1086 
ern RR ret 
Fig.‘ 1. 
Fin ——— 
formed in front of the body and leaving it at S to enter the fluid, is very thin, 
so that it represents approximately a surface of discontinuity for the distribution 
of the velocity. This sheet can be seen on photo’s of the flow in the immediate 
neighbourhood of the body. Sometimes the stream lines are bent so sharply that 
we may speak of a discontinuity. 
Thus a vortex layer leaves the body at both sides; as will be known these 
layers (at least for the two-dimensional flow) have the property to cur! themselves 
in the way of spirals to a row of vortex threads. The vortex threads formed from 
both layers are placed more or less regularly (von Karman, Phys. Zeitschrift 18 
p. 49, 1912 and others). 
This may be demonstrated by coloured fluid flowing from holes at the front 
surface of the body. This fluid comes into the boundary or vortex layer, partici- 
pates its motion and in this way makes its form manifest. Especially fine cinema- 
