72 BATHYMETRICAL SURVEY OF 



(53 at 4 p.m.), the thermometer in 3 feet of water then reading 52-9, 

 while that in 1 foot of water read 53*3. The temperature recorded in 



1 foot of water was lower than that recorded in 3 feet of water in the 

 forenoon and late evening, but at noon and 1 p.m. it was higher, at 



2 p.m. and 3 p.m. it was identical, and from 4 p.m. till 8 p.m. it was 

 higher, the greatest difference recorded being 1 0> 3 at 9 p.m. (52-8 at 



3 feet, and 51'5 at 1 foot). 



The temperatures taken beneath the surface have been collected 

 together and arranged chronologically in the foregoing table, which may 

 be useful for future reference and comparison. The great majority of 

 them were taken in the small 80 -feet basin towards the west end of the 

 loch, while one series was taken near the east end on April 3, and three 

 series were taken towards the middle of the loch, opposite Craiganour, 

 on May 1, 2, and 23. The table shows well the heating up of the 

 water with the advance of summer. The readings taken near the west 

 end in March are all below 39 Fahr. that is, below the maximum 

 density point, though surface temperatures exceeding 39, and in one or 

 two cases exceeding 40, were recorded near the shore during the last 

 days of March. On April 2 and subsequently, the temperature was 

 above that of maximum density (39'l), but the observations taken near 

 the east end 011 April 3 showed that the temperature of the water from 

 surface to bottom was just below maximum density point. The water 

 in the small western basin had a temperature under 40 up to April 10, 

 and was practically uniform from top to bottom, but on April 21 and 

 subsequently the temperature rose, and there was a considerable range 

 between the temperature of the upper and lower layers. The water in 

 the main basin had a temperature of 40 at 200 and 300 feet on May 1 ; 

 on May 2 the temperature was 40-2 at 200 feet; and on May 23 it 

 was 41-9 at 100 feet. By the beginning of June the water near the 

 surface had attained a temperature of 50, and by June 21 that 

 temperature extended down to 50 feet, the upper 10 feet having on 

 that date a temperature of 52. 



Loch Lyon (see Plate XXI.). Loch Lyon lies at a high elevation 

 at the head of Glen Lyon, amid grand and mountainous scenery, its 

 waters being carried by the river Lyon into the river Tay a short 

 distance above the mouth of Loch Tay; it contains both salmon and 

 trout. It trends in a north-east and south-west direction, and is 

 extremely simple both in outline and in the conformation of the bottom. 

 It is of nearly uniform width, except for a cone of alluvium, brought 

 down by the river, on the south-eastern shore. It is about 1| miles in 

 length, with a maximum breadth of over a quarter of a mile, the mean 

 breadth being over one-fifth of a mile, or 12 per cent, of the length. 

 Its waters cover an area of about 236 acres, or over one- third of a square 

 mile, and it drains an area of over 10J square miles, an area nearly 



