The Tay Bridge. 25 



1. That the train was blown off the rails, and, coining 

 into collision with some important member of one^ of the 

 girders, broke it, and so lei the structure fall. The first 

 question to be settled with reference to this hypothesis is 

 whether the force of the wind was likely to effect such a 

 displacement. 



The velocity and pressure of wind have long been a sub- 

 ject of continuous observation at meteorological observatories, 

 and values of the latter of about 40 lbs. per square foot have 

 been recorded on various occasions, while 55 lbs. is given in 

 Trantwine's Engineers Pocket Book as having been once 

 realised at Glasgow. The highest result on the books of the 

 Melbourne Observatory is 35 lbs. A value higher than 

 any of those mentioned was obtained at Sydney during 

 the great " Dandenong" gale, but has not yet been con- 

 firmed by any corroborative testimony. During the Tay 

 Bridge gale a pressure at Glasgow of 25 lbs. was registered 

 more than once, and occasional gusts of even greater intensity 

 are believed to have occurred. 



Taking a general view of the data available, it appears 

 that we may certainly anticipate in exposed positions occa- 

 sional wind pressures of 40 lbs. to the square foot, and that 

 it would be prudent in such cases to design structures so as 

 to be safe up to half a hundredweight to the square foot, or 

 a ton to 40 square feet. These numbers have the advantage 

 of facilitating computation. 



We next inquire what wind pressure would be needed to 

 overturn ordinary railway rolling stock. Taking an ordinary 

 first-class carriage, as used on the Victorian railways, we have 

 the following results : — 



"Weight, empty ... ... ... ... 7*5 tons. 



„ with 32 passengers, at 168 lbs. ... 9*9 tons. 



Moment of stability = weight x £ distance of centres of rails. 



= weight X 2 '5 ft. (if the gauge be 4 ft. 8| in. ) 

 = 18-75 ft.-tons, if empty, or 24*75, if full. 

 Area of side of carriage, 24 ft. x 8 ft. = 192 sq. ft. 

 Height of centre of pressure from rails, 7 ft. 

 Moment of overturning, in ft.-tons = 7 X 192 x /? = 1344/?. 

 When p = wind pressure in decimals of a ton to the sq. ft. 



Equating this expression with the values of the moment 



of stability last obtained, we have— 



18-75 24-75 



^=j3^=-0139ton = 31 lbs.; andp=y^== -0184 ton = 41 lbs. 



as the pressures at which the carriage would be on the point 



