THE FRESH-WATER LOCHS OF SCOTLAND. 
77 
approximately the outline of the loch, approaching very close to the 
west en(f, where between the mouths of the Ogle and Kendrum burns 
a sounding of 57 feet was taken about 300 feet from the shore, giving a 
slope of 1 in 5’3. At the opposite end of the loch the 50-feet contour is 
met with about one-third of a mile from the bridge across the river at 
St. Fillans. The 100-feet basin approaches to within less than a quarter 
of a mile from the west end, and less than half a mile from the east end, 
and is over 5 j miles in length ; it covers an area of nearly square 
miles. The 200-feet basin is miles in length, stretching from three- 
quarters of a mile from the west end to miles from the east end, and 
covers an area of IJ square miles. The 250-feet basin is nearly 2 miles 
in length, and a quarter of a mile in maximum width, extending from 
1| miles from the west end to 3 miles from the east end. The maximum 
depth of 287 feet was observed near the centre of the loch, between the 
mouths of the Allt Bhacaidh on the north and the Allt Dhunain on the 
south, about 2| miles from the west end, and 3| miles from the east 
end. The area of the lake-floor covered by less than 100 feet of water 
is about 926 acres, or 38J per cent, of the total area of the loch ; the 
area between the 100- and 200-feet contour-lines is about 755 acres, or 
per cent. ; and the area covered by more than 200 feet of water is 
about 700 acres, or 30 per cent. The flat-bottomed character of the 
loch is indicated by the last-mentioned percentage, which is nearly 
equal to the preceding one, though the interval of depth is only 87 feet 
as compared with the previous interval of 100 feet. The comparatively 
uniform average slope from the shore down to a depth of 100 feet is 
shown by the fact that the areas on each side of the 50-feet line are 
nearly equal, viz. 477 acres (or nearly 20 per cent, of the entire area of 
the loch) between the shore and the 50-feet contour, and 449 acres (or 
nearly 19 per cent.) between the 50- and 100-feet contours. 
Loch Earn was surveyed on May 14 to 19, 1902, and the level of the 
surface of the water was determined by levelling from Ordnance Survey 
bench-mark as 317*2 feet above sea-level. This is identical with the 
level determined by the surveyors of the Ordnance Survey on August 
25, 1899. 
Temperature Observations . — On May 14, at 3.45 p.m., the tem- 
perature of the surface water near Lochearnhead was 46°*1 ; at 6 p.m. 
the surface temperature was 44°, and at 7 p.m. near the shore 47°*2. 
On May 15, at 11.30 a.m., the surface temperature near shore about 
a mile east of Lochearnhead was 44°. On May 16, at 10.30 a.m., the 
surface temperature at the St. Fillans end of the loch was 44°*1, and 
at 1*30 p.m. it was 44°. On May 17, at 5 p.m., the surface temperature 
off Dalkenneth over the deepest part of the loch was 43°*8. The range 
observed in the temperature of the surface water during those four 
days was thus 3°*4, from 43°*8 to 47°*2, the range in the air temperature 
during the same period being 7°*5, from 40°*5 to 48°. 
