TRA NS ACTIONS. 



The Fressure of the Wind on Roofs and Inclined 



Surfaces. 



By J. H. KiN-EALY. 



The formula generally used to calculate the normal pressure of 

 the wind on a surface inclined at an angle to the direction of the 

 wind is the one deduced by Unwin from experiments made by 

 Hutton in 17S6. 



This formula is 



»r T, , . .V I.S42 COS i — I, 



A' = F (sin I) ^ 



where iVis the normal pressure per square foot of surface ; z, the 

 angle of inclination with the direction of the wind ; and Z', the 

 pressure per square foot of the same wind upon a surface perpen- 

 dicular to its direction. 



In making the series of experiments from which the above for- 

 mula was deduced, a complete description of which is in Tract 

 36, 17SS, titled "Resistance of the Air to Bodies in Motion," 

 Hutton used an apparatus that consisted essentially of a small 

 wooden lever fastened at one end to a cylinder that turned easily 

 about a vertical axis. The cylinder was made to turn on the ver- 

 tical axis by means of weights fastened to a cord wound about 

 it. A small rectangular plane was fastened by one of its edges 

 to the outer end of the lever. By noting the weights necessary 

 to turn the cylinder, and making due allowances for friction, Hut- 

 ton measured the resistance of the air to the plane when it was 

 inclined at various angles to the horizon. It is evident from his 

 description of the manner of making the experiment, that, al- 

 though he does not mention the matter, Hutton, by the experi- 

 ments, found not the normal pressure of the air on the plane, but 

 the horizontal compoiient of the normal pressure. 



As a result of his experiments he gives the formula 



„ r> , . .. 1.S43 cos /, 

 R ^ P (sm t) ^ 



where P is the resistance when the plane is perpendicular to its 



V. — 3-1 [May 14, 1SS9. 



