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XXII.— ON THE TEMPEEATUEE AT VAEIOUS DEPTHS IN 

 LOUGH DEEG APTEE SUNNY WEATHEE. By GEO. F. 

 FITZGEEALD, F.T.C.D., F.E.S. 



[Read, April 21, 1886.] 



The measurements upon which this Paper is founded were made by 

 me in the month of July, 1876, and I would have hardly thought 

 them worth recording only that I have lately seen it noticed as a 

 new fact that the isothermal surfaces in the Lake of Geneva are 

 not level surfaces ; and that this was so in Lough Derg was one of 

 the special features I remarked in my observations of nearly ten 

 years ago. 



I made experiments with a maximum and minimum thermo- 

 meter, attached to a sounding-line, and the differences of tempera- 

 ture observed were so great that there could be no doubt, even 

 with rough experiments. 



The observations were made after a long continuance of hot, 

 sunny weather, during which the day temperatures ranged from 

 73° F. to 75° F., and the night temperatures from 55° F. to 65° F. 



The temperature of the surface of the lake rose rapidly during 

 sunshine, at a rate of nearly a degree per hour. In the deep 

 water the temperature of the surface did not rise so fast as in the 

 shallow water. About 3*30 in the day the temperature of the 

 surface water in the deep parts was 71° F., and in the shallows 

 75° F. From a calculation of the amount of heat that enters the 

 water, it seems that only about -gV^h, or less, was used in heating 

 it, the rest being probably spent in evaporation. During the 

 evening the temperature of the surface fell slowly, until in the 

 morning it was uniform, to a depth of about five yards, this being 

 the depth, apparently, that the convexion currents during the 

 night reached. This temperature was, on the night I observed 

 it, eleven degrees above the night temperature of a thermometer 

 exposed on grass. In the shallow water the temperature fell more 

 rapidly until it was about 2° colder than the surface water in deep 

 parts, and nearly the same as that of the water at the bottom of 



