ATMOSPHERIC FRICTION, WITH SPECIAL REF¬ 
ERENCE TO AERONAUTICS. 
BY 
A. F. Zahm. 
[Read before the Society February 27, 1904.] 
MEASUREMENTS. 
The experiments here described were made to determine 
the magnitude of the friction of air flowing over even sur¬ 
faces, both smooth and rough ones, and the law of its varia¬ 
tion with the speed of flow, the length, and quality of surface. 
The primary purpose of the investigation was to establish a 
basis for calculations in engineering, and particularly in 
aerial navigation; but it is hoped that the measurements 
are sufficiently accurate to be of value also to the general 
dynamics of fluid motion. 
It has long been known to marine science that in a well- 
formed vessel one of the chief elements of resistance is the 
skin-friction of the water on its sides; and, by analogy, it 
was surmised that a fair-shaped body in the air might be 
retarded in a similar way by the tangential drag of that 
fluid. But the measurements of several prominent experi¬ 
menters led them to affirm that the skin-friction of the air 
is negligible, even for bodies of fair outline. The present 
research, however, seems to prove that the frictional resist¬ 
ance is at least as great for air as water, in proportion to 
their densities. In other words, it amounts to a decided 
obstacle in high-speed transportation. In aeronautics it is 
one of the chief elements of resistance, both to hull-shaped 
bodies and to aero-surfaces gliding at efficient angles of 
flight. It seems important, therefore, that the main laws of 
this resistance should be carefully determined. 
36-Bull. Phil. Soc., Wash., Vol. 14. 
(247) 
