been very much cooled, to the grass under the handkerchief, must have deprived the latter of part of its heat; another, that the handkerchief, from being made colder than the atmosphere by the radiation of its upper surface to the heavens, would remit less heat to the grass beneath than what it received from that substance. But still the sheltered grass, notwithstanding these drawbacks, was upon one night 80, and upon another 11° warmer than grass fully exposed to the sky, which are differences sufficiently great, to explain the utility of a very slight shelter to plants, in averting or lessening injury from cold, on a still and serene night.

In the next place, in order to learn whether any difference would arise from placing the sheltering substance at a much greater distance from the ground, I had four slender posts driven perpendicularly into the soil of a grass field, so as to be six feet eminent above the surface, and to form the angles of a square having sides eight feet in length. Over these was thrown an old ship flag of a very loose texture. Concerning the experiments carried on by means of this disposition of things, I shall only say, that they led to the conclusion, as far as the events of different nights could rightly be compared, that the higher shelter had the same efficacy with the lower, in preventing the occurrence of a cold upon the ground, in a clear night, greater than that of the atmosphere, provided the oblique aspect of the sky was equally excluded from the spots on which my thermometers were laid.

'On the other hand, a difference in temperature, of some magnitude, was always observed on still and serene nights, between bodies sheltered from the sky by substances touching them, and similar bodies, which were sheltered by a substance a little above them. I found, for example, upon one night, that the warmth of grass, sheltered by a cambric handkerchief raised a few inches in the air, was 3° greater than that of a neighbouring piece of grass which was sheltered by a similar handkerchief actually in contact with it. On another night, the difference between the temperatures of two portions of grass, shielded in the same manner as the above mentioned, from the influence of the sky, was 4o. Possibly, experience has long ago taught gardeners the superior advantage of defending tender vegetables from the cold of clear and calm nights, by means of substances not directly touching them; though I do not recollect ever having seen any contrivance for keeping mats or such like bodies at a distance from the plants, which they were meant to protect.

Walls, I believe, as far as warmth is concerned, are regarded as useful during a cold night, to the plants which touch them, or are near them, only in two ways; first, by the mechanical shelter which they afford against cold winds, and secondly, by giving out the heat which they had acquired during the day. It appearing to me, however, that on clear and calm nights, those on which plants frequently receive much injury from cold, walls must be beneficial in a third way, namely, by preventing, in part, the loss of heat which they would sustain from radiation if they were fully exposed to the sky; the following experiment was made for the purpose of determining the justness of this opinion.

A cam

A cambric handkerchief was placed perpendicularly to a grassplat, by means of two upright sticks, at right angles to the course of the air, and a thermometer was laid upon the grass close to the lower edge of the handkerchief, on its windward side. A thermometer thus situated was several nights compared with another lying on the same grassplat, but on a part of it fully exposed to the sky. On two of these nights, the air being clear and calm, the grass close to the kandkerchief was found to be 4° warmer than the fully exposed grass. On a third, the difference was 6°. An analogous fact is mentioned by Gersten, who says, that a horizontal surface is more abundantly dewed, than one which is perpendicular to the ground.'

Dr. Wells has been singularly fortunate in illustrating the formation of ice in warm climates, which he has shown to depend on the radiation of heat, and not, as had generally been supposed, on the refrigerating effect of evaporation. It is necessary, for the success of this process, that the air should be still, which is a circumstance unfavourable to evaporation; it is found to succeed best in dewy nights, when the quantity of evaporation must be inconsiderable; the straw on which the pans containing water are placed, must not be wet, in order that it may not communicate heat from the ground, and the pans must be porous for a similar reason. A cold of 14°, or more, is often required for the purpose, and Dr. Wells found that evaporation in still air, at a low temperature, did not produce a cold of above a degree or two. He succeeded in freezing water in this country without any evaporation, when the air a few feet above the ground was at 37° or even 39°; the temperature of grass fully exposed being at the same time 30°. In Mr. Williams's experiments, the straw, on which the pans stood, appeared warmer than the water, because it was much sheltered by them from the sky. Dr. Wells found that the bottom of an empty pan kept pace in cooling with the pans of water, until the congelation took place; some moisture was deposited on it, which afterwards froze; and in another experiment, the water itself had gained some grains in weight, while part of it was frozen, in an atmosphere of 37°.

In reasoning respecting the heat transmitted by mists, Dr. Wells observes, that since the diminution of light, as ascertained by Leslie's photometer, is small, it will readily be granted that the sanie state of the atmosphere will also give transit to radiant heat :' it must however have occurred to him on reflexion, that the indications of Leslie's instrument depend immediately on radiant heat, and are only applied indirectly to light; so that there is no occasion for the analogy from which he has derived his argument. It is not with a view of detracting from the merit of our author's laborious series of experiments, that we feel ourselves compelled to enter a protest against the total novelty of the opinions which

they

they have so amply illustrated and confirmed. Dr. Wells appears, in his historical account of the doctrines relating to the nature and causes of dew, to have undertaken to afford us complete information respecting the sentiments not only of Aristotle and Theophras tus, but also of the most distinguished' philosophers of modern times, p. 131 some of the works, however, of the persons whom he mentions, and some of the latest, have most unaccountably escaped his attention.

'Mr. Prevost of Geneva,' says Dr. Wells, in his work on radiant heat, has already in this way accounted for the effect of clouds, in diminishing the cold of the atmosphere at night; but he seems not to have known, that they have a much greater effect of the same kind on the temperature of bodies upon the surface of the earth. My explanation of the latter operation of clouds is a direct consequence from the facts which I had observed respecting the prevention of cold on the ground from radiation, by the interposition of solid bodies between it and the heavens, and occurred to me in 1812. Mr. Prevost's work, indeed, was published in 1809, but I did not see it before the summer of 1813, when it was lent to me by his relation Dr. Marcet of London, who at the same time said, that he believed there was no other copy of it in Great Britain, except one, which had been sent by himself to Edinburgh. p. 79.

Now we have at this moment before us a copy of Mr. Prevost's Recherches Physicomécaniques sur la Chaleur, printed at Geneva in 1792; from which, for the sake of greater authenticity, we shall extract some passages in the original language.

'SECT. 24. Phénomène. La nuit, lorsque le ciel est serein, l'air est généralement plus froid près de la terre. Au printems et en automne, il gèle peu lorsque le ciel est couvert. Souvent enfin, par une nuit séreine, s'il vient à passer un nuage par le zénith de l'observateur, à l'instant il voit monter le thermomètre.

SECT. 25. Essai d'explication. L'air même le plus dense, tel que celui de nos plaines, est perméable à la chaleur rayonnante; car c'est danscet air qu'on observe celle-ci. L'air rare des régions supérieures de l'atmosphère est encore plus perméable; il est en quelque sorte transparent, ou plutôt transcaloreux. Mais l'eau ne l'est pas, ni la vapeur vésiculaire. Les nuages sont opaques pour la chaleur comme pour la lumière. Ils absorbent l'une et l'autre, et ne la laissent passer que lentement.

'Ainsi la chaleur rayonnante de la terre traverse avec facilité l'atmosphère pure, mais elle est interceptée par les nuages. Ceux-ci font donc pour la terre une espèce de vêtement. Ils empèchent l'écoulement de sa chaleur rayonnante; et en la recevant vers leur partie inférieure, ils s'échauffent de ce côté-là, comme un habit s'échauffe du côté du corps, et par conséquent ils renvoient à la terre un peu plus de chaleur rayonnante que ne peut faire l'air transparent.

La surface supérieure du nuage se refroidit, au contraire, par l'émission facile de sa chaleur dans un air raréfié. Mais le passage lent

VOL. XII. NO. XXIII.

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de la chaleur gênée, qui serpente de l'une à l'autre surface, ne peut rétablir l'équilibre incessamment rompu par la source inépuisable de chaleur du côté de la terre, et par le gouffre toujours ouvert où elle se précipite de l'autre.

" Tout nuage la nuit est donc exactement comparable à un vêtement très épais, qui recouvre un corps maintenu chaud par une cause interne et perpetuelle (tel qu'est, par exemple, le corps humain.) La surface intérieure est chaude, la surface extérieure participe à la température froide de l'air ambiant. Et l'application du vêtement sur le corps y maintient la chaleur.

On n'a pas lieu d'être surpris de la promptitude de l'effet, parceque tout le jeu de la chaleur rayonnante, allant et revenant de la terre au nuage et du nuage à la terre, s'exécute en un instant indivisible. D'ailleurs à l'instant où le nuage arrive au zénith, il arrive en quelque sorte tout préparé. Sa partie inférieure a déjà acquis une chaleur excédante. Déjà elle émet plus de chaleur rayonnante que pareille étendue d'air de la même région. C'est un lambeau de vêtement, qui passe d'une partie du corps à l'autre. Ainsi à l'instant même où ce vêtement chaud vient couvrir l'observateur, le thermomètre doit accuser sa présence.

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66

SECT. 142. Le phénomène météorologique, indiqué au Sect. 24, a été remarqué par M. Pictet, et consigné dans ses journaux d'observation, C'est ce qu'atteste l'extrait suivant, qu'il en a transcrit textuellement, et auquel il a joint une remarque importante. Janvier, 1777. Dans la nuit du 4 au 5, le thermomètre étoit à —12 [5°] à 10 heures du soir; le tems s'étant couvert ensuite, il n'étoit plus qu'à -10 [830] à 11 h. du soir. Je me rappelle distinctement, au sujet de cette note, (ajoute M. Pictet, en me la communiquant,) un fait que je ne trouve pas enregistré, c'est que le haussement de température dont il est question, fut simultanée avec l'apparition d'un nuage assez voisin, mais peu étendu, aux environs du zenith.

Un autre fait, observé par tous les agriculteurs, et relatif à l'influence prompte et presqu'immédiate des nuages sur le sol, (indépendamment de leur effet pour intercepter les rayons solaires,) est celui-ci; on sait que dans les circonstances les plus favorables d'ailleurs à l'apparition de la rosée, elle est nulle, ou presque nulle, si le ciel est couvert ; et que les blanches gêlées, si redoutables au printems et en automne, n'ont pas lieu à même température, si le temps est couvert.

Tous les faits mentionnés dans cette remarque de M. Pictet, s'expliquent naturellement par les principes posés au Sect. 25, c'est à dire, en considérant les nuages comme le vêtement du sol, et en ayant égard à la chaleur rayonnante.'

Nor were these doctrines by any means unknown in our own country; we find, for instance, in a Course of Lectures published in London seven years ago, that when the weather has been clear, and a cloud passes over the place of observation, the thermometer frequently rises a degree or two almost instantaneously: this has been partly explained by considering the cloud as a vesture, pre

venting

wenting the escape of the heat which is always radiating from the earth, and reflecting it back to the surface.'

It is true that the theory could only be completed by the application of Professor Leslie's discoveries to the circumstances of the phenomenon: bút it is remarkable that this very application was made, in a case confessedly similar, by the author of the same work which we have last quoted.

'I once intended,' says Dr. Wells, p. 105, to add here an explanation of some very curious observations by Mr. Prévost of Besançon on dew, which were published first by himself, in the 44th number of the French Annals of Chemistry, and afterwards by Mr. Prévost of Geneva, in his Essay on Radiant Heat; but fearing to be very tedious, I have since given up the design. I will say, however, that, if to what is now generally known on the different modes, in which heat is communicated from one body to another, be added the two following circumstances, that substances become colder than the air before they attract dew, and that bright metals, when exposed to a clear sky at night, become colder. than the air much less readily than other bodies, the whole of the appearances observed by Mr. Prévost may be easily accounted for.

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It has been observed,' says the author of the Course of Lectures published in 1807, that a piece of metal, placed on glass, usually protects also the opposite side of the glass from the deposition of dew; and Mr. Benedict Prévost has shown, that, in general, whenever the metal is placed on the warmer side of the glass, the humidity is deposited more copiously, either on itself, or on the glass near it,' [as in the case of the shutter]; that when it is on the colder side, it neither receives the humidity, nor permits its deposition on the glass; but that the addition of a second piece of glass over the metal destroys the effect, and a second piece of metal restores it. It appears that, from its properties with respect to radiant heat, the metallic surface produces these effects by preventing ready communication either of heat or of cold to the glass.'

Had Dr. Wells been as solicitous to attend to the labours of his contemporaries as he has been very laudably anxious to recur to those of his predecessors, he might have said, not that the experi ments of Mr. Prévost might be easily accounted for' from the properties which he mentions, but that they actually had been explained in a similar manner by one of his own countrymen, There are, however, some modern philosophers, who, whether from their own fault, or from that of their hearers and readers, or from both, appear to be perpetually in the predicament of the celebrated prophetess of antiquity, who always told truth, but was seldom understood, and never believed: and the author of the Lectures in question has not unfrequently reminded us of the fruitless vaticinas tions of the ill fated Cassandra.