5 
1907-8.] The Temperature Changes in Fresh-Water Lochs. 
water, eventually disappear at the surface. Thus the upper layer of water 
is gradually cooling and increasing in depth until the whole basin again 
reaches the first phase of the cycle and becomes of uniform temperature. 
These are the three typical phases to which, in an experimental investi- 
gation, it is necessary to approximate, and in what follows I wish to explain 
the methods used. 
Apparatus and Methods. 
The apparatus * at my disposal consisted of a glass trough 152 cm. long, 
10*5 cm. wide, and 12 5 cm. deep. This is the same trough as was used by 
Messrs White and Watson for their Seiche experiments.]- Use was also 
made of the parabolic bottom which they employed. A continuous 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 2 inches of the 
top. The wind-current was directed along the channel between the cover 
of the trough and the surface of the water. The arrangement used is 
shown in fig. 3. 
It was not found possible to experiment with water of varying tempera- 
ture. The temperature gradient in a loch (or the rate of change of tem- 
perature with depth) is small. If the temperature gradient in the experi- 
mental tank were made the same as in a natural basin, very small differences 
of temperature would require to be experimented with. Where the tem- 
perature 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 lochs are mainly due to the 
difference in density of water at various temperatures, and if in experiment- 
ing the differences in temperature are very small, the differences in density 
will be too small to make experiments depending on these differences 
practicable. I therefore fell back on the device of imitating the differences 
in temperature by differences in density. In this way it is easy to exag- 
* The apparatus which I used belongs to the Natural Philosophy Department of the 
University, and was put at my disposal by Professor MacGregor. 
f Proc. Roy. Soc. Ed ., vol. xxvi., part iii., p. 142 . 
