574 PROCEEDINGS OP^ SECTION H. 



money was going on in Victoria leading American engineers were 

 constructing the gigantic Kinzua viaduct, more than ;300ft. hijfh, 

 and giving it a resistance to wind pressure per square foot little 

 more than half that possessed by the bridge so ruthlessly con- 

 demned. 



Move recently still a Royal Commission reported on certain iron 

 bridges on a railway in Tasmania and condemned them as weak 

 under wind pressure. The designer of the bridges protested, and 

 further advice was sought: The matter being referred to the 

 author of this paper, he calculated the resistance and found that it 

 would take no less than 2001bs. per square foot to overturn the 

 girders! And yet the Royal Commission condemned them, but 

 never dreamt of condemning the railway carriages that passed over 

 them, and which a similar calculation showed would infallibly 

 capsize under a pressure of only 251bs. Under these circumstances 

 of conflicting authorities and wildly inconsistent practice it appeared 

 to the writer that a paper on wind p.ressure, comparing and dis- 

 cussing the data and rules given in the books and detailing results 

 of his own inquiries, calculations, and experiments, could not fail to 

 be of use and interest. 



For information upon wind pressure recourse is usually had to 

 observatories equipped tor meteorological work. These institutions 

 usually possess some appliance professing to indicate the velocity 

 of the wind, and more rarely an apparatus recording its pressure 

 against a vertical surface. The instrument for determining the 

 Telocity is in all cases known to the author, either Robinson's or 

 Hagemann's anemometer. At the Melbourne Observatory both 

 are in use. In those cases in which the pressure itself is directly 

 measured Osier's anemometer is employed. Robinson's anemo- 

 meter consists of four hemispherical cups attached to arms 

 radiating: from a vertical axis, the rotational velocity of which is 

 supposed to bear a constant relation to the speed of the wind. 

 Hagemann's is simply a vertical tube, in which a partial vacuum is 

 formed by the passage of the air over its open end, which ]3artial 

 vacuum is measured and recorded by suitable mechanism below. 

 Osier's is a thin plate of metal, placed perpendicular to the direc- 

 tion of the wind, and jorovided with a spring behind, the com- 

 pression of which shows at once on a suitable scale the pressure in 

 pounds per square foot. It is if properly made by far the most 

 satisfactory instrument of the three from an engineering point of 

 view. The principal results obtained by the use of the above 

 instruments at different observatories are as follows : — 



BRITISH OBSERVATORIES. 

 Bidstone, near Liverpool, 90lbs. per square foot ; Greenwich, 

 5 1 lbs. per square foot ; Glasgow, 48lbs. per square foot ; Edinburgh, 

 271bs. per square foot. All these results are by Osier's anemo- 

 meter. 



