258 
ZAHM. 
studied by Maxwell, the conditions are different. He found 
that when one plane moved edgewise near and parallel to 
another plane, at a constant speed below one-twelfth of an 
inch per second, the friction is independent of the pressure 
and proportional to the absolute temperature for such at¬ 
mospheric conditions as prevail near the earth’s surface. 
Some measurements were made with the four-foot friction- 
board covered with various materials to observe the effect of 
quality of surface upon the tangential resistance. Practi¬ 
cally the same friction was observed, whether the board was 
covered with dry varnish, or wet, sticky varnish, or sprinkled 
with water, or covered with calendered or uncalendered 
paper, or glazed cambric, or sheet zinc, or old English 
draughting paper, which feels rough to the touch. But when 
the plane was covered with coarse buckram, having sixteen 
meshes to the inch, the friction at ten feet a second was 10 
to 15 per cent, greater than for the uncovered surface, and 
the friction increased as the velocity raised to the power 2.05, 
or approximately as the square of the speed. 
The fact that such a variety of materials exhibit practi¬ 
cally the same friction seems to indicate that this is a shear¬ 
ing force between the swiftly gliding air and the compara¬ 
tively stationary film adhering to the surface, or embedded 
in its pores. If, as seems to be true, there is much slipping, 
this means that the internal resistance of the air is less at 
the surface than at a sensible distance away. As the shear¬ 
ing strength of a gas is due to the interlacing of its mole¬ 
cules, owing to their rapid motion normal to the shearing 
plane, it may be that the diminution of shear near a boundary 
surface is due to the dampening, within the film, of the com¬ 
ponent of molecular translation normal to the surface. 
To summarize the results attained thus far, it may be said 
that, within the ascribed limits of size and wind speed 
(1.) The total resistance of all bodies of fixed size, shape 
and aspect is expressed by an equation of the form 
