PHYSICAL LABORATORY OF GREAT BRITAIN. 



353 



be assumed a.s from oiu-e to twice the area of the front surfaee accord- 

 ing to the extent of the openino-.s in the hittice girders, (3) that a 

 factor of safety of 4 for the iron work and of 2 for the whole bridge 

 overturning be as.sumed. These recommendations were not based on 

 any special experiments. The question had )>een investigated in part 

 by the late Sir \Vm. Siemens. 



During the construction of tlie Forth Bridge Sir R. Hakei- con- 

 ducted a series of observations. 



T<if,b- II. 



The results of the iirst two years' observations are shown in Table 

 II, taken from a paper read at the British Association in 1884. Three 

 gauges were used. In No. 1 the surface on which the wind acted was 

 about li 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 of 

 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 sur- 

 face 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 fnid the large fixed gauge set opposite, these being arranged 

 for easterly and westerly w inds. 



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 kindl}" shown me some further 

 records with a small gauge. 



According to these, pressures of over 50 pounds have been i;^gistered 

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

 as registered reached from 10 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 ou 

 the small area. 



SM 1901- 



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