ON THE TEMPERATURE OF THE FRESH-WATER LOCHS OF SCOTLAND. 411 



Augustus, and they continued the observations till the end of April 1905, when the 

 station was dismantled. 



The detailed thermometrical survey was made, as already mentioned, from the 

 Rhoda, and was only carried on down to a depth of 200 feet, primarily because of 

 the difficulties in the way of observing regularly at greater depths ; but it appeared, 

 from observations made at other times, that the behaviour of the water at greater 

 depths than 200 feet could always be explained by reference to the movements pro- 

 ceeding at lesser depths. One difficulty which threw doubt on the observations arose 

 from the fact that the observation station was in close proximity to two of the largest 

 rivers which enter the loch — the Oich and the Tarff. Very careful observations were 

 made to determine how far the influence of these rivers made itself felt, and, with 

 certain very interesting exceptions, which will be referred to afterwards,* it was found 

 that the effect of these rivers was negligible at the Rhoda. 



II. Yearly Changes. 



The problem which confronts anyone who wishes to study the temperature of a 

 loch is, broadly, — How does the loch gain or lose heat, and what phenomena present 

 themselves during the cycles of changes which take place? What is the effect of 

 wind and sunshine and other factors at different periods of the year ? As a first step 

 towards answering these questions I have prepared a table t showing the monthly means 

 of temperatures at Fort Augustus, at different depths down to 200 feet, for the whole 

 period during which observations were made. Where more than one series of observa- 

 tions had been taken in one day the first of these series alone was used in computing 

 the mean temperature, so as to avoid giving too great weight to the observations of 

 any particular day. 



* See page 427. 



t The mean Air Temperature in these tables is taken from the Kegistrar-General's Reports. 



[Table. 



