420 ON THE TEMPERATURE OF THE FRESH- WATER LOCHS OF SCOTLAND. 



most effect when the water is warm. If in early autumn there is a cold spell of weather 

 with little sunshine, the quantity of heat in the loch in the succeeding winter may be 

 much less than in another year with a much colder winter. The point to which the 

 loch is cooled down will of course vary slightly from year to year.* 



In early spring, the temperature is very nearly the same throughout the loch, and 

 the temperature at this time will approximately determine the abysmal temperatures in 

 the succeeding year, but even the lowest layers of Loch Ness have a range of about two 

 degrees in any one year. I have made a rough calculation of the amount of heat which 

 enters the loch during the year, by assuming that in the coldest time of year the loch 

 has a uniform temperature of 41*2° F., and arriving at the maximum amount of heat in 

 the loch from the detailed temperature observations made in September 1904. This 

 calculation gives the quantity of heat which enters the loch as approximately 1/9 x 10 1G 

 gram calories for the whole loch. Dr Knott t states that the total quantity of heat 

 supplied is about 7 "2 x 10 16 gram calories, or about four times the amount of heat which 

 according to the above calculation actually enters the loch ; and this seems a very 

 reasonable proportion for the amount of heat supplied to bear to the amount of heat 

 stored up in the loch. 



V. Temperature Seiche. 



I have in a cursory manner discussed the temperature changes which take place 

 throughout the year. I now wish to direct attention to some of the phenomena which 

 accompany these changes. The most important of these which calls for 'consideration 

 is what I may call the temperature seiche. The existence of a temperature seiche was 

 not suspected when the observations were begun at Fort Augustus. It very soon 

 became apparent, however, from the numerous observations which were made that 

 movements of great magnitude were in progress, and for a long time the reason of these 

 changes was sought in vain. At first it was tried to explain the changes by simple 

 currents and then by the influence of rivers, but it was impossible to get a workable 

 theory based on any such considerations. Gradually it came to be recognised that the 

 changes were consequent on the unstable conditions set up by the action of the wind in 

 piling up warm water at one end of the loch, and the observers were able with some 

 degree of accuracy to predict the changes which would follow a storm or a change 

 of wind ; but it was some time before the periodic nature of these changes was 

 fully understood. Mr E. R. Watson, in a paper published in the Geographical 

 Journal for October 1904, his results having been previously communicated to 

 this Society, sought to give an explanation of the observations, and though many 

 limnologists have expressed themselves sceptical of the soundness of his deductions, 

 no other explanation has been put forward which can seriously be called an explana- 

 tion. Put briefly, the theory is this. In autumn, when the Sprungschicht is in 



* The waters of Loch Ness have never heen known to freeze save round the shores, as the water is never cooled 

 down to the maximum density point. 



t Proc. Roy. Hoc. Edin., 1900-1, vol. xxiii. p. 296. 



