Continents have a colder atmosphere than islands situated in the same degree of latitude, and countries lying to the windward of the superior classes of moun tains, or forests, are warmer than those which are to the leeward. Earth, always possessing a certain degree of moisture, has a greater capacity to receive and re tain heat than sand or stones, the latter therefore are heated and cooled with more rapidity: it is from this circumstance that the intense heats of Africa and Ara bia, and the cold of Terra del Fuego are derived. The temperature of growing vegetables changes very gradually ; but there is considerable evaporation from them : if these exist in great numbers, and congregated, or in forests, their foliage preventing the rays of the sun from reaching the earth, it is perfectly natural that the immediate atmosphere must be greatly affected by the ascent of chilled vapours.
Our next object is the ascent and de cent of water. The first mentioned ope ration of this fluid has been noticed al ready. See Evarossrtow.
Dews, the effect oftlio 'WIC Cattle, are variously accounted for by different ob servers of nature ; the general result, however, seems to be, that they are the last feeble efforts of evaporation, which, deprived of their warm stimulus by the approach of night, fall through the chill of the air in extremely small and distinct globules, covering every substance with that trembling and brilliant lustre, which rain is incapable of affording through the weight pf each drop. According to Hales, 3.28 inches of dew annually falls on the earth ; but Dalton asserts, that the quan tity is about five inches in the same pa ned. M. Prevost made some curious ex periments, to ascertain why dew should be deposited on glass, when it did not adhere to metal almost in contact : plates of metal fixed on glass are sometimes covered by dew, and at others the case is reversed ; in the latter instance they are bounded by a dry zone : if the other surface of the glass is exposed, the part opposed to the metal remains perfectly dry, and if the metal is applied again, it will not prevent the deposition. The ex periment may be made at a window, when moisture attaches to either side. M Prevost observes, that glass is cover. ed externally, even when the air is warm est within the house, and that metal fix ed internally receives more moisture than the glass. After pursuing the subject to its utmost limits, this gentleman con cludes that the phenomena are entirely the effect of the action of heat. That de scription of dew known by the name of honey-dew is attributed to insects.
The strata of air near the surface of the earth unquestionably contains more mois ture, or vapour, than the higher parts of the atmosphere. The regions above the summits of mountains are probably very dry; and De Luc and Saussure say, the air on those they explored was less im pregnated with vapour in the night than during the day ; for as the stratum next the earth condenses and cools at the former period, there can be no doubt that each stratum descends, yet as clouds are seen to tower far above the most ele vated peaks, vapour must at particular times rise to an amazing height.
Rain never descends till the transparen cy of the air ceases, and the invisible va pours become vesicular, when clouds form, and at length the drops fall : clouds, in stead of formingually at once throughout all parts ogre horizon, gene rate in a particular spot, and impercepti bly increase till the whole expanse is ob scured. It is singular, that clouds collect And spread at a considerable height ia the atmosphere, where the air is drier than in the lower strata, which are generally overcharged with moisture. "It is equal ly remarkable," says a late writer, " that the part of the atmosphere at which they form has not arrived at the point of ex treme moisture, nor near that point, even a moment before their formation." Thus it appears, that their formation does not proceed from a greater quantity of va pour accumulating than could remain in the atmosphere without passing its maxi mum. M. De Luc asserts, that the heat of clouds exceeds that of the surround ing air in some particular instances ; hence their formation cannot arise from the capacity of air for combining with moisture being decreased by cold, as clouds may frequently be observed, which, after floating through the atmo sphere during the heat of the day, disap pear at night when the heat diminishes : thus we might proceed to prove that clouds do not originate in the way sup posed by many observers, and that we arc still ignorant in what manner vapour is disposed of after it enters the atmosphere; and why it rejects its assumed form, re turns again to vapour, and falls in rain ; and why evaporation should prevail dur ing very hot and dry seasons, without vi sibly saturating the whole atmosphere. Theories in this instance are of very little use, as the subject is evidently placed too far out of our reach for experiment : in this state of uncertainty we must have re course to facts.