SUPPLEMENTARY REPORT ON METEOROLOGY. 65 
77. Another method of measuring, or at least comparing the 
intensity of solar radiation, would be by the use of photographic 
aper*, 
78. The next problem which radiation experiments may be 
expected to solve, is the quantity of heat actually received from 
the sun in a year on the surface of the earth. According to 
Pouillet’s Jast estimate, this amounts to a quantity of heat 
capable of melting 31 metres’ thickness of ice all over the 
globet+. Such estimates must be received with considerable 
diffidence. 
79. The excess of heat received during the day is given off at 
night, or rather there is a perpetual radiation of heat from the 
globe towards the celestial spaces, which, granting the con- 
stancy of climate from age to age, must exactly equal the quan- 
tity of heat received. Observations of this kind may be made 
in various ways. The simplest is by exposing a thermometer 
to the aspect of the open sky, laying it on some freely radiating 
substance, such as snow, wool, or swandown. In this manner 
were conducted the experiments of Six, Wilson, and Wells. 
Wells noticed a difference of two thermometers, one in air, the 
other placed on swandown, amounting to 8°3 centigrade de- 
grees{ (15° F.). Boussingault, in his observations amidst the- 
Andes, has recorded a depression of 6°1; but the radiating 
thermometer was laid simply on turf, the other was suspended in 
the air at a height of 1°6 met. (5 ft. 4in.)§. His observations 
were carried to a height of 4600 metres, where we should expect 
the effect of nocturnal radiation to be greatly increased, owing 
to the excessive transparency of the atmosphere. The same 
author mentions the curious fact, that to defend their crops 
from the intensity of the nocturnal cold, the natives of South 
America often make artificial clouds by means of smoke. 
80. Leslie applied his differential thermometer to the measure 
of radiation by exposing one ball in the focus of a parabolic 
mirror, which he then called an ethrioscope. The conduct of 
systematic experiments of this kind is a matter of considerable 
difficulty. The only continuous series with which I am ac- 
quainted were made at Geneva during several years succeeding 
1836, and published amongst the regular and excellent obser- 
vations preserved in the Bibliotheque Universelle, a journal in 
* See Herschel, Phil. Trans. 1840, p. 46. “‘ Description of an Actinograph 
or self-registering Photometer for Meteorological purposes.” 
¢ Mémoire sur la Chaleur Solaire, p. 9. His former estimate was 14 metres 
only (see First Report, p. 222). 
£ Arago, Annuaire, 1836, p. 261. See also Annuaire, 1838, p. 214, &c. 
§ Ann. de Chim. lii. 260. 
VOL. 1x. 1840. F 
