DEPTH 
3 
1907-8.] The Temperature Changes in Fresh-Water Lochs. 
ture of whose waters at no time falls below 39° F., the maximum density 
point of fresh water ; and it is with such lochs that I am at present con- 
cerned. Taking Loch Ness as the type of such lochs, the three phases of 
the temperature cycle are : (1) December to April, (2) May to July, 
(3) August to November. During the first phase the water in the loch is 
all of uniform, or nearly uniform, temperature. During the second phase 
there is a gradation of temperature from top to bottom ; but the tempera- 
ture gradient (or the rate of change of temperature with depth) is most 
considerable near the surface, and falls away towards the bottom of the lake. 
FIRST PHASE 
TEMPERATURE 
SECOND PHASE 
THIRD PHASE 
TEMPERATURE 
Fig. 1. 
In the third phase the discontinuity layer has made its appearance, with a 
layer of water above of nearly uniform temperature, and below another 
layer also of nearly uniform temperature, but colder than the upper layer, 
and consequently of greater density. These three phases are illustrated by 
figs. 1 and 2. Fig. 1 represents the three phases by means of the usual 
temperature-depth diagrams. The first phase is represented by a vertical 
straight line ; the second by a smooth curve, showing a rapid fall in 
temperature near the surface, and a more gradual change towards the 
bottom ; while in the third phase the first portion of the curve is 
nearly vertical ; then there is a discontinuity in the curve — a very rapid 
