Friction on Even Surfaces. 61 
the net friction on each board is known for any one velocity, 
it can readily be computed for any other velocity. 
Fig. 2.—Relation between Wind-speed and Friction. 
#0009 
PER SQUARE 
7000368 
G 
ue sbelel 
“00932 
5 i is 
9 22 
In practice the computations illustrated in Table I. 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 
chosen on each line representing a wind-speed of 10 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 observed values 
are given in the subjoined table :— 
TABLE I. 
Skin-friction at 10 ft. per sec. for various Lengths of Surface.. 
Length of 9 . p f 
Friction Board. a7 +. o> 12. LG: 
Average Friction, ANNK9: i 3 4 Sane 
Ee ek ho 000524  0-000500 0-000475  0:000467 0-000457 
Knowing, then, the friction, at the same speed, on planes 
of five different lengths, there remained to determine the 
