Notes on Air Resistance and Pressure. 



BY E. C. MURPHY. 



(With Plate II). 



The following experiments were made by the writer in the ma- 

 chine shop of this University for the purpose of determining the 

 relative resistance and pressure of air on surfaces of different de- 

 grees of curvature. The object in view was to determine the 

 advantage gained in the use of curved instead of plane fans or 

 slats in windmills, and more especially that for the jumbo mill 

 which is now being used to some extent in Western Kansas. Many 

 experiments have been made to determine the relation between 

 pressure and velocity in plane surfaces. The results do not agree, 

 though the results of recent experiments agree more nearly than 

 those of earlier date. Very few experiments have been made to 

 determine the relation between velocity and pressure for curved 

 surfaces, and the results differ widely. These differences are prob- 

 ably due largely to the method of measuring the velocity of the 

 wind. The velocity as found by the cup anemometer may be in 

 error 20 per cent, or over for velocities of 30 miles or more per 

 hour; and small pressure plate anemometers may be in error 40 

 per cent, or more. (See Eng. JVcius, Vol. 33, p. 183.) 



Space will not permit us to speak even briefly of the results of 

 other experiments. Captain W. H. Bixby, in his report on "Max- 

 imum Spans Practicable for Suspension Bridges," reprinted in 

 Eng. N'c^os, Vol. 23, gives a brief review of the history of past in- 

 vestigations in this subject. The work of 65 experimenters and 

 writers is reviewed. A series of experiments not mentioned in 

 Captain Bixby's report, made by J. Irminger, C. E., are worthy of 

 special mention, in that they measure not only the pressure on the 

 windward side of surface, but also the section on the leeward side. 



Figures i and 2 show the apparatus for judging the resistance. 

 Fig. I shows a part of a lathe, the belt A passes over the pulleys 

 A, B, C and D, thus giving to the pully K, which drives the fans 

 four velocities. The belt K passes over the pulley L in Fig. 2. 

 The power is transmitted from the face plate E to the pulley K 

 through the spring balance I, thus the number of pounds pull 

 required to run the fans at any speed is seen. 



({i3) KAN. UNIV. yUAR., VOL. IV., NO. 1, JULY 1, 189.5. 



