52 BATHYMETRICAL SURVEY OF 



lines drawn in at equal intervals, and the percentages to the total area, 

 for it will be observed that the area between the 25-feet and 50-feet 

 contours is smaller than in the deeper zones, while the area of the lake- 

 floor covered by more than 100 feet of water is very considerable : 



Feet. Acres. Per cent. 



Oto 25 107 ... 30-9 



25 50 59 ... 17-0 



50 75 70 ... 20-1 



75 100 63 ... 18-3 



Over 100 48 ... 13-7 



347 100-0 



Temperature Observations. A series of temperatures taken in the 

 deepest part of the loch at 5 p.m. on the date of the survey gave the 

 following results : 



Surface 51'0 Fahr. 



20 feet 51-0 



50 50-5 



80 47'2 



100 46-8 



These observations give a total range of temperature throughout the 

 body of water of 4'2, the " sprungschicht " occurring between 50 and 

 80 feet, where a fall of 3-3 was recorded. 



Dabli Loch (see Plate XVIII.). Dubh Loch lies about 3 miles to the 

 south-east of Loch Gairloch, and is almost continuous with Loch Bad an 

 Sgalaig, the stream between them being about 300 yards in length, and 

 the difference in level about 4 feet. The loch trends in a south-west and 

 north-east direction, and is nearly a mile in length, varying considerably 

 in width, the maximum breadth being one-third of a mile. Its waters 

 cover an area of about 99 acres, and it drains an area of nearly 2J square 

 miles. The maximum depth of 68 feet was observed near the centre of 

 the loch. The volume of water is estimated at 136 million cubic feet, 

 and the mean depth at 31| feet. The loch was surveyed on August 2, 

 1902, when the elevation of the lake-surface above the sea was ascertained 

 to be 357-35 feet. 



Dubh Loch forms a simple basin, the bottom sloping down on all sides 

 towards the deepest part in the centre ; the deeper water, however, is 

 found nearer to the south-western than to the north-eastern end. The 

 contour-lines follow approximately the trend of the shore-line, but approach 

 closer to the north-western shore than to the opposite one, indicating a 

 steeper slope in that direction. Thus, near the middle of the north- 

 western shore a sounding in 36 feet was taken about 30 feet from shore, 

 giving a gradient exceeding 1 in 1, and towards the south-west end another 

 sounding in 36 feet was taken about 40 feet from shore. The 50-feet 

 basin is half a mile, and the 25-feet basin three-quarters of a mile, in 



