254 
ZAHM. 
planes of any length could be made by adding extra boards, 
the lengths most employed being 2, 4, 8, 12, and 16 feet. 
The method of using the planes to determine the surface 
friction was as follows: The total force was measured, at 
various velocities, using the prow and stern first on the 16-foot 
board, then successively on the 12,8, 4, and 2 foot boards, and 
finally with nothing between them. Subtracting this last 
force from each of the others gave the friction on those five 
lengths. It may not be absolutely true that the end resist¬ 
ance was the same for each of those lengths, but the error 
of this assumption is regarded as very slight for several 
reasons: (1) the end resistance is but a small part of the 
total; (2) the stream lines are so slightly disturbed that the 
flow about the ends must be practically the same in all 
cases; (3) the results harmonize very well with those ob¬ 
tained by other methods. 
Table II, taken from the laboratory note-book, exhibits 
the observed and computed values for the two-foot friction- 
board. The mouth of the tunnel was screened with cheese¬ 
cloth to steady the flow of the air, in order to obviate wab¬ 
bling in so small a board. The velocity was thus reduced, 
it is true, but sufficient values are given to make a reliable 
diagram. 
The following data may be prefaced : surface of the two- 
foot plane without prow and stern, 8.83 square feet; weight 
of plane with end pieces, 17 pounds; 1 inch swing of 
plane = 0.0862 pounds wind force; barometric pressure, 
29.80 inches; mean temperature of experiment, 24°.2 C. 
Table II. 
Surface Friction by 24" X 25.5" Pine Board with Prow and Stern. 
Speed 
of 
fan. 
Swing of 
plane. 
Force 
causing 
swing. 
Force on 
prows. 
Net 
friction. 
Pressure 
tube 
anem. 
Wind 
speed. 
Friction 
per 
sq. ft. 
Rev. min . 
In. 
Lbs. 
Lbs. 
Lbs . 
Mg. sq. cm. 
Ft. sec. 
Lbs. • 
200 
0.050 
0.00431 
0.00233 
0.00198 
22.8 
6.38 
0.000224 
250 
0.080 
0.00690 
0.00365 
0.00325 
37.0 
8.12 
0.000368 
800 
0.120 
0.01<»34 
0.00553 
0.00481 
58.0 
10.18 
0.000545 
350 
0.158 
0.01362 
0.00734 
0.00628 
78.5 
11.80 
0.000710 
400 
0.205 
0.01767 
0.00949 
0.00818 
103.5 
13.52 
0.000925 
450 
0.260 
0.02240 
0.01188 
0.01052 
132.0 
15.34 
0.001188 
500 
0.310 
0.02586 
0.01379 
0.01207 
155.0 
16.61 
0.001366 
