26 The Tay Bridge. 



of overturning when empty or full respectively. Both of 

 these values, it will be seen, are within the limits of actually 

 recorded observations, and from this point of view the theory 

 is a perfectly admissible one. There are, however, certain 

 considerations tending to show that the theory, though 

 reasonable, is not the true one — First, the wind was much 

 more likely to overturn the carriages when running over the 

 first, or southern, part of the bridge than when partially 

 sheltered inside the cage, and yet we know that that part 

 was traversed in safety. Second, the diagonals of the girder 

 were few in number, and of massive construction; so that it 

 is probable that the comparatively slight wooden body of a 

 railway carriage would be powerless to injure them, espe- 

 cially as it would not strike a fair blow against a bar inclined 

 45° to the horizon. The engine itself, weighing four 

 times as much as a carriage, and presenting less surface to 

 the wind, would not incur any danger of displacement. 

 Third, in accordance with this view the injured span only 

 should have fallen, and not 3000 feet of bridge. Fourth, the 

 wreck would have been found on the site of the bridge, and 

 not some twenty yards to the east. 



But while this theory is rejected as an explanation of the 

 Tay Bridge disaster, its consideration is useful as directing 

 attention to what is, possibly, a serious danger to railway 

 traffic. Suppose, for example, a partially empty train were 

 caught on the Moorabool Viaduct by a heavy gale ; it is not 

 at all impossible that the carriages might be swept into the 

 valley beneath. It is true that our somewhat wider gauge 

 gives our rolling stock a little more stability than possessed 

 by English vehicles, but allowing for this and computing as 

 above, the critical pressure is found to be only 34J lbs. per 

 square foot for ordinary carriages when empty, and only 

 26 lbs. for the large American bogie stock, both of which 

 are within the range of recorded pressures at the observatory. 

 Under these circumstances it becomes a question whether 

 a special semaphore actuated by wind pressure should not 

 be erected on our largest and most exposed structures. 

 Such a machine could be simply constructed of a semaphore 

 arm, a plate at right angles to the bridge to receive the 

 wind pressure, and a balance weight; there need be only 

 one axis of motion, and this could be of small diameter and 

 loose fit, so as to need no lubrication, and if of gun-metal 

 there would be no danger of its being stuck fast by rust. 

 The balance-weight should be such a size that it would turn 



