ATMOSPHERIC FRICTION. 
261 
varnished wooden surfaces in air and water, but appears to 
be wholly untrue for calico surfaces. In default, therefore, 
of an adequate physical theory of surface friction the mag¬ 
nitude in any given case can be determined only by direct 
experiment. 
To complete the theory of the skin-friction board, two steps 
further remain to be taken. First, the equations of motion 
for a viscous fluid must be integrated to find the velocity at 
all points in the disturbed region about a thin material 
plane. Then the speed of flow must be measured at all 
points next the plane and at some distance away. The 
writer expects soon to map the stream-lines and measure the 
velocity. If, then, the equations can be integrated so as to 
give the speed as a function of the space coordinates, the 
computed and observed values can be directly compared. 
It is hoped that some one may obtain sufficiently general 
solutions of the equations to be of practical value, particu¬ 
larly for the simpler case in which the plane is indefinitely 
wide and in which the edge conditions are negligible. The 
integrals, if sufficiently general, will be of great importance 
to the science of surface friction, and may at once be applied 
to the mass of accurate data that for a generation has been 
accumulating in the laboratories of marine engineers. 
APPLICATIONS. 
The laws of skin-friction have both theoretical and prac¬ 
tical application. They serve theory by explaining some 
apparently anomalous phenomena and by leading to more 
complete formulse for total resistance. They are of practical 
use because in many of the forms employed in transporta¬ 
tion the skin-friction is a large part of the total resistance. 
It has been the common practice of writers on air resist¬ 
ance to employ the Newtonian formula, 
R = av 2 , 
in which a is regarded as constant for a surface of fixed form 
and aspect; but this equation is far from true (1) for blunt 
