gauges were used. In No. 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 hydrodynamical 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 pounds 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 the 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