THE FRESH-WATER LOCHS OF SCOTLAND. 137 



when the elevation was 810'5 feet above the sea, nearly identical with the 

 elevation determined by the Ordnance Survey on May 18, 1896, viz. 810*4 

 feet. 



Temperature Observations. The following temperatures were taken in 

 the deepest part of the loch : 



Surface 46-0 Fahr. 



5 feet 44-3 



10 44-0 



25 44-0 



56 43-0 



The range from surface to bottom was 3, a fall of 1'7 being observed 

 between the surface and a depth of 5 feet, while the readings at 10 and 25 

 feet were identical. 



St. Mary's Loch (see Plate XLIX.). St. Mary's Loch is situated about 

 10 miles south of Peebles, 12 miles south-west of Selkirk, and 13 miles 

 north-east of Moffat, and is somewhat crescentic in outline, the narrower 

 upper portion trending nearly north and south, while the wider lower 

 portion trends in a north-easterly direction. It is 3 miles in length, the 

 maximum breadth exceeding half a mile, the mean breadth being one-third 

 of a mile. The superficial area is about 635 acres, or 1 square mile, while 

 the drainage area, including the area draining into the Loch of the Lowes, 

 extends to about 42 square miles. The maximum depth of 153 feet was 

 observed in the wider part of the loch, about 1^ miles from the northern 

 end. The volume of water is estimated at 2018 millions of cubic feet, and 

 the mean depth at 73 feet, or nearly half the maximum. There are two 

 deep basins exceeding loO feet in depth, separated at the entrance of the 

 Megget water by a ridge, on which a maximum depth of 88 feet was found : 

 the larger and deeper one towards the lower end is nearly 1J miles in 

 length, while the smaller one towards the southern end has a maximum 

 depth of 112 feet, and is over half a mile in length. The areas between 

 the contour-lines, and the percentages to the total area, are as follows : 



Feet. Acres. Per cent. 



Oto 50 229 ... 35-9 



50 100 207 ... 32-6 



100 150 193 ... 30-4 



Over 150 6 ... 1-1 



635 100-0 



The loch was surveyed on May o and 6, 1905, when the elevation was 

 found to be 809*5 feet above the sea, or more than a foot higher than the 

 elevation determined by the Ordnance Survey officers on May 18, 1896, viz. 

 808-2 feet. 



Temperature Observations. The following temperatures taken in the 

 deepest part of the loch at 6^30 p.m. on May 5, 1905, showed that the 

 range from surface to bottom was only 1*8 Fahr. : 



