spheric temperature are the same, the amount of moisture floating in the air is at its maximum, i. e., no more will be suspended without an increase of temperature; but this does not often occur, the atmosphere being generally a few degrees warmer than the dew-point, and consequently able, either to suspend a larger amount of vapour, or to fall in temperature before the precipitation of that moisture, in a visible form, will take place. The difference between the dew-point and mean diurnal, mensual, or annual temperature is the mean dryness of the same respective periods. The mean dew-point is always below that of mean temperature, and it is less in January than in August; whereas the mean dryness is greatest in May and June in this country, where the former ranges between 34°.3 and 55°.3 F., and the latter between 1°.6 and 8° F. throughout the year. We give a diagram of the mean dew-point and mean dryness, observed throughout the year. 127. Dalton ascertained the dew-point by the plan of Le Roi. Having filled a thin glass vessel with cold spring water, he observed if dew was found upon the outside, and if so, the temperature of the water was too low. Pouring this out and carefully drying the vessel, he replaced it, having allowed it to gain a little heat from the ambient air. Repeating these manipulations till dew ceased to be deposited, he marked the temperature of the water at that instant, and thus obtained the required indication. Instruments which we shall hereafter describe, called Hygrometers, have been invented for more easily ascertaining the humidity of the air, but when carefully performed, the method described, with the aid of a formula, is the best. When the hygrometer falls below 15°, the air feels damp; when ranging between 30 and 40° it is dry; when from 50° to 68°, very dry; and when above 70°, intensely dry. An apartment to be comfortable, should range between 35° and 60° degrees as the extremes. In this climate, it oscillates in winter from 5° to 25°, and in summer, from 15° to 55°. In Upper India it has been noticed so very high as 160 hygrometric degrees. The question whether aqueous vapour is chemically combined or merely blended with the atmosphere, has not been determined-Berzelius, Berthollet, Saussure, and Thomson support the former theory; Dalton, Henry, and author, the latter.' 1 Count Claude Louis Berthollet,-Stat. Chimique; Henry,-Syst. of Chem.; Nicholson's Jour. vols. viii. and ix.; Thomson's Ann. of Philos. vols. vii. and xii. . Dalton, Syst. Chem. Philos; &c. CHAPTER VII. 130. Analogous pheno 128. Dew. 129. Circumstances attending its formation. mena. 131. Sweating statues. 132. Requisites for the deposition of dew. 133. Quantity dependent upon colour. 134. Amount at various periods, and on different bodies. 135. Theory. 136. Substances resembling dew. 137. Hoarfrost. 138. Fogs and mists. 139. Nature of the vesicles of fogs. 140. Theory of mists. 141. Electricity of fogs. 142. Dry fogs; remarkable one of 1783. 143. Cause. 144. Luminous fogs. 145. Frost smoke. 146. Clouds; how suspended. 147. Their electricity. 148. Green-coloured clouds. 149. Altitude. 150. Motion and velocity. 151. Influence on temperature. 152. Nomenclature. 153. Effect of perspective. 154. Transformations. 155. Mountain-caps. CLOUDS. "I am the daughter of the earth and water, And the nursling of the sky; I pass through the pores of the ocean and shores; I change, but I cannot die. For after the rain, when, with never a stain, The pavilion of heaven is bare, And the winds and sunbeams, with their convex gleams, Build up the blue dome of air, I silently laugh at my own cenotaph, And out of the caverns of rain, Like a child from the womb, like a ghost from the tomb, 128. Dew is the spontaneous appearance upon objects freely exposed to the atmosphere, of moisture which was invisible. From the remotest ages, an explanation of this meteor has been sought, and poets have often sung its praises. It was early noticed that it was deposited only on nights of cloudless beauty and serenity; hence arose the fancy that it was the produce of the stars, and partaking of such celestial origin, it is not to be wondered that the most exalted veneration and surprising properties should have been ascribed to it. Aristotle,' when he theorized that it was humidity detached in minute particles from the clear chill air, showed that light was gleaming, but soon to be darkened: the human mind was not prepared to receive such a simple truth. Pliny repeats the observation, and tells us that "neque in nube, neque in flatu cadunt rores.' Against these rational hypotheses, we would place in apposition the chimera of its telluric rise. The colours from dew-drops have been investigated by Scoresby." To witness the phenomenon, it is necessary to use a telescope of short focus, and turn the back to the sun, taking the shadow of the observer's head as a guide. Varying the position of the instrument, most brilliant globules will be seen, dazzling like diamonds, and shining in all the gorgeous beauty of the spectrum. 5 129. Dr Wells is our chief authority on dew, and his theory is founded upon calorific radiation. This philosopher paid great attention to the subject, and presented the public with the result of two years' experiments in the vicinity of London, in one of the most beautiful inductive essays in our language.* Dr Wells noticed what was pointed out by Dr P. Wilson of Glasgow, that this meteor is deposited on bodies whose temperature is lower than that of the surrounding air; but he correctly assigned a different explanation, having observed that dew does not gather till after the temperature of the body is reduced,—that it is the effect, not the cause of the lowering of the temperature of the bodies bedewed. Recently this has been well illustrated by the delicate experiments of Melloni. It is from the invisible elastic vapour diffused around, that the meteor is derived. 130. Analogous to this phenomenon in cause and effect, is the moisture upon the roof and walls of a heated apartment, when the temperature of the humid air within rises above 1 Meteor. lib. i. cap. x. et De Mundo, cap. iii. a Jameson's Journ. No. 61. 2 Hist. Nat. xviii. 29. Essay on Dew. Lond. 1814. Ed. Roy. Soc. Tr. 1788-Hoarfrost. 6 Compt. Rend. No. xiii. p. 531,-Let. to Arago. that of the building; or upon the windows when a fall of hail or rain suddenly reduces the temperature of the external air; or upon the human body on entering a moist and highly heated hot-house; or upon the walls of rooms when a humid thaw succeeds a protracted frost; or when we breathe upon a cold metallic or silicious substance. 131. We are told by Virgil, Tibullus, Ovid, Lucan, and other ancient writers,' of "sweating statues," and weeping images! "Et mæstum illacrymat templis ebur, æraque sudant." VIRGIL, Georg. i. 479. "In temples mourning, Iv'ry wept, and Brass Sweated." TRAPP. But what were these? It does not seem a mere poetical licence, but a grave scientific fact, a fine illustration of the gathering of dew upon bodies colder than the ambient air, which, in the instance quoted, was then intensely humid, for in the same passage, descriptive of the "wonders" seen at Cæsar's death, mention is made of floods which swept woods away and cattle with their stalls." It is no sufficient objection that the metal figures were also wetted, for though metallic bodies do not speedily receive dew, yet if the circumstances are favourable and sufficiently continued, they will become bedewed. A similar phenomenon was observed in 1506 in Italy.' 132. For the deposition of dew the surrounding atmosphere must suspend as much moisture as its particular temperature will allow; and the bodies upon which it falls must be colder than the ambient air. The night must be cloudless, or nearly so, and perfectly serene, with circumstances favourable to terrestrial radiation into the planetary regions: for, according to the theory of Prevost,3 radiation is always taking place from every object, and consequently when clouds are inter 1 Virgil,-Georg. lib. i. v. 479; Tibullus,-Op. lib. ii. 5: Livy,-lib. xliii. cap. 13; Plutarch,-Coriol., &c. Holinshed, Chron. v. iii. p. 793, fol. 3 Rech. sur la Chaleur; Ph. Tr. 1802, 443. |