1888.] 



of the Vibration of the new Tay Bridge. 



397 



base of the box. A small brass ball attached to the lower mass b fits 

 easily but without shake in a cylindrical hole in a, so that the two 

 mast swing together. The masses of a and b are proportioned, with 

 respect to their distances from their respective supports, so that the 

 equilibrium of the compound system is nearly neutral, and by way of 

 final adjustment the upper disk a may be raised or lowered by turn- 

 ing the pins at the top until the margin of stability is as small as 

 may be wished. The recording lever c is held by a gimbal joint in a 

 bracket d, fixed to the side of the box, and capable of adjustment 

 vertically and horizontally. The bottom of the lever is a ball which 

 gears into the hole in a, and at the top there is a hinged index of 

 straw with a needle-point to write the record. To reduce friction, 

 part of the weight of the straw is borne by a spring e. The smoked 

 glass plate / stands on a shelf which projects from one of the sides 

 of the case, which is a triangular box. In the particular instrument 

 employed at the Tay Bridge, the ground's motion was magnified six 

 times. 



The seismograph was set upon the ground in the six-foot way 

 between the two pairs of rails at the middle of the length of the 

 southernmost high girder, at a distance of about 1-| mile from the 

 Dundee end of the bridge, and f mile from the Fife end. The girders 

 are there 245 feet long, and stand at a height of about 110 feet above 

 the bottom of the river and 135 feet above the foundations of the 

 piers. Between this and the Fife shore there are 28 piers ; towards 

 Dundee there are 57 piers, and at that end the bridge forms a 

 curve of 21 chains radius by which its direction is turned through 

 nearly a right angle as it approaches the shore.* In this position 

 observations were made while eight trains crossed the bridge. There 

 was no wind, and, until a train came on, the recording index of the 

 seismograph stood perfectly at rest. 



As soon, however, as a train entered. the bridge — from either end — 

 the index began to move. The movements were at first so minute that 

 it was difficult to estimate their range with any accuracy ; allowing 

 for the multiplication given by the lever, the movement began with 

 longitudinal shaking through something like of an inch. In the 

 case of trains coming from Dundee this was transmitted round the 

 bend of the brido-e and was noticed long before the train had reached 

 the straight part. At first the movement was wholly longitudinal, and 

 it was not until the train had come much nearer that lateral oscilla- 

 tion began to be felt. The interval by which longitudinal vibrations 

 preceded transverse vibrations was much greater than could be 

 explained by difference in their velocity of transmission. Near the 



* For particulars of the dimensions of the bridge, reference should be made to 

 Mr. Kelsey's paper in the 1 Proceedings of the Institute of Mechanical Engineers,' 

 August, 1887. 



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