VI. Atmospheric Friction on Even Surfaces. 
By A. ¥. Zaum, Ph.D* 
TUXHE aim of the experiments here described is to determine 
_ the friction of free air flowing swiftly over even surfaces, 
and the law of its variation with the s speed of flow, the length 
and quality of surface. The work differs from that ‘of Maxwell 
and others on viscosity, in some essential features. The air 
flows in uniform current past a thin plane at speeds ranging 
up to forty feet a second. The primary purpose of the 
research was to establish a basis for calculations in aero- 
nautics ; but it is hoped that the measurements are sufficiently 
accurate to be of interest also in the general dynamics of 
fluid motion. 
It is well known, from the investigations of Froude and 
others, that the resistance of a fair-shaped vessel in water is 
due largely to surface friction; and, by analogy, it was 
surmised that in the atmosphere a like relation should obtain. 
But the measurements of several prominent experimenters led 
them to affirm that the tangential resistance of air is negligible, 
even for bodies of fair outline ; while the present research 
tends to prove that such friction is fully as great for air as 
for water, in proportion to their densities. It therefore seems 
important that the main laws of this resistance should be 
carefully determined. 
To measure the tangential force of the air on even surfaces, 
various skin-friction planes were suspended inside a wind 
tunnel, by means of two fine steel wires attached to the top 
Fig. 1.—Friction Plane suspended in Wind-tunnel. 
of the laboratory, as shown in fig. 1. The tunnel itself, 
standing on the ‘floor of the laboratory, measures 40 Peat 
* Communicated by Lord Rayleigh. 
