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Part of this impulse can be received back when the motion of 
the body is retarded; viz. the part given by classical hydrodynamics, 
for which may be put: 
(“apparent mass”). (velocity of the body). 
Of the rest a small part can be received back; the greater part, 
however, is lost. *) 
When we have to do with an tdeal fluid (absolutely without 
friction) these considerations need not be changed, when only we 
say that the vortices always remain in an infinitely thin layer at 
the surface of the body. They do not diffuse and are not washed away. 
The impulse therefore is always seated in this layer and has the 
value: 
(“apparent mass’’). (velocity of the body); 
this amount can be totally received back when the motion of the 
body is retarded. 
In order to obtain an “irreversible” resistance viz. to give an 
impulse to the fluid that cannot be received back, the vortex motion 
must come outside this layer, there must be diffusion of the vorticity, 
be it to a low degree. 
1) O. ReynoLps mentions the following simple experiment (Scientific Papers I, 
p. 188), which may be repeated easily: a body moving in a fluid is suddenly 
slopped; when directly afterwards it is released, it proceeds still a short distance 
in its original direction. The motion in the fluid present after the stopping has 
therefore still exerted a force on the body in the direction of the motion and has 
given back impulse to the body. 
