ATMOSPHERIC FRICTION. 
251 
Table I. 
Skin-friction on Plane Measuring 16' X 4' X 4" - 
Speed of fan. 
Swing of plane. 
Force causing 
swing. 
Differential 
pressure in 
shields. 
End-thrust on 
plane. 
Pressure - tube 
anemometer. 
Wind speed. 
! Friction per 
square foot. 
Rev. min. 
In. 
Lbs. 
Mg. sq. cm. 
Lbs. 
Mg. sq. cm. 
Ft. sec. 
Lbs. 
150 
0.27 
0.080 
0.0 
0.0 
70 
11.11 
.000579 
200 
0.41 
0.121 
0.0 
0.0 
105 
13.63 
.000875 
250 
0.54 
0.160 
0.0 
0.0 
155 
16.16 
.001156 
300 
0.76 
0.225 
1.0 
0.003 
225 
19.46 
.00165 
350 
0.95 
0.277 
1.7 
0.005 
295 
22.30 
.00203 
400 
1.19 
0.352 
3.9 
0.011 
375 
25.14 
.00262 
450 
1.45 
0.428 
7.1 
0.019 
465 
28.0 
.00324 
500 
1.74 
0.515 
9.5 
0.026 
570 
31.0 
.00392 
550 
2.04 
0.603 
13.7 
0.037 
670 
33.6 
.00463 
600 
2.39 
0.701 
16.5 
0.045 
815 
37.0 
.00539 
The force in the third column is computed from the ob¬ 
served swing of the plane. Adding the end-thrust, since 
the differential pressure opposed the deflection of the plane, 
there results the actual skin-friction on the exposed surface. 
Dividing by the area of the surface gives the values recorded 
in the last column. The wind speed is computed from the 
pressure-tube readings by a theoretical formula, which has 
been carefully verified by a special series of experiments 
which were published in the Physical Review , December, 1903. 
The values of the wind velocity and skin-friction have been 
plotted on logarithmic cross-section paper, as shown in figure 
3. Their relation in this, as in subsequent experiments, is 
invariably expressed by a straight line—that is, by the rela¬ 
tion, 
F av n . . . («) 
in which F is the total friction, v the wind speed, a, n, nu¬ 
merical constants. The concrete relation obtained from the 
numerical values of table I is, for a plane 16 feet long, 
f = 0.00000671 ?; 1,85 . (v = ft. sec.), 
/ = 0.00001363 v 1,85 ....... (v = mi. hr.), 
