200 
THE METHODS OF WIND MEASUEEMENT. 
From the engineering point of view the state of knowledge is 
still most unsatisfactory, as shown by a perusal of the “ Report 
of the Committee to consider the question of Wind Pressure on 
Railway Structures,” formed in consequence of the Tay Bridge 
disaster. From this Report it would seem that a pressure of 
nearly 90 lbs. per square foot (corresponding to the utterly 
incredible velocity of 190 feet per second) was recorded at 
Bidstone, near Liverpool, but though the Commissioners, all 
eminent men of science, believed this to have been actually 
registered, and not due to any unforeseen action of the springs 
of the pressure board, they considered it was abnormal, and 
caused by local circumstances. Bidstone is no doubt very 
exposed, but the reasons which led the Commissioners to fix 
upon 56 lbs. per square foot as the maximum pressure of 
wind to which a structure should be ever exposed, are not at 
all clear, unless it be that some definite figure is required for 
calculation, and this number, which is used by some French 
engineers, has always proved sufficiently high. Two of their 
number, however. Prof. Stokes and Sir Wm. Armstrong, affixed 
a statement that there was no evidence to shew what the extent 
of the lateral pressure was-— that is, of the extent of surface 
upon which the maximum pressure really acted at one time. 
According to Mr. Hawkesley, past Pres. Inst. C.E., a 
pressure of 40 lbs. per square foot is unknown in these islands, 
as it would be sufficient to have overthrown most of the long 
existing factory chimneys, to have overset past wind-mills, etc., 
to have scattered the slighter built domestic and other structures ; 
and this would be evident when it is remembered that 40 lbs. 
per square foot means, upon a building 40 feet by 30 feet, more 
than 20 tons. Again, it is well known that a pressure of 
between 30 and 40 lbs. per square foot will overturn an ordinary 
railway carriage, but there is not an authorised instance of this 
having happened. In America the confident belief of engineers 
