248 
ZAHM. 
To measure the tangential force of the air on even sur¬ 
faces, various skin-friction planes were suspended inside a 
wind-tunnel by means of two fine steel wires attached to the 
top of the laboratory, as shown in figure 1. The tunnel 
itself, standing on the floor of the laboratory, measuring 40 
feet long by 6 feet square, has a 5-foot electric suction fan 
at one end, and a cheese-cloth screen, or two, at the other, to 
straighten the current of inflowing air. A boy with a rheo¬ 
stat and tachometer holds the fan at any desired speed, ac¬ 
curately to a fraction of 1 per cent., thus giving an even 
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C X=. : =: =..=.=.=--zq n 
Fig. 1. —Section of Wind-tunnel with Suspended Skin-friction Plane. 
flow of air of like constancy. As the wind-friction moves the 
plane endwise the displacement is determined by the motion 
of a sharp pointer attached to one suspension wire and travel¬ 
ing over a fine scale lying on top of the tunnel. The swing 
of the plane can be measured accurately to five-thousandths 
of an inch, and the force on the plane is exactly proportional 
to the scale readings. The wind-speed is usually measured 
by a pressure-tube anemometer, though other kinds have 
been employed for comparison. 
In the first attempt to determine the coefficient of skin- 
friction a thick plane was used, having wind-shields fore and 
