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 18J 

 per cent., and that over 400 feet about 65 acres, or nearly 1 J 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 officers of the Ordnance Survey on July 19, 1860, found the level 

 of the loch to be 667 - 5 feet above the sea. 



Temperature 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 Rannoch 

 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 



