336 On the Distribution of Temperature in 



- may be taken as the earliest dates of maximum tem- 

 ure at the depths indicated. 



If \ve compare the mean temperature of the whole column 

 of 100 fathoms of water at Inversnaid on the 5th and the 

 JJIH! September, we have on the 5th the mean temperature 

 44-54 F., and on the 22nd September 44-52, the difference 

 being 0-02 F. Remembering that heat is lost more quickly in 

 autumn than it is gained in spring, and considering that these 

 temperatures are nearly identical, we shall not be far wrong if 



lit the epoch of maximum heat in the water of the deepest 

 part of the lake as occurring some time in the third week of 

 September. This brings it very near the date of the equinox, 

 and it seems natural to expect the heat to accumulate in the 

 water so long as the day is longer than the night, and to decrease 

 so soon as the conditions are reversed. This conclusion is sup- 

 ported by the observations of Fischer, Forster, and Brunner, 

 who made a most interesting series of observations on the distri- 

 bution of temperature in the Lake of Thun, in Switzerland, 

 during the years 1848 and 1849. They found hardly any 

 increase of heat between 3rd February and 28th March, but 

 after the latter date the influx of heat was ver}' rapid. It is 

 probable, therefore, that the temperature of the bottom water 

 in our deepest lakes depends chiefly on the temperature of the 

 air between the preceding autumnal and vernal equinoxes. Two 

 causes combine, namely, the greater meridian altitude of the 

 sun and the greater length of the day in the summer than in 

 the winter half year. The latter cause has the effect that the 

 water is exposed to heating for a greater portion of the 24 hours 

 than it is to cooling, while the former cause ensures a greater 

 supply of heat per minute during the day in summer than in 

 winter. 



Further, the rate of loss of heat, due to radiation alone, 

 must be very much greater during a winter night than during 

 a summer one. 



The rise of temperature in the bottom water and deeper 

 layers is made more apparent by considering the actual readings 

 of the thermometer employed as given by the millimetre scale 

 on the stem. 



