246 H. A. Hazen — Wind Velocity and Pressure. 



The value of the constant in seven of the published results 

 ranged from "0039 to "0054:. The mean of the seven agrees 

 very closely with that of Rouse. 



Recent Experiments. — Experiments have been tried in France 

 on the resistance of the air to flat boards, suspended laterally 

 from a train, movable on axles and counter-weighted. The re- 

 sults have been given in the Railroad and Engineering Journal 

 for February, 1887. The train was run at increasing velocities 

 and the instant was noted when the resistance of the air, pre- 

 ponderating, caused the lifting of the weight and reversal of 

 the board. The resistance on a plate a decimeter square, in 

 absolutely calm weather, was found to be '52 kilogram, at a 

 speed of 44*5 miles per hour. The data give : 



£>= -00535 SV a . 



For other velocities the variation is sensibly as the square of 

 the speed ; and within practical limits the extent of the surface 

 does not sensibly affect the result. 



Theoretical formula. — Prof. Unwin in the " Encyclopaedia 

 Britannica," in the article Hydromechanics, at p. 517, gives the 

 following formula for the resistance encountered by a thin 

 plate moving through the air, or through an indefinitely large 

 mass of still water, in a direction normal to its surface 



V 2 



J 2a- 



& 



In this : R= total resistance, expressed in pounds, due to an 

 excess of pressure in front augmented by an increase of pres- 

 sure due to a partial vacuum in the rear of the plate ; /== a 

 coefficient to be determined by experiment ; G = density of the 

 fluid; S= surface of plate in square feet ; Y= relative velocity 

 of fluid and plate in feet per second ; g = force of gravity. 

 For a plate moving in still fluid, /=1*3, and for a current im- 

 pinged upon a fixed plane it is 1*8, whether the fluid be air or 

 water. Prof. Unwin thinks this difference is, perhaps, due to 

 errors in the experiment. Thibault, with plates of 1*17 to 2*50 

 square feet area, exposed to wind pressure, found / to vary 

 from 1*568 to 2*125, the mean being 1*834. Using this value 

 fovf, and making the proper substitution in the above formula, 

 we find by our notation, 



^=•00496 SV 2 . 



It will be noticed that Prof. Unwin's description of Thibault's 

 researches differs slightly from that already given, presumably 

 of the same experiments. The latter were taken from a trans- 

 lation of Morin's mechanics and may be in error. Prof. 

 Unwin finds that all experiments with whirling machines have 

 shown a steady increase of resistance per square foot with the 



