120 THE FRESH- WATER LOCHS OF SCOTLAND 
Stormy weather has the effect of emphasising the discontinuity, 
and also of increasing the depth at which it is to be found. The 
thickness of the uniform surface layer gradually increases as the 
season progresses. The difference in temperature between the upper 
and lower layer diminishes until eventually the discontinuity of 
temperature vanishes altogether. 
Effect of Winds 
The effect of wind in producing currents and in forming the discon- 
tinuity layer has been studied experimentally^ by driving a current 
of air along the surface of the water contained in a long glass trough. 
The apparatus consisted of a glass trough 152 cm. long, 10*5 cm. 
wide, and 12*5 cm. deep, fitted with a parabolic bottom. A con- 
tinuous blast of air could be driven along the trough by means of an 
electrically driven rotary fan. The top of the trough was covered 
over for nearly its whole length, and the trough was as a rule filled to 
within about two inches of the top. The wind-current was directed 
along the channel between the cover of the trough and the surface 
of the water. 
It was not found possible to experiment with water of varying 
temperature. The temperature gradient in a loch (or the rate of 
change of temperature with depth) is small. If the temperature 
gradient in the experimental tank were made the same as in a natural 
basin, very small differences of temperature would require to be 
experimented with. Where the temperature gradient is made large, 
conduction and convection currents become of very much greater 
importance than they are in a natural loch ; and as the depth to 
which the disturbance of surface-waves is felt is relatively much 
greater in an experimental trough than in a natural basin, the 
equalising effect of surface disturbances is also much greater. If the 
gradient in the experimental trough is made comparable to the 
natural gradient, the range of temperature is very small — so small 
that the experiments would not have been possible. 
The temperature changes occurring in lakes are mainly due to 
the difference in density of water at various temperatures, and if in 
experimenting the differences in temperature are very small, the 
differences in density will be too small to make experiments depending 
on these differences practicable. The device adopted was that of 
imitating the differences in temperature by differences in density. 
In this way it is easy to exaggerate the differences in density which 
in a lake are due to temperature, and so to make the experiments 
more manageable, and the effect of conduction is thus eliminated. 
1 See Proc. Roy. Soc. Ediii., vol. xxviii. p. 2, 1907. 
