ATMOSPHERIC FRICTION. 
255 
The third column gives the whole force on the friction- 
board with its prow, stern, and suspension wires. The fourth 
column gives the force on the latter alone, which, deducted 
from the whole force, gives the friction on the sides of the 
two-foot length. Dividing this net friction by 8.83, the area 
of the true friction surface, gives the values in the last 
column. 
Similar tables were obtained for the other friction-boat 4 s, 
of lengths 4, 8,12, and 16 feet respectively. When the values 
from the five tables are plotted on logarithmic cross-section 
paper they give five separate straight lines, all having the 
same inclination as the one shown in figure 3, in which the 
slope tangent is 1.85. This means that, for all the velocities 
and lengths of surface employed in this research, the skin- 
friction is expressed by an equation of the form 
F=av u85 .0), 
a being a numerical constant, and v the wind speed. Hence 
if the net friction on each board is known for any velocity, it 
can readily be computed for any other velocity. 
In practice the computations illustrated in tables I and 
II were obviated, for all the tables, by a simple expedient. 
The observed anemometer readings and swing of the plane 
were plotted while the measurements were in progress, 
giving five straight lines, all of the same slope. Then a 
point was selected on each line representing a wind speed of 
ten feet per second, and the corresponding friction per square 
foot of surface noted. From these values the numerical 
equations between F and v can at once be written. The ob¬ 
served values are given in the subjoined table. 
Table III. 
Skin Friction at 10 Feet per Second for Various Lengths of Surface . 
Length of friction board. 
2 
4 
8 
12 
16 
Average friction,pounds 
per square foot... 
0.000524 
0.000500 
0.000475 
0.000467 
0.000457 
37—Bull. Phil. Soc., Wash., Vol. 14. 
