26 NOTES ON FLYING-MACHINES. 



These observations were plotted (Fig. 9) and tabulated, after 

 which computations were made from the formula P = *0023 

 V 2 x sin X, relating to finding the force of wind impinging on a flat 

 surface where P = pressure in pounds per square foot. V = 

 velocity of wind in feet per second. X = angle of incidence of 

 direction of the wind with the plane of the surface. The results 

 were plotted on the same diagram (Fig. 9) in dash and cross curves, 

 so that a comparison can be made at a glance between the theoretical 

 curves and the actual observations. 



It will be remarked how closely the observations at 5° and 

 10° agree with the theoretical curves and diverge at 15°, 20°, and 

 25°. Instrumental errors would be most likely to show at the 

 higher speeds and fine angles, where the difficulty of measuring 

 small quantities is greatest. Centrifugal force is not answerable 

 for the non-agreement, that would, if it was appreciable at all, 

 tend to bring the observations closer to the computed curves, as. 

 the movable arm carrying the plane was above and behind the 

 radius of the whirling machine when recording the higher 

 pressures, all the other parts being carefully counterpoised. 



The difference is therefore understood to show graphically that 

 the actual resistance of the air is greater than is given by theory, 

 (see Brande and Cox : Die. Science, Literature, and Art, article- 

 on Parachute) and that when the resistances are resolved, the 

 horizontal resistance is less than the theoretical resistance, and 

 the observed lift is greater than the theoretical lift. 



It may be interesting to state the view taken of the possibility 

 of a man driving a machine similar to Figs. 1, 2, 3, fast enough to 

 lift his own weight, viz,, that a powerful athlete exerting a force of 

 25,000 foot-pounds per minute, could create for a short time with a 

 machine similar to Figs. 1, 2, 3, a thrust of 23 pounds or 6,750 

 foot-pounds per minute ; and that if the total area of the planes, 

 was 400 square-feet, and the weight of the machine and man was 

 225 pounds, the 23 pounds thrust would theoretically have to 

 drive the planes at 5° angle with the horizon, at a speed of 122 

 miles per hour in order that the machine and man might be 

 air-supported. 



Supposing the plane to be perfect, a thrust of 19*6 pounds 

 would be absorbed in driving it alone, without counting the 

 resistance of the body of the experimenter and the framework of 

 the carriage, which at a speed of 122 miles per hour would be 

 theoretically 73 pounds per square-foot. 



Discussion. 

 Professor Threlfall said : — With reference to the relation between 

 speed and resistance in the wings that are moving through the airr 

 hydro-dynamical considerations, in which the viscosity of the air 



