ATMOSPHERIC FRICTION. 
253 
in which f is the average friction in pounds per square foot 
of surface, and v is the wind velocity in the units indicated 
within the parentheses. This relation was corroborated by 
later experiments in which no wind-shields were used. 
Having fairly established the law of variation of the skin 
friction with the air velocity, an effort was made to discover 
its variation with the length of surface. A simpler method 
was then adopted which had been considered, but was dis¬ 
carded in the beginning as appearing hardty delicate enough 
to measure such extremely small forces as the friction was at 
first conceived to be. 
Fig. 4. — Skin-friction Plane with Sharp Ends, Suspended in Wind-tunnel. 
Planes were now constructed similar to those commonly 
used to determine the ^kin-friction of water. The first 
was a pine board 4 feet long, 25.5 inches wide, and 1 inch 
thick, carefully trued and varnished, and suspended in 
the wind tunnel, as usual, by steel wires 0.025 of an inch 
in diameter. It was provided with a 7-inch pine prow 
and stern, both of ogival form. These were held on by 
dowel pins, as shown in figure 4, and each terminated in a 
sharp edge, from the center of which a steel pin protruded 
along stream between guides to steady the plain against 
wabbling. As the doweling was carefully executed, straight 
