ATMOSPHERIC FRICTION. 
257 
practice, the following table has been derived from the equa¬ 
tion f = 0.0000158 l 0,07 v 1-85 . The friction for any inter¬ 
mediate velocity, or length of surface, may be found by in¬ 
terpolation. If the surface is of variable length it may be 
divided into longitudinal strips, the force on each strip being 
the product of the area of the strip multiplied by the average 
friction for its particular length. Only the values in heavy 
type lie within the range of the experiments above described. 
Table IV. 
Friction per Square Foot for Various Speeds and Lengths of Surface. 
T3 
CD 
<D 
Pu 
OQ 
Average friction in pounds per square foot. 
S 
S 
V plane. 
2 / plane. 
V plane. 
8' plane. 
16 / plane. 
32 7 plane. 
mi. hr. 
5 
0.000303 
0.000289 
O 000275 
0.000262 
0.000250 
0.000238 
10 
0.00112 
0.00105 
O OOlOl 
0.000967 
O 000922 
0.000878 
15 
0.00237 
0.00226 
O 00215 
0.00205 
O 00195 
0.00186 
20 
0.00402 
0.00384 
0.00365 
0.00349 
0.00332 
0.00317 
25 
0.00606 
0.00579 
0.00551 
O 00527 
0.00501 
0.00478 
30 
0.00850 
0.00810 
0.00772 
0.00736 
0.00701 
0.00668 
35 
0.01130 
0.0108 
0.0103 
0.0098 
0.00932 
0.00888 
40 
0.0145 
0.0138 
0.0132 
0.0125 
0.0125 
0.0114 
50 
0.0219 
0.0209 
0.0199 
0.0190 
0.0181 
0.0172 
60 
0.0307 
0.0293 
0.0279 
0.0265 
0.0253 
0.0242 
70 
0.0407 
0.0390 
0.0370 
0.0353 
0.0337 
0.0321 
80 
0.0522 
0 0500 
0.0474 
0.0452 
0.0431 
0.0411 
90 
0.0650 
0.0621 
0.0590 
0.0563 
0.0536 
0.0511 
100 
0.0792 
0.0755 
0.0719 
0.0685 
0.0652 
0.0622 
It may now be inquired what other circumstances alter 
the surface friction. Perhaps the chief of these are the atmos¬ 
pheric changes of density and the unevenness of surface. 
No effort was made to determine the relation between the 
density and skin-friction of the air, partly for want of time, 
partly because, with the apparatus in hand, too great changes 
of density would be needed to reveal such relation accu¬ 
rately. Doubtless the friction increases with the density, 
since it is due to the inertia of the fluid near the friction sur¬ 
face. Of course, in steady motion at low velocity, such as 
