82 BATHYMETRICAL SURVEY OF 



about a quarter of a mile from the south-west end to about one-third of 

 a mile from the north-east end, and is very nearly 14 miles in length. 

 The 200-feet basin stretches from about 1| miles from the north-east end 

 to about 2i miles from the south-west end, and is about 11 miles in 

 length. The main 300-feet basin approaches to within less than two 

 miles from the north-east end, and is over 7| miles in length; it is 

 separated, by a slight shoaling of the bottom over an interval of about 

 a mile, from a small subsidiary 300-feet basin (based upon soundings of 

 301 and 305 feet), which is over half a mile in length. The 400-feet 

 basin is over 3J miles in length, lying in the northern half of the loch, 

 and approaching to about 3| miles from the north-east end. The 

 deepest part of the loch lies between Skiag on the south-eastern shore 

 and Cragganruar on the north-western shore, about 5J miles from the 

 north-east end of the loch, or about 6 miles by road from Kenmore, 

 where there is a small basin over 500 feet in depth, two soundings of 

 508 feet being recorded about midway between the two shores. 



The area of the lake-floor covered by less than 100 feet of water is 

 estimated at about 1972 acres (over 3 square miles), or over 30 per cent, 

 of the total area of the loch ; the area between the 100-feet and 200-feet 

 contour-lines is about 1532 acres (nearly 2J square miles), or 23J per 

 cent. ; the area between the 200-feet and 300-feet contours is about 

 1390 acres (over 2 square miles), or over 21 per cent. ; the area between 

 the 300-feet and 400-feet contours is about 1017 acres (over 1J square 

 miles), or 15J per cent. ; the area between the 400-feet and 500-feet 

 contours is over 600 acres (rather less than one square mile), or over 

 9 per cent. ; while the area covered by more than 500 feet of water is 

 about 9 acres, or a small fraction of 1 per cent, of the entire area of the 

 loch. These gradually decreasing areas between the contour-lines drawn 

 in at intervals of 100 feet indicate that the average slope of the bottom 

 becomes steeper and steeper on proceeding from the shore out into deep 

 water; this is also clearly shown by a comparison of the two shallow 

 zones on both sides of the 50-feet contour-line, the area between the 

 shore and the 50-feet line being about 1161 acres, while the area 

 between the 50-feet and 100-feet lines is about 811 acres, or respectively 

 about 18 and 12J per cent, of the total area of the loch. The slope of 

 the bottom of the loch is shown on the natural scale and exaggerated 

 five times on the longitudinal and cross sections on the map. 



Loch Tay was surveyed on April 29 to May 7, 1902, and the level 

 of the surface of the water was determined by levelling from bench- 

 marks as being 349-1 feet above the sea. The officers of the Ordnance 

 Survey found the level to be 347-9 feet above the sea on August 12, 

 1899. 



Temperature Observations. Many surface temperatures were taken 

 during the progress of the survey from April 29 to May 7, 1902, the 

 readings varying from 41-8 Fahr. to 47-5 a range of 5-7. The higher 



