418 
BATHYMETRICAL SURVEY OF 
to the Royal Society of Edinburgh.* The results arrived at are briefly 
as follows : — 
The yearly cycle of changes in a loch such as Loch Ness is very much 
the same from year to year. Fig. 69 shows graphically what may be 
called the typical curves for each month of the year, being drawn from 
the monthly means of the temperature readings during the period of the 
observations. From these curves it appears that in September there is 
the greatest quantity of heat in the loch. Thereafter the loch cools 
gradually till March or April, when the water again begins to gain heat. 
This is the time when the mean air temperature begins to be higher than 
the surface temperature. From May till August the increase in tem- 
perature at various depths proceeds regularly, and the typical curves 
representing the temperature of the water to a depth of 200 feet are 
practically straight lines. Below that depth it is probable that the 
temperature increases less rapidly in proportion to the depth, but 
even in the deepest waters of Loch Ness there is a range in temperature 
of about 2° Fahr. The lowest recorded temperatures in the deepest 
parts of the loch are in April, and the highest in the middle of 
November. During the period in which the loch gains heat, the 
most remarkable changes are those taking place at and near the surface. 
Rapid changes are of frequent occurrence, and are probably due to 
convection currents. On one occasion the temperature at the point of 
observation rose 6° Fahr. in two minutes. 
When the mean air-temperature falls below the surface temperature, 
which is usually in August, the loch begins to part with its heat. This 
is shown in the change of type in the typical curve for September. The 
surface layers lose heat, while lower down the water still continues to 
rise in temperature ; as already mentioned, the highest temperature at 
700 feet was observed in November, or about three months after the 
loch began to lose heat. In August the discontinuity between the upper 
and lower layers of the loch usually becomes well marked. As the upper 
layers of water become colder, there is a layer at the surface of nearly 
uniform temperature, and of gradually increasing depth. Below this 
layer there is a sudden change of temperature — a discontinuity layer — 
below which there is the colder water in the loch. As the season 
advances this discontinuity layer gradually sinks lower, and the layer of 
uniform temperature above it increases in depth, until finally the whole 
loch is of nearly uniform temperature. 
Before the discontinuity layer makes its appearance, the currents 
produced by winds are distributed through the whole loch. There is 
* “ The Temperature of the Fresh- water Lochs of Scotland, with special reference to 
Loch Ness, with an appendix containing observations made in Ijoch Ness by members 
of the Scottish Lake Survey,” Tram. Boy. Soc. Edhi..^ vol. 45, p. 407 (1907); “An 
Experimental Investigation of the Temperature Changes occurring in Fresh-water Lochs, 
Ptoc. Boy. Soc. Edin., vol. 27, p. 2 (1907). 
