[53] Ammianus Marcellinus ascribes the longevity and robust health of mountaineers to their exposure to the dews of night. Dew was employed by the alchemists in their experiments on the solution of gold. The ladies of old collected the “celestial wash,” which they imagined had the virtue of preserving their fine forms, by exposing heaps of wool to the influences of night radiation. It was supposed that the lean features of the grasshopper arose from that insect feeding entirely on dew: “Dumque thymo pascentur apes, dum rore cicadæ,” Virgil, Eclog.
See some curious remarks by Boyle, On the Power of Dew in Working on Solid Bodies: Works of the Honourable R. Boyle, vol. v. p. 121. 1744.
[54] See the Researches on Heat, by Professor James Forbes, in the Transactions of the Royal Society of Edinburgh; also Melloni’s papers on the same subject in the Annales de Chimie, several of which have been translated into the Scientific Memoirs, edited by Mr. Richard Taylor.
[55] The phenomena of dew have constantly engaged the attention of man. Aristotle, in his book De Mundo, puts forth some just notions on its nature. An opinion has almost always prevailed that dew falls. Gersten appears to have been the first who opposed this motion. He was followed by Musschenbroek, and then by Du Fay. The researches of Leslie were of a far more exact character. Dr. Wilson, in the Transactions of the Royal Society of Edinburgh, 1st vol., published a Memoir on Hoar Frost of much interest; but the questions involved remained unsettled until the researches of Dr. Wells, which were published in his Essay on Dew.
[56] By far the most complete set of experiments on the radiation of heat from the surface at night, which have been published since Dr. Wells’s memoir On Dew, are those of Mr. Glaisher, of the Royal Observatory at Greenwich. Instruments of the most perfect kind were employed, and the observations made with sedulous care. The results will be found in a memoir On the Amount of the Radiation of Heat, at night, from the Earth, and from various bodies placed on or near the Surface of the Earth, by James Glaisher, Esq., Philosophical Trans. for 1847, part 2.
[57] Dr. Wells noticed the practical fact that very light shades protected delicate plants from frost, by preventing radiation. Mr. Goldsworthy Gurney has made a series of interesting experiments, and he imagines that by shading grasslands with boughs of trees, or any light litter, a more abundant crop is produced. The subject has been discussed in the journals of the Royal Agricultural Society. May not the apparent increase be due entirely to the succulent condition in which a plant always grows in the shade?
[58] This paper of Melloni’s will be found in the Bibliothèque Universelle de Genève, for 1843. The conclusions are highly ingenious, but they rest entirely on the analogy supposed to be discovered between the relations of heat, like light, to the coloured rays of the spectrum. This, it must be remembered, is not the case, since even Sir William Herschel showed that red light might exist with only a minimum of calorific power, notwithstanding the fact, that the maximum heat-ray of the spectrum coincides with the red rays.
[59] Dr. Robinson, of Armagh, in his Memoir On the Effects of Heat in lessening the Affinities of the Elements of Water.—Transactions of the Royal Irish Academy, vol. xxi. part 2.
[60] On this subject consult Robert Were Fox, On the Temperature of the Mines of Cornwall.—Cornwall Geological Transactions, vol. ii.; W. J. Henwood, on the same subject, Ib. vol. v.; Reports of the British Association, 1840, p. 315; Edinburgh New Philosophical Journal, vol. xxiv. p. 140.
[61] On the causes of the temperature of Hot and Thermal Springs; and on the bearings of this subject as connected with the general question regarding the internal temperature of the Earth: by Professor Gustav Bischoff, of Bonn.—Edinburgh New Philosophical Journal, vol. xx. p. 376; vol. xxiii. p. 330. Some interesting information on the temperature of the ground will be found in Erman’s Travels in Siberia, translated by W. D. Cooley, vol. i. p. 339; vol. ii. p. 366. Sur la Profondeur à laquelle se trouve la couche de Température invariable entre les Tropiques, by Boussingault: Annales de Chimie et de Physique, 1833, p. 225. Reference may also be made to Humboldt’s Cosmos, Otto’s translation; and to the excellent article on Meteorology, by George Harvey, in the Encyclopædia Metropolitana. These chthonisothermal lines, as they are called, have been traced by Humboldt and others over extensive districts.