68 BATHYMETRICAL SURVEY OF 



soundings run from 2 to 5 feet; the south-western expansion is the 

 shallowest, with a maximum depth of 3 feet to the west of the central 

 large island (Eilean na Coille), the bottom to the east and south of that 

 island being covered by only 1 foot of water. Over 90 per cent, of the 

 entire lake-floor is covered by less than 10 feet of water, and less than 

 2 per cent, by over 20 feet of water. The surface temperature in the 

 eastern expansion at 11 a.m. on April 21, 1902, was 44-2. 



Loch Rannoch (see Plate XIX.). Loch Rannoch, one of the larger 

 and more important of the lochs in the Tay basin, was the headquarters 

 of the Lake Survey for nearly four months, from March 20 to July 10, 

 1902, and during that period a great many soundings, as well as 

 observations on the temperature of the water, on the biology, and on the 

 rise and fall of the surface of the loch, were taken, all the members of 

 the staff taking part in the work. The lake trends in an east-and-west 

 direction, and is a lovely sheet of water, the hills on both sides, and the 

 woods clothing its shores in many places, adding beauty to the scene. 

 The famous Black Wood of Rannoch on the south side is of great 

 antiquity. The loch contains many small trout, and is famed for large 

 Salmo ferox. It is nearly 9| miles in length, considerably over a mile 

 in maximum breadth, the mean breadth being about three-quarters of 

 a mile, or about 8 per cent, of the length. Its waters cover an area of 

 over 4700 acres, or nearly 7 \ square miles, and it drains directly an area 

 of about 130 square miles, but, since it receives the outflow from Loch 

 Ericht, Loch Eigheach, Loch Laidon, Loch Ba, &c., its total drainage 

 area is about 243| square miles, or 33 times the area of the loch. 



Over eight hundred soundings were taken in Loch Rannoch, the 

 maximum depth observed being 440 feet, or 20 feet deeper than the 

 maximum depth recorded by Mr. Grant- Wilson during his survey in 

 the year 1888, when he took about 320 soundings. The volume of 

 water contained in the loch is estimated at about 34,387,131,000 cubic 

 feet, or less than a quarter of a cubic mile, and the mean depth at 167| 

 feet, or 38 per cent, of the maximum depth. The length of the loch 

 is 116 times the maximum depth, and 306 times the mean depth. The 

 loch is widest and deepest in the eastern half, narrowing and shallow- 

 ing towards the west on approaching the island Eilean nam Faoileag, 

 then deepening again to the west of that island. It consists of one 

 large main basin, with two subsidiary small basins over 50 feet in 

 depth towards the west end, separated from the main basin by the 

 shallow water in the neighbourhood of Eilean nam Faoileag. The 

 larger of the two subsidiary basins is about three-quarters of a mile in 

 length, stretching from south of the island An t-Eilean Fearna, at the 

 entrance of the river Ericht, towards the west end of the loch, and the 

 maximum depth recorded in it was 84 feet; the smaller basin lies 

 between the two islands mentioned and towards the northern shore, 



