634 Proceedings of the Royal Society of Edinburgh. [Sess. 
For with different strengths of wind the return current must come to the 
surface at different points. 
Fig. 4 shows the typical direction of isotherms in a thermally stratified 
lake during a moderately strong wind whose direction is shown by the 
arrow, and it is easy to imagine from it that the return current may take 
place very near the surface at the windward end of the lake, and also that 
slight variations in the intensity of the wind, or the rise and fall of the 
temperature seiche, may cause variations, not only in the strength of the 
return current, but also in the depth at which it is found. 
To gain further knowledge of the behaviour of currents when the lake 
was of uniform temperature, arrangements were made for Mr Wm. 
DIRECTION OF WIND 

Macdonald to observe during January and February 1909. The observa- 
tions were not taken with any very definite object in view. Mr Macdonald 
was instructed to make observations at stated depths and intervals. Had 
there been any change of wind during the period he worked, better results 
might have been obtained. As it was, the wind blew persistently from the 
S.W. of all strengths to a gale, and in practically all cases the observations 
show greater velocities in the very deep water than at intermediate depths, 
and persistently in a S.W. direction, with numerous cross-currents. The 
following examples will serve to show how consistently S.W. the currents 
in the deep water were. This must have been an accident, due to the point 
at which the observations were made. The position of the return current 
was not detected at all, and the presumption is that it has been forced up 
into shallow water by the shallowing of the lake, as indicated on page 631. 
