from the Surface of the Ground and over a Sheet of Water. 11 



land. The following experiments made during last October on 

 the Lake of Geneva entirely corroborate this view. 



Three Centigrade mercurial thermometers, capable of showing 

 the tenth part of a degree, were fixed at different heights round 

 a vertical pole about 16 feet long. Each thermometer was 

 attached to the extremity of a horizontal rod fixed to the pole, 

 and sufficiently long to ensure the thermometers being sus- 

 pended, not over the boat from which the experiments were to 

 be made, but directly over the surface of the water. One of the 

 thermometers was placed at a height of about 3 inches above 

 the surface of the water, the second at about 6 feet, and the 

 third at 15 feet. Three series of observations were made du- 

 ring the evenings of the 26th and 28th of October, at about 

 600 yards from land, under the most favourable circum- 

 stances — the sky being beautifully clear, and the surface of the 

 lake unruffled by the slightest breeze. The observations were 

 commenced a quarter of an hour before sunset, and renewed 

 every half hour until three-quarters of an hour after sunset. 

 The following is the average result of the observations noted 

 during the evening of the 26th of October : — 

 Temperature of the atmosphere 3 inches above the water. 11*65 C. 

 „ 6 feet „ . 11-62 



15 feet „ . 11-80 



Temperature of the water at the surface of the lake . 12 



The average result obtained during the evening of the 28th 

 was as follows : — 



Temperature of the atmosphere 3 inches above the water. 1 1 -35 C. 

 „ „ 6 feet „ . 11-29 



„ „ 15 feet „ . 11-32 



Temperature of the surface of the water . 12*75 



The consequence to be drawn from these results is, I appre- 

 hend, that the comparative temperature of the successive strata 

 of air above the surface of the lake up to the height of 15 

 feet, undergoes no sensible change from the effects of nocturnal 

 radiation. The almost imperceptible differences indicated by 

 the thermometers in no case exceeding a few hundredths of a 

 degree, may be fairly attributed to errors arising from accidental 

 currents, which it is difficult to guard against completely in 

 observations of this nature. 



The following is the mean result of comparative observations 

 made at the same moment in the centre of a large field about 

 700 yards from the borders of the lake : — 



Temperature of the surface of the ground . . . 6*98 C. 

 Temperature of the air 3 inches above the ground . 8'00 

 6 feet „ . 9-10 



„ j } 15 feet „ . 9*65 



