WIND PRESSURE. 579 



all these experiments, which were very numerous, is unnecessary. 

 General results were as follow : — 



Cube. — The pressure on a cube was found to be as nearly as 

 possible the same, whether the wind was parallel to a side or a 

 diagonal, and was '9 of the pressui'e on a square card equal in size 

 to one face of the cube. Modulus = 'O. 



Rectancfular blocks of A'arying proportions : x = length in direc- 

 tion of wind ; y and ;: the other dimensions. 



X ■= 2y =. 2z modulus -8 



.c = 3y =: 3;: " "7 



y = 2'^ = 2~ " -9 



'y = Zx =z 'iz " -9 



A block, representing a tower square in plan, and having a 

 height equal to three times its width of base, gave a modulus of -9 

 Avhen the wind was perpendicular to one face. When the wind 

 was in the direction of a diagonal the effect was, as nearly as could 

 te measured, the same. 



Pyramid. — A pyramid of square base, having a height of about 

 three times its base, and representing fairly well a not uncommon 

 form of church spire, gave a modulus of '8 when a side was 

 presented to the wind. When one angle was presented to the 

 wind the total pressure was increased by about one-fourth. 



Cylinder. — Two cylinders of different sizes were tested and 

 found to give a modulus as compared with planes of the same 

 height and width of -S'i, or slightly more that one half. The 

 usual rule is in this case fairly accurate. 



Octagonal Prism. — One of these was tested corresponding in 

 size to one of the cylinders, and was found to experience a 

 pressm'e about 10 per cent, greater than its circumscribing cylinder 

 did. This was doubtless due to the disturbance of the current 

 of air by its angles. This result will be of value in connection 

 with towers and chimneys, such as those used by the Melbourne 

 tramways. 



Cone. — A cone having its height about three times its base gave 

 a modulus of "50. 



>S;»Aere.— Modulus = -36. 



Hemispherical (hip. — As used in Robinson's anemometer: — Con- 

 vexity to wind. '36 ; concavity to wind, I'lo. 



Roofs. — The results given in Stoney. p. 524, as to pressure on 

 roofs of various pitches were verified for pitches steeper than 45°. 

 At low angles the results were rather higher than the table, but the 

 experiments were not altogether satisfactory. The roofs were 

 further tested in connection with vertical walls of a height rather 

 greater than that of the roof. These walls tend to deflect the 

 wind upward and relieve the roof of pressure to a very marked 

 degree. With a roof of 60° pitch the pressure was thus reduced 

 40 per cent., and with a roof of 45° pitch 80 per cent., while at 



