170 Scientific Proceedings, Royal Dublin Society. 



the deep parts of the lake. It thus appears that the cold water 

 supply for the bottom of the lake may be kept up by the cold 

 night water from the shallows. 



On laying out a series of afternoon isothermal lines, it appears 

 that they are closer together in the shallow water than in the deep 

 water, the bottom in the shallower water being in general colder 

 than at the same depth in deep water, though, of course, in the 

 very shallow water, where the surface was several degrees hotter 

 than elsewhere, the whole of this very shallow water was warmed 

 up, and was hotter than water at the same level elsewhere. The 

 rate of change of temperature downwards was very regular, from 

 a depth of from five to six yards, to the bottom. At the depth of 

 five to six yards, it changed more rapidly, and from that up to the 

 surface was the region that was affected by the diurnal changes of 

 temperature. During the day the upper layers in this region 

 became much hotter, and during the night the whole of this 

 region gradually became of the same temperature throughout. 

 The depth of this region was observable, during the days I 

 observed it, by the variation in the rate of change of temperature 

 that occurred at this depth ; the change of temperature was more 

 rapid here than in either the subjacent or in the immediately 

 superincumbent layers. 



From the rate of decrease of temperature in the superficial 

 layers I calculated that the coefficient of absorption of heat per 

 yard was -71, but as it is known that this is very different, for 

 different rays of the spectrum, it is probable that the coefficient of 

 absorption of the first layers is very much greater. I had not any 

 sufficiently accurate method of measuring the temperatures at near 

 points to determine the rate of change of temperatures for small 

 distances near the surface, but it was certainly very much more 

 rapid than even at a short distance below the surface. 



