ALA REPORTS ON THE STATE OF SCIENCE, ETC. 
cleaves the air, and constrains the fluid to move in curved paths round 
it: if we could calculate those paths, we should have a sufficient theory. 
of lift and drag. 
The Importance of Viscosity. 
38. Now if air were only frictionless, so that it offered no resistance to 
the sliding of one layer over another, we should have, already, a sufficient 
theory ; for, although the labour is heavy, we do know now how to 
calculate, in these special circumstances, the paths which are taken by the 
fluid. My next illustration (Fig. 2) shows a simple example, the theoretical 
stream-lines (as they are called) for air moving past a long circular 
cylinder. 
Fic. 2. 
Irrotational Flow past Circular Cylinder. 
And air is substantially frictionless, so far as concerns the sliding of 
air on air. Mr. Bryant and Mr. Williams have explored the paths actually 
taken by air in its passage past a very long aerofoil (or wing), and they 
have compared these with the predictions of theory (Fig. 3): except in 
the immediate neighbourhood of the surface of the aerofoil, and in a 
narrow region behind it (termed the ‘ wake ’), the motion is such as could 
occur in a fluid practically devoid of friction. 
Mr. Fage and Mr. Simmons have recently extended this work to the 
case of a short aerofoil (Fig. 4). Again, except in the immediate neigh- 
bourhood of the aerofoil, and in the ‘ wake’, the effects of air friction are 
negligible.” 
8 In Fig. 4, the full lines show the lines of flow in a transverse plane (situated at 
a distance behind the aerofoil equal to one-third of the total span), as these would be 
seen by an observer facing ‘up’ or ‘down wind’. 
