664 



Mr. Richard T. Glazebrook 

 Table II. 



[May 24, 



In preparing the table the mean of all the readings of the re- 

 volving gauge between and 5, 5 and 10, &c, lbs. 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) only one reading of more than 

 32-5 lbs. was registered, and this, it is practically certain, was 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 more than 50 lbs. have been 

 registered on three occasions since 1886. On two other occasions the 

 pressures, as registered, reached from 40 to 50 lbs. 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 observa- 

 tions 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 direc- 

 tion 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 question is looked at 

 as a hydrody mimical problem ; the lines of fluid near a small obstacle 

 will differ from those near a large one, and the distribution of pres- 

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

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



