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odes, after lying for years neglected in the bookseller's warehouse, have become the storehouse from which manufacturing poets extract epithets to debase and misapply them; Beattie's delightful Minstrel; Charlotte Smith, whose descriptions, whether iu prose or verse, have always the charm of well-selected truth; Cowper who, though he is indebted for half his popularity to other causes than his real merit, is not more popular than he deserves to be; and who, in his Task, may be said to have created a new species of blank verse, so entirely does it differ in character from all former specimens. We would fain speak of the Della Cruscan swarm, who, like ephemeræ, had their summer's day; who were the heroes of newspapers and reviews for a brief season, and are now remembered only in the Baviad and Mæviad. To borrow a phrase from the Methodists, there has been a great revival in our days-a pouring out of the spirit. The publication of Percy's Reliques led the way. The passion of tragedy has been restored by Joanna Baillie, and the language of the old comedy by Tobin, who did not live to hear of the success which had so long been the object of his ambition. But we have already trespassed upon our limits, and have no room to dwell upon these topics; nor to advert to what has absurdly been called the New School, farther than to observe that the Aristarchs who for twelve years past with equal pertinacity and pertness have directed their censures against the founders (as they are pleased to style them) of that school, have by this time probably found reason to suspect that they have not been more fortunate in poetical criticism than in political prophecy.
ART. IV.-An Essay on Dew, and several Appearances connected
with it. By William Charles Wells, M.D.F.R.S. London, 1814. Svo. pp. 150. HE experiments, related in this Essay, have very clearly illus
trated the nature and formation of dew, and very satisfactorily established the ingenious author's theory respecting it; a theory, which, if not altogether so original as he supposes it, has certainly never been brought forwards in so striking and simple a form; por indeed was it possible that it should be completed, at any time previous to the important discoveries, respecting the radiation of heat, which have been made within the last ten years; although, when it is understood that the properties of all bodies, with regard to cooling, are the exact counterparts of those which they exhibit in heating, the whole difficulty of the subject vanishes.
The sun's rays pass through the atmosphere, in the absence of clouds, with little immediate effect on its temperature; they strike
on the earth, and the earth is much more heated by them than the air: in a clear night the reverse of this happens; the surface of the earth throws off heat by radiation more rapidly than the air, .and when there are no clouds to intercept and reflect it, this surface is reduced to a temperature lower than that of the air in its neighbourhood : the difference is still more marked in light substances, in imperfect contact with the earth, and Dr. Wells has shown that, in such cases, it often actually amounts to 15 or 20 degrees.
It being once established that such a cause is sufficient for the production of a greater degree of cold at the surface of the earth than elsewhere, we may easily pursue its operation through all its consequences and combinations, which however are often very complicated; but in all instances it appears, that the production of cold must be previous to the deposition of moisture, and is not, as has sometimes been suspected, a consequence of that deposition, which, on the contrary, as Dr. Wells has very fully shown, like almost all other instances of condensation, is actually attended by the extrication of a certain portion of heat.
From calculations, founded on the experiments of Mr. Dalton, and other earlier observers, we infer that air, at the temperature of the freezing point, is capable of containing, when saturated with moisture, about zbo of its weight of water in an invisible form: its capacity is doubled by raising its temperature 20°; again doubled by an elevation of 22°; then of 24°, 26°, 28°; and so on in succession. Thus at 52°, the air of a jar inverted in water will contain Too of its weight of moisture; at 74o, 5o; and at 98°, about 25. The air of the atmosphere is generally in such a state as to require a depression of a few degrees for the deposition of a portion of the moisture which it contains : a glass of pump water, or a pot of porter, from a cool cellar, becomes covered with a real dew in miniature, when brought into a room, by cooling the air in immediate contact with it. If humid but transparent air at 74° were cooled to 52°, it would deposit too of its weight of water, and zdo more if cooled again to 32°; and at all common temperatures, the depression of a single degree will occasion a deposition of a little more than 3 of the whole moisture contained in the air. Hence it is obvious that the differences of temperature, observea by Dr. Wells, must be amply sufficient to account for the deposition of dew under the circumstances which are commonly observed to occasion its appearance.
Professor Leslie, in his late work on the Relations of Air to Heat and Moisture, has estimated the quantity of water capable of being contained in air at the freezing point, from his own experiments, as equal to ado of the weight of the air, and has supposed this quan
tity tity to be always doubled by each successive addition of 27° of temperature ; so that the moisture would amount to go at 59°, and at 86° to do, instead of 5, which would be the result of our mode of determination : his estimate is therefore a little greater than ours in one case, and a little less in another; but we are disposed to prefer our own mode of calculation, because it is founded on more general views of the subject, which are sufficiently supported by a variety of experiments of different kinds.
The theory, advanced by Dr. Wells, is a consequence so simple and obvious of the principles deduced from the discoveries of Mr. Leslie, and other observers, and now generally admitted, that it only requires to be distinctly stated, and clearly understood, in order to be considered as satisfactory. At the same time it would scarcely be just to omit inserting some account of the various arguments and experiments by which our author has thought it right to enforce his doctrines ; and in pursuing this detail, we shall find a number of miscellaneous facts and remarks, which are by no means unimportant.
It was observed by Aristotle, that dew appears only on clear and calm nights: when the weather is both cloudy and windy, it is scarcely ever deposited : and Dr. Wells has found, that whatever diminishes the exposure of any substance to the unclouded sky proportionally diminishes the quantity of dew that it receives; thus ten grains of wool, placed upon a horizontal board, acquired, in the course of a night, fourteen grains of moisture, while a similar quantity, attached to the lower surface of the board, gained only four grains. Light and detached substances also receive dew much more abundantly than those which are more completely in contact with the solid earth : thus, while ten grams of wool, placed on a grassplat, gained sixteengrains in weight, another portion, placed on a gravel walk, gained only nine, and on the mould of a garden, eight: not was dew ever deposited ou the bare ground, however exposed. Polished metals seldom exhibit the appearance of dew on their surface, although pieces of metal and glass, exposed at equal temperatures to the steam of hot water, exhibited equal dispositions to attract it; so that nothing analogous to elective attraction can be supposed to take place in such cases. What is said of dew, is equally applicable to hoar frost, which, as Aristotle truly observed, is merely frozen dew.
The second step of Dr. Wells's investigation was to ascertain the thermometrical differences attending the phenomena. He once observed a thermometer, placed on the grass, 14° lower than another four feet above it; but the passage of a cloud often raised the teinperature of the grass several degrees. The wool above the board was 7° colder than the same substance immediately below it. The
surface of a gravel walk was 164° warmer than the neighbouring grass, which was similarly exposed, although the earth an inch below the grass was even warmer than the air. A very important fact in meteorology was also ascertained by these experiments, that a thermometer, fully exposed to a clear sky, often represents the temperature of the neighbouring substances 2°, 3°, or 4o below the truth; and that in order to avoid this source error, it is necessary to prevent the radiation of its heat into the empty space, by covering its bulb with gilt paper, if it is intended to ascertain the actual temperature either of the air, or of any other substance in contact with it; and an error of a contrary nature may also sometimes occur, when heat is radiated copiously by the surrounding bodies, even in the absence of the sun's direct rays. A plate of metal, lying on a plat of grass, was observed to be 10° warmer than the grass surrounding it. One of the substances which exhibited the greatest degree of comparative cold was swansdown, which was once found 15° colder than the air a few feet above it. Mr. Wilson of Glasgow had once observed snow as much as 16° colder than the atmosphere; and to this difference 2° may be added, for the correction of the temperature of the air as indicated by the thermometer. Dr. Wells even thinks it probable that, in cold and exposed countries, substances near the surface of the earth may be 30° or 40° colder than the air at a considerable height in the atmosphere.
The temperature of wool, exposed to the sky in dewy nights, was always found to be depressed below that of the neighbouring air, before it began to acquire any additional weight, and this depression was again often diminished while the dew was deposited; so that 5° or 6° of cold seem to be frequently prevented in this manner Hence it happens that the difference between the temperature of the surface of the earth and the air is less in summer than it winter, when there is less moisture to be deposited. A second caution of importance, in practical meteorology, relates to the use of the hygrometer; which, if fully exposed to the sky, may become much colder than the surrounding air, and thus exhibit a very erroneous indication, in consequence of the deposition of moisture, from air not previously saturated with it.
Theophrastus remarks, that the effects of cold are generally most hurtful in hollow places; and our author explains this phenomenon from the greater stillness of the air in confined situations, allowing the process of cooling to go on without interruption from the approach of fresh portions of air, which would afford heat both by direct communication, and in consequence of the deposition of moisture. That the air is not wholly incapable of emitting and receiving heat by radiation, as well as by direct communication, is proved by the heat of the atmosphere observable in the day time, during calm weather, in the middle of the largest oceans, while the water below it is considerably colder. Dew has sometimes been supposed to originate altogether from vapours rising out of the earth : thus a metal will often collect dew on its lower surface only, when it is of the same temperature with the air immediately surrounding it; but it is sufficiently obvious, from the experiments which have been related, that the most copious source of dew is the moisture previously contained in the atmosphere. An inside shutter often favours the deposition of moisture on a window in the night time, by preventing the radiation of heat from the room. It will be easily understood, that the effects of a clear sky must sometimes he perceived in the human body, producing, by means of the uncompensated radiation of heat, a greater sense of cold, than could be expected from the temperature of the air as exhibited by the thermometer.
' I had often,' says Dr. Wells, p. 120,“ in the pride of half knowledge, smiled at the means frequently employed by gardeners, to protect tender plants from cold, as it appeared to me impossible, that a thin mat, or any such flimsy substance, could prevent them from attaining the temperature of the atmosphere, by which alone I thought them liable to be injured. But, when I had learned, that bodies on the surface of the earth become, during a still and serene night, colder than the atmosphere, by radiating their heat to the heavens, I perceived immediately a just reason for the practice, which I had before deemed useless. Being desirous, however, of acquiring some precise information on this subject, I drove into the earth of a grassplat four slender sticks, in such a manner, as to make them rise six inches perpendicularly above the grass, and form the corners of a square, the sides of which were two feet long. Over the upper ends of these sticks were drawn lightly the four corners of a fine cambric handkerchief, rendered by long wear still thinner than it had been originally, and having here and there a slight rent. In this disposition of things, therefore, nothing existed to prevent the free passage of air from the exposed grass, to that which was sheltered by the handkerchief, except the four small sticks, and there was no substance to radiate heat downwards to the covered grass, except the handkerchief itself. The temperature of the grass, which was thus shielded from the sky, was upon many nights examined by me, and always found warmer than that of neighbouring grass, which was uncovered, if this was colder than the air. When the difference in temperature, between the air several feet above the ground and the unsheltered grass, did not exceed 5°, the sheltered grass was about as warm as the air; if that difference, however, exceeded 5°, the air was found to be somewhat warmer than the sheltered grass. Thus, upon one night, when fully exposed grass was 11° colder than the air, the latter was 30 warmer than the sheltered grass ; and the same difference existed on another night, when the air was 14° warmer than the exposed grass. One reason for this difference was, that the air, which passed from the exposed grass, by which it had