Lakes 299 



principal basins of very unequal depth: the large expanse of 

 water studded with islands at the lower end, the Balloch basin ; 

 the middle or Luss basin ; and the upper and deepest or Tarbet 

 basin. In the last we have 600 feet of water, in the Luss basin 

 200 feet, and in the Balloch basin a maximum of 72 feet of 

 water. On 23rd September 1876 the bottom temperature in 

 the Tarbet basin was 41-4, and in the Luss basin 46-4. Loch 

 Tummel, a much smaller lake, consists of three basins, each 

 of them being from 100 to 120 feet deep, and in them we have 

 bottom temperatures of 46-3, 46-9 and 45-2, the lowest 

 temperature being nearest the outlet. 



It might have been expected that the bottom temperature 

 in lakes similar as regards size and depth would be lower at 

 ter elevations and higher nearer the sea-level. This does 

 not, however, hold universally; thus Lochs Tummel and 

 Garry are very similar in size and depth; they are only 12 

 miles from each other, but Loch Tummel is 450 feet and Loch 

 Garry 1330 feet above the sea; yet at 102 feet in Loch Garry 

 the temperature on the i8th August 1876 was 53-9, and in 

 Loch Tummel at the same depth on the i6th August 1876 it 

 was 45*4. The difference of elevation is nearly 900 feet, 

 and, instead of the higher lake holding the colder water, its 

 water is 8-5 warmer than that of the lower one. Similarly 

 in Loch Ericht, 1153 feet above the sea, the bottom tem- 

 perature at 324 feet was 44-7, and in Loch Rannoch, 668 I ret 

 above sea, at the same depth it was 44-0. These examples 

 will suffice to show that many circumstances concur in deter- 

 mining tin- temperatures of the waters of lakes. There is one 

 factor whi'-h is often neglected, namrlv, the amount of change 

 of water. This depends on the drainage area of its tributary 



ams, and necessarily varies greatly. 



In comparing the bottom temperature in lake* with the 



i tnnjHT, mires of the coldest halt ..[ the year, we find that 



the two approach each other more neailv the hnjin th<-r 



temprratun-s are. \Vhrn the tnnp( latuiv of the air falls for 



a lengthened period below the tempi-ratine <.| maximum 



ity of water (39-2 Fahr.), then the m hani< al < 

 prodi much the same as if the tempnaturr had been 



