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 Loch Ness by members 

 of the Scottish Lake Survey," Trans. R<j. Soc. Edin., vol. 45, p. 407 (1907); "An 

 Experimental Investigation of the Temperature Changes occurriug in Fresh-water Lochs, 

 Pror.. Rm/. fine. Edin., vol. 27, p. 2 (1907). 



