's memoir on "Earthquake Measurement" cited above. A complete 

 three-component instrument is described in ' Nature,' vol. 34, p: 343. 



