60 » Dr. A. F. Zahm on Atmospheric 
Table 1., 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 air, in crder to obviate wabbling 
in so small a board. The velocity was thus reduced, it is 
true, but sufficient values are given to make a reliable 
diagram. With the sixteen-fceot plane speeds up to forty 
feet a second were employed, with like results. 
The following data may be prefaced :—surface of the two- 
foot plane without end-pieces, 8°83 square feet ; weight of 
plane and ends 17 pounds ; one inch swing of plane =0°0862 
pounds wind-force; barometric pressure 29°80 inches ; mean 
temperature of experiment 24°°2 C. 
Tape ft. 
Surface Friction by 24 in. x 25°5 in. Pine Board with sharp ends. 
| 
= = 5 Favs 
Speed | Swing Force _ Force Pressure- | wing Friction 
Net 
ea) Ge causing OW | sao Ae per 
Fan. Plane. | Swing. Ends. | a prte Anem. | Ss a Sq. Ft. 
————_ | | -|—— peice —-——= ——— 
rey. min.| in. lbs. lbs. | lbs. | gm.sq.cem.| ft. see. | ibs. | 
200 |; 050 | 0:00431 , 0:00233 0:00198 22°8 6°38 (0:000224 | 
250 0-080 | 0:00690  9:00365 | 0:00325 370 812 0°000368 
300 | 0120 | 0:01034 000553 0-:00481 58-0 10°18 (0-000545 
350 | 0158 | 0:01362) 0:00734| 000628; 78°5 11:80 |0:000710 
400 | 0-205 | 0:01767  0:00949 | 000818 1035-1352 0000925 
450 | 0-260 
0:02240 | 0:01188! 0:01052, 1320 15°34 {0001188 | 
500 | 0-310 | 002586 0:01379| 001207, 1550 | 16°61 o-onnes 
The third column gives the whole force on the friction- 
board with its ends and suspension-wires ; the fourth column 
gives the force on the ends and wires 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 boards, 
of lengths 4, 8, 12, 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 fig. 2 (plotted from 
Table I.), 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 : 
Wi wat, isl. 6 + ve 
a being a numerical constant, v the wind speed. Hence if 
