70 
BATHYMETRICAL SURVEY OF 
of a mile in length, and enclosing the maximum depth of the loch (440 
feet) ; a short distance to the west (opposite Craiganour) is a second 
small basin based upon a sounding of 404 feet ; and three-quarters of a 
mile farther west is the third basin, with a maximum depth of 421 
feet. The area of the lake-floor covered by less than 50 feet of water 
is about 1200 acres (nearly 2 square miles), or 25 per cent, of the total 
area, while the area between the 50-feet and 100-feet lines is about 750 
acres, or 16 per cent., showing a relatively rapid descent beyond the 
50-feet line. The area between the 100- and 200-feet lines is about 
877 acres, or nearly 19 per cent, of the entire area. The area between 
the 200- and 300-feet lines is about 950 acres, or over 20 per cent., 
the area between the 300- and 400-feet lines is about 875 acres, or 18^ 
per cent., and that over 400 feet about 65 acres, or nearly 1^ per cent., 
of the total area of the loch. 
On commencing the survey of Loch Rannoch, the height of the 
surface above sea-level was determined from Ordnance Survey bench- 
marks as 668 feet ; the level of the loch fluctuated during the progress 
of the survey, but the soundings have all been reduced to this datum. 
The ofiicers of the Ordnance Survey on July 19, 1860, found the level 
of the loch to be 667‘5 feet above the sea. 
Teni'perature Observations . — -Very many temperature observations 
were taken between March 20 and July 10, 1902. The surface tem- 
peratures need not be discussed in detail ; the lowest reading recorded 
was 37°*9 on March 28, and the highest 59°-8 on June 23, showing a 
range of 22° in the temperature of the surface water during the period 
of three months. An interesting series of hourly observations on the 
temperature of the air and of the surface water at the pier at Hannoch 
Lodge was taken on June 9. One thermometer was immersed in 3 feet 
of water outside the pier, and another in 1 foot of water inside the pier, 
and they were read simultaneously with an air-thermometer at intervals 
of one hour from 9 a.m. to 10 p.m. The temperature of the air rose 
gradually, though irregularly, from 48° at 11 a.m. to a maximum of 
53° at 4 p.m., falling gradually again to 44°*5 at 9 p.m., and 45° at 
10 p.m. The thermometer in 3 feet of water showed a gradual rise in 
the temperature from 51°*9 at 9 a.m. to 53° at 11 a.m., then a slight fall 
at noon (52°‘7) and at 1 p.m. (52°'5), the maximum (53°*6) being 
recorded at 2 p.m., falling to 52°*9 at 4 p.m., rising to 53°‘3 at 5 p.m., 
falling gradually to 52°-l at 8 p.m., then rising to 52°-8 at 9 p.m., and 
53° at 10 p.m. The thermometer in 1 foot of water showed a gradual 
rise in the temperature from 51°’4 at 9 a.m. to the maximum of 53°*6 at 
2 p.m., whence it fell gradually to 51°-5 at 9 p.m., the reading at 
10 p.m. being 52°. The maximum temperature of the water was 
recorded in each case at 2 p.m., while the maximum temperature of the 
air was recorded at 4 p.m., and the temperature of the air was always 
lower than that of the water, except when the air was at its maximum 
