THE FRESH-WATER LOCHS OF SCOTLAND. 
253 
beiug respectively 10 and 5 feet, the mean depth in each case being one- 
half of the maximum. The volume of water in Duddingston is about 
4 million cubic feet, and in St. Margaret’s about half a million cubic feet. 
They were surveyed on June 27, 1903, when the temperature of the water 
in Duddingston was 61°'l Fahr., and in St. Margaret’s 60°'7. 
Harperrig Meservoir (see Plate CXI.) is situated at the base of the 
Pentland hills, about 12 miles south-west of Edinburgh. It exceeds a 
mile in length from south-west to north-east, with a maximum breadth 
of half a mile, the superficial area being about 226 acres. It is, on the 
whole, comparatively shallow and flat-bottomed, with a small area of deep 
water near the outflow, the maximum depth of 30 feet being recorded 
close to the weir. More than one-half of the bottom is covered by water 
between 10 and 20 feet in depth, while only 4 per cent, is covered by 
more than 20 feet of water. The volume of water is estimated at 
108 million cubic feet, and the mean depth at 11 feet. When surveyed 
on July 21, 1903, the elevation was 891'9 feet above sea-level. 
Threipmuir Reservoir (see Plate CXII.) lies about 4 miles to the 
north-east of Harperrig reservoir, and is practically continuous with 
Harelaw reservoir, though standing about 25 feet higher. These two 
contiguous basins form a perfect contrast in conformation, Threipmuir 
covering an area six times greater than that of Harelaw, but being 
relatively shallow. Indeed, the south-western portion is merely a swamp. 
Threipmuir reservoir is IJ miles in length from south-west to north-east, 
excluding a narrow arm branching off to the east ; from the extremity of 
this arm to the extremity of the south-western swampy portion is fully 
2 miles. The superficial area is about 192 acres, while the drainage area 
exceeds 6 square miles. The maximum depth of 17 feet occurs near the 
outflow, whence the water shallows gradually on proceeding to the south- 
west or along the narrow arm to the east. The volume of water is esti- 
mated at 66 million cubic feet, and the mean depth at 8 feet. When 
surveyed on July 1, 1903, the elevation was 831 ’5 feet above the sea, and 
the temperature of the water was uniform, the reading at the surface being 
59°*3 Fahr,, and at 10 feet 59°‘2. 
Harelaio Reservoir (see Plate CXlI.) is irregular in outline, and exceeds 
half a mile in length from south-west to north-east, covering an area of 
only 30 acres, and draining an area of 7 square miles, including Threip- 
muir. The depth increases gradually on proceeding from the inflow 
towards the outflow, where a maximum of 54 feet was recorded. The 
volume of water is estimated at 30 million cubic feet (nearly half that 
of Threipmuir), and the mean depth at 23 feet (nearly three times that of 
ThreqDmuir). When surveyed on July 1, 1903, the elevation was 806*0 
feet above the sea. Temperatures taken in the deepest part gave some 
