132 
BATHYMETRICAL SURVEY OF 
4 square miles. Castle Loch is simple in conformation, and of a flat- 
bottomed character, the deeper water, over 15 feet, occupying the central 
and eastern parts of the loch, three soundings at the maximum depth of 
18 feet being recorded to the north-east of the island. Of the entire lake- 
floor, only 28 per cent, is covered by less than 5 feet of water, while 
38 per cent, is covered by more than 10 feet of water. The mean depth 
is estimated at 8^ feet, and the volume of water at 72 million cubic feet. 
The loch was surveyed on April 27, 1905, when the elevation was 135‘7 feet 
above the sea, as compared with 137T feet determined by the Ordnance 
Survey on February 22, 1 899. The temperature of the water was uniform 
throughout, readings at the surface giving 46°*9 Fahr., and at 18 feet 46°’8. 
Rightae Mill Loch (see Plate XL VII.). — A small loch, pyriform in 
outline, to the south of Castle Loch. Though the smallest of the group, it 
drains the largest area, the overflow being carried by the Mill burn into 
the Valison burn just before entering the river Annan. The loch trends 
from north-west to south-east, tapering gradually towards the outflow at 
the southern end, and nearly one-third of a mile in length, the superficial 
area being about 19 acres, and the drainage area about 6^ square miles — 
an area over two hundred times greater than that of the loch. The basin 
is simple, the deeper water lying in the broadest part of the loch towards 
the upper end, the maximum depth of 13 feet having been found a short 
distance from the north-eastern shore. The mean depth is 7| feet, or more 
than half the maximum, and the volume of water 6 million cubic feet. The 
elevation was 137*2 feet above the sea on the date of the survey (April 29, 
1905), wdien identical readings of 49°*0 Fahr. were taken at the surface 
and at 12 feet. 
From the following table it will be seen that in the fourteen lochs under 
consideration, 599 soundings were taken, and that the aggregate area of 
the water-surface is about If square miles, so that the average number 
of soundings per square mile of surface is 335. The aggregate volume of 
water contained in the lochs is estimated at 652 millions of cubic feet. 
The area drained by these lochs is about 24| square miles, or 13 J 
times the area of the lochs. 
