THE FRESH-WATER LOCHS OF SCOTLAND. 
227 
west to east, where the maximum depth of 7 feet is found. The mean 
depth is feet, the area about 149 acres, and the volume 28 millions of 
cubic feet. The drainage area is about a square mile. The outflow is 
by Mill burn, a quarter of a mile long, issuing from the south-west corner 
of the loch. The surface was 13*9 feet above sea-level on August 17, 
1903, and the surface temperature was 58°-5 Fahr. 
Loch of Swannay (see Plate XCIII.). — The Loch of Swannay (or 
Swona, or S wanna) is a large loch of elliptical form, in the extreme north 
of Pomona, at Costa head. It is surrounded by moorland, in part 
cultivated. The shores are paved with flat stones. Under Costa hill 
are ranges of crags. The Muckle Holm island is stony, and there are 
many stony shoals, - especially south of the island, and along the west 
shore. The length is 2 miles, and the greatest breadth two-thirds of 
a mile. Around the Muckle Holm the bottom is irregular and shoaly. 
South of the island is a considerable flat of 8 feet deep. North of the 
island is a large flat-bottomed basin, gradually deepening from 12 feet 
to 16 feet near the north end. The mean depth is 9J feet, the area about 
603 acres, or .nearly one square mile, and the volume 242 millions of 
cubic feet. The drainage area is fully 5 square miles. A small burn 
enters at the southern end, and the Swannay burn flows out at the north- 
west corner, over a dam with a sluice, which is now disused. There is 
rock in the stream at the outflow. The bottom is visible to a depth of 
8 or 10 feet, and is stony, with little or no deposit of mud. The surface 
was 134*55 feet above sea-level on August 31, 1906. The temperature 
at the surface was 57°*7 Fahr., at 15 feet 57°*0. 
Loch of Boardhouse (see Plate XCIII.).— The Loch of Boardhouse, 
which, in accordance with the Orcadian custom of naming a loch in- 
differently from any conspicuous feature in its neighbourhood, also receives 
the appellations Loch of Birsay, Loch of Twatt, Loch of Kirbuster, Barony 
Loch, and perhaps yet others, is a large and broad loch in the parish of 
Birsay, about half a mile east of the village of that name. It lies amid 
fields gently sloping up from the shore, with patches of moorland. Ravie 
hill, on the south-west, and Kirbuster hill on the north-east, are heather- 
topped hills of no great elevation. The shores are stony, and so shallow 
that they are difficult to approach in a boat — the grass slopes come quite 
to the water’s edge, with hardly any beach. The only rock seen was at 
the original outflow. The length is 2 miles, and the .greatest breadth 
near the eastern end two-thirds of a mile. The bottom is flat, and 
gradually deepens from 5 feet at the north-west end to the* maximum 
depth of 9 feet close to the east end. The mean depth is 6 feet, the area 
nearly a square mile, and the volume 150 millions of cubic feet. The 
drainage area is 13^ square miles. The Kirbuster burn, a fair-sized river, 
enters at the east end. From the north-west corner the Birsay burn flows 
