ON THE TEMPERATURE OE THE FRESH- WATER LOCHS OF SCOTLAND. 419 



at Dores, which is at the N.E. end of the loch, and at Inverfarigaig, which is near the 

 centre of the loch. About 16th September the wind changed from N.E. to S.W. This 

 was followed by an increase in temperature at 100 feet, and by a relatively much 

 greater change at 150 feet. This I attribute to the fact that the return current takes 

 place along the surface of separation at the Sprung schicht, gradually losing itself 

 among the surrounding layers. At this time of year the Sprung schicht is nearly at 

 150 feet, which explains why the increase should be relatively so much greater at that 

 depth than at 100 feet. There is hardly any appreciable increase in temperature at 200 

 feet. At Inverfarigaig there is nothing in the curves which would lead one to expect 

 that there had been a change of wind. At Fort Augustus the change affects practically 

 only the surface layers above the Sprung schicht, and these layers fall greatly in 

 temperature. This is probably because the water which has been blown along the surface 

 is replaced by cold water rising from the neighbourhood of the Sprung schicht. At 

 Invermoriston, five miles from Fort Augustus, there seemed also to be a corresponding 

 fall in temperature through the first 100 feet, as though the influence of the return 

 current was felt thus far. Thereafter it would seem to lose itself entirely, and indeed 

 it is not possible to assert that it is felt to this distance. Mr Watson * states that 

 the amount of water at any given temperature is nearly constant during all the changes 

 which occur, and he arrived at this conclusion by measuring the areas between isotherms 

 drawn in a diagram such as is shown in fig. 6. I think that Mr Watson's observations 

 were not sufficiently complete to warrant this statement, and I can only say that with the 

 more detailed observations at my disposal I have not been able to verify this statement. 

 He is also of the opinion that in stormy weather the loch loses heat much more rapidly 

 than in calm weather, and as to this also I have not been able to satisfy myself. 



The next step in the temperature changes which occur is that the Sprungschicht 

 gradually descends. Above the Sprungschicht there is always the layer of water at 

 nearly uniform temperature, but the temperature of this layer is always decreasing. 

 Below the Sprungschicht there is a gradually decreasing layer of water the temperature 

 of which is gradually increasing both by convection and by conduction. So the 

 Sprungschicht gradually sinks until finally it reaches the bottom of the loch, and at 

 last the loch is of practically uniform temperature throughout. 



IV. Quantity of Heat in the Loch. 



The yearly cycle of changes is of course very largely dependent on the meteorological 

 conditions of the year. A cold spring will mean that the time at which the loch begins 

 to gain heat will be delayed. A warm spring will mean that the water will heat up 

 rapidly, as it is in spring that the water is coldest. In a very warm spring we may 

 expect that the bottom temperatures will be considerably higher in the year which 

 follows than if the heating had taken place gradually. Also extremes of cold will have 



* Geographical Journal, October 1904. 



