I40 POPULAR SCIENCE MONTHLY. 



gauges were used. In ISTo. 1 the surface on which the wind acted was 

 about 1% square feet in area; it was swiveled so as always to be at 

 right angles to the wind. In No. 2 the area of surface acted on was 

 oi the same size but was fixed with its plane north and south. No. 3 

 was also fixed in the same direction but it had 200 times the area, its 

 surface being 300 square feet. 



In preparing the table the mean of all the readings of the revolving 

 gauge between and 5, 5 and 10, etc., pounds per square foot have 

 been taken and the mean of the corresponding readings of the small 

 fixed gauge and the large fixed gauge set opposite, these being arranged 

 for easterly and westerly winds. 



Two points are to be noticed: (1) There is only one reading of over 

 32.5 pounds registered, and this it is ^practically certain is due to faulty 

 action in the gauge. Sir B. Baker has kindly shown me some further 

 records with a small gauge. 



According to these pressures of over 50 pounds have been registered 

 on three occasions since 1886. On two other occasions the pressures as 

 registered reached from 40 to 50 pounds per square foot. But the 

 table, it will be seen, enables us to compare the pressure on a small 

 area with the average pressure on a large area, and it is clear that 

 in all cases the pressure per square foot as given by the large area is 

 much less than that deduced from the simultaneous observations on the 

 small area. 



The large gauge became unsafe in 1896 and was removed, but the 

 observations for the previous ten years entirely confirm this result, the 

 importance of which is obvious. The same result may be deduced from 

 the Tower Bridge observations. Power is required to raise the great 

 bascules and the power needed depends on the direction of the wind. 

 From observations on the power some estimate of the average wind 

 pressure on the surface may be obtained, and this is found to be less 

 than the pressure registered by the small wind gauges. 



Nor is the result surprising when the matter is looked at as an hydro- 

 dynamical problem — the wind blows in gusts — the lines of flow near a 

 small obstacle will differ from those near a large one ; the distribution of 

 pressure over the large area will not be uniform. Sir W. Siemens is 

 said to have found places of negative pressure near such an obstacle. 

 As Sir J. Wolfe Barry has pointed out, if the average of 56 pouuds to 

 the square foot is excessive then the cost and difficulty of erection of 

 large engineering works is being unnecessarily increased. Here is a 

 problem well worthy of attention and about which but little is known. 

 The same too may be said about the second of the Board of Trade rules. 

 What is the effective surface over which tlie pressure is exerted on a 

 bridge? On this again our information is but scanty. Sir B. Baker's 

 experiments for the Forth Bridge led him to adopt as his rule double 



