[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.