Radiant Heat to Aqueous Vapour. 41 



Observation proves the radiation to augment as we ascend a 

 mountain. Martins and Bravais, for example, found the lower- 

 ing of a radiation-thermometer 5°*7 Cent, at Chamouni; while 

 on the Grand Plateau, under the same conditions, it was 13 0, 4 

 Cent. The following remarkable passage from Hooker's Hima- 

 layan Journals, 1st edit. vol. ii. p. 407, bears directly upon this 

 point : — " From a multitude of desultory observations I conclude 

 that, at 7400 feet, 125°- 7 or 67° above the temperature of the 

 air, is the average maximum effect of the sun's rays on a black- 

 bulb thermometer These results, though greatly above 



those obtained at Calcutta, are not much, if at all, above what 

 may be observed on the plains of India [because of the dryness 

 of the air. — J. T.] . The effect is much increased with the ele- 

 vation. At 10,000 feet, in December, at 9 a.m. I saw the mer- 

 cury mount to 132° [in the sun], with a difference [above the 

 shaded air] of 94°, while the temperature of shaded snow hard 

 by was 22°. At 13,100 feet, in January, at 9 a.m. it has stood 

 at 98°, with a difference of 68°'2, and at 10 a.m. at 114°, with 

 a difference of 81°'4, whilst the radiating thermometer on the snow 

 had fallen at sunrise to o, 7." This enormous chilling is fully 

 accounted for by the absence of aqueous vapour overhead. I 

 never under any circumstances suffered so much from heat as in 

 descending on a sunny day from the so-called Corridor to the 

 Grand Plateau of Mont Blanc. The air was perfectly still, and 

 the sun literally blazed against my companion and myself. We 

 were hip deep in snow; still the heat was unendurable. Im- 

 mersion in the shadow of the Dome du Goute soon restored our 

 powers, though the air of the shade was not sensibly colder than 

 that through which the sunbeams passed. Notwithstanding the 

 enormous daily accession of heat from the sun, terrestrial radia- 

 tion at these altitudes preserves an extremely low temperature at 

 the earth's surface, 



Without quitting Europe we find places where, even when the 

 day temperature is high, the hour before sunrise is intensely 

 cold. I have often experienced this even in Germany ; and the 

 Hungarian peasants, if exposed at night, take care, even in hot 

 weather, to prepare for the nocturnal chill. The range of tem- 

 perature augments with the dryness, and an u excessive climate " 

 is certainly in part caused by the absence of aqueous vapour. 



Regarding Central Australia, Mr. Mitchell publishes extremely 

 valuable tables of observations, from which we learn that, when 

 the days are at the same time calm and clear, the daily thermo- 

 metric range is exceedingly large. The temperature at noon 

 being 68° on the 2nd of March 1835, that at sunrise next 

 morning was 20°, showing a difference of 48°. The 7th and 

 8th were also clear and calm ; the difference between noon and 



