tion of dew, for they had both gained the knowledge of the important circumstance that the deposition of dew is attended with cold in the bodies upon which it forms. They imagined, however, that this cold was occasioned by the formation of dew, whereas the existence of cold in the dewed body is ascertained to be prior to the formation of that liquid, and is, in fact, the cause We may illustrate this view of the subject by the most familiar examples. A wine-cooler standing on a table in a heated room, converts into dew the water which exists in the air of the room, in the state of pellucid vapour; exactly in the same way that substances, on the surface of the earth, which have become colder than the atmosphere, attract from that source the refreshing liquid now under consideration. The glass panes in our windows exhibit the same phenomenon whenever the air of our apartments is heated much beyond the external air; for the glass, being in contact with the outer air, is cooled several degrees below the temperature of the room; and thus the principle operates which produces the deposition of invisible vapour, presenting, according to circumstances, water, hoar-frost, or dew. The same thing is exemplified when a very cold liquid is poured into a bottle in a warm room; moisture forms on the outside, which according to its quantity will either stand in small drops, or run down in the form of water.

The means of explaining these facts have been afforded us by the successful labours of Count Rumford and Mr. Leslie on the subject of heat. Every one is inclined to ask, how do bodies become colder than the air which surrounds them, and how do some particular bodies acquire a greater degree of cold than others almost in contact with them? The answer is this: all bodies have less or more the property of radiating or sending forth their heat, and it has been ascertained, that such as most imperfectly conduct heat, give it out most freely by radiation. In the absence of the sun, during the night, the heat radiated by bodies on the surface of the earth, ascends into the atmosphere, and, if the sky be clear, rises, it is imagined, to a very great height; for, it is found out, that the clearer the sky is, the more rapid is the process of radiation, and the greater, of course, degree of cold produced in the radiating substances. Hence, it comes to pass that certain bodies, such as wool, flax, swandown, and vegetables, have their temperature very much diminished in a serene, calm night, and, consequently, in agreement with the law already stated, become fitted for attracting dew; while, on the contrary, metals which are among the worst radiators of heat, lose less of their temperature, and, of course, have less dew condensed upon them. In short, the more a body binks in temperature below that of the air, the more dew is

the

formed

formed on it, and the more nearly it remains on an equality with the air in point of heat, the less will it be susceptible in regard to that liquid.

It is perfectly unnecessary to say another word in explanation of Dr. Wells's theory: it is one of those discoveries which accounts for all the phenomenon connected with its subject, and at the same time derives support from all. There are, indeed, numerous facts and illustrations adduced by the author in support of his opinions, and to confirm their coincidence with natural appearances by the facility of their application in explaining them. We leave them, however, to be studied by such as take a particular interest in enquiries of this nature, and shall merely form an abstract, from the third part of the treatise, of the several appearances connected with dew. One morning in winter, Dr. Wells observed that the panes in the windows of his bedchamber were all of them moist in the inside, but that those which had been covered by an inside shutter during the night, were more moist than the others which had been uncovered, and upon examination, he found that the covered panes were 3° colder than the uncovered. The reason of this fact did not immedi ately present itself. He soon, however, perceived that the closed shutter shielded the glass from the heat radiated by the walls and furniture of the apartment, which, as nothing intervened to intercept it, would dart against the uncovered panes. On this account, and in complete unison with the general principles of his theory, the covered panes were found by our author to be colder than the others.

2d. Gardeners are known to spread a thin mat, or some slight covering, over delicate plants to protect them from the cold, and this precaution is equally well known to be attended with success, although, till lately, neither gardener nor philosopher could tell how the effect was produced. It is now understood, however, that the use of the covering is not to keep off the cold of the atmosphere, but to prevent the escape of heat by radiation from the plants; and that it will have this efficacy, was completely proved by means of a direct experiment. Dr. Wells extended a very thin cambric handkerchief over a portion of a grass plat, the skreen being six inches above the grass, and found that the spot thus covered was several degrees warmer than the neighbouring grass, which remained uncovered. The effect of a covering, however, is not so great when it touches the body protected, as when it is a few inches raised above it.

3d. It is in the same way that a fall of snow, covering the earth, protects vegetable substances during the frosts of a severe winter; it prevents the radiation of their heat.

4th. Pliny and Plutarch, among the ancients, entertained the opinion,

opinion, that the light of the moon accelerates the putrefaction of animal substances, and the same notion appears to prevail among the negroes in Africa and the West Indies, at the present day. The origin of this mistake is very probably connected with the obvious fact, that clear moonlight nights are remarkable for a copious deposition of dew-(hence roscida et rorifera luna) and dead animal substances wetted with this moisture, in hot climates, will unquestionably decompose on that account with greater rapidity.

5th. The last part treated of in this Essay, is the formation of ice during the night in Bengal, while the temperature of the air is above 32°, or the freezing point of water. It is known to all our readers, that this process is performed by placing in shallow pits, or excavations, small unglazed earthen pans, a quarter of an inch thick and 1 inch deep, filled generally with boiled soft water. The bottom of the pits are covered over with dried sugar canes, or stems of India corn, to the thickness of eight inches or one foot; and if the nights be serene and calm, ice is frequently procured even when the thermometer stands above forty. This result has been usually ascribed to the evaporation of the water from the sides of the earthen pans; but we are inclined to agree with Dr. Wells, who appears to have proved by the most satisfactory experiments, that the formation of ice in such circumstances is entirely owing to the radiation of heat from the surface of the exposed water. As a proof of this, we may mention that wind, which greatly promotes evaporation, prevents the freezing altogether, and that when the straw, placed in the bottom of the pits, becomes wet, (a circumstance which would likewise promote evaporation, and consequently the process of ice-making, if it depended on evaporation,) it is necessary to remove it, and procure a layer of dry materials, in order to insure success. Indeed, the litter of straw and canes, under the pans, seems to have no other use but that of preventing the conduction of warmth from the earth into the water. It is moreover admitted on all hands, that ice is procured on those nights ouly which are favourable for the production of dew; and, as Dr. Wells remarks, it is not to be thought that one portion of air will be depositing moisture, from possessing a superabundance of it, while another in the immediate vicinity is receiving, by means of evaporation, moisture in great quantity in the state of pellucid vapour. Nor has it been at all proved, that the cold produced by evaporation is sufficiently intense to freeze water. With the view of ascertaining this point, our author placed on a feather bed, situated between the door and window of his room, two china plates, into one of which as much water was poured as covered its bottom to the depth of a

quarter

quarter of an inch: the other plate was kept dry. Upon applying the bulb of a small thermometer to the inside of the bottom of each plate, he found that when the temperature of the air in the room was 75°, the thermometer in the plate containing water was between 6° and 7° lower than the one in the dry plate; that the difference between these thermometers diminished gradually as the air became colder; and that when the temperature of the air was 40°, the lowest of which he had taken any note, the difference was only one degree and a half. At 32°, therefore, the difference of the temperature of the wet plate, when evaporation was going on, and of the dry plate would have been very small, if at all perceptible; and this supposition agrees with an observation made by Mr. Wilson of Glasgow, who found that no cold was produced by evaporation from snow, possessing a temperature of 27°, though the air in the ueighbourhood was purposely much agitated by him to promote evaporation.

We feel grateful to Dr. Wells for the addition which he has made to our knowledge on an interesting and important subject. He has handled the matter in a very philosophical style; his statements are in general clear, and his reasoning conclusive and satisfactory; qualities of much more consequence than fine writing, which he seems rather to have contemned.

ART. III. Sermons, by the Rev. John Venn, M. A. Rector of Clapham. 2 vols. 8vo. pp. xxxii. 368 and 410. 21s. Rivingtons and Hatchard. 1814.

THE preface to these volumes contains a sketch of the life of the author, with a particular and animated draught of his character and virtues, extracted from the sermon preached on the occasion of his death. We are perfectly persuaded that Mr. Venn was a worthy, well meaning man, sincere and zealous in the discharge of his professional duties, and of attainments sufficiently respectable. The discourses before us were not prepared for the press by the author. They were selected from his manuscripts, he himself before his death having recommended such a selection as might best suit the sentiments of his friends, who inform us that the volumes before us may be received as a fair exhibition of his manner, and sentiments, and doctrine." Sermons published under such circumstances are entitled to a large share of candour, for we never can be certain that the author's selection would have been the same, and it is always probable, that in preparing for the press, he might have altered

not

Our general judgment of

not only phrases but sentiments. these discourses is, that they are generally unobjectionable in the doctrine taught, and correct in the moral practice inculcated. The author is evidently of that class of ministers which has absurdly at least, if not injuriously, assumed the exclusive and distincuve denomination of evangelical. He is more moderate and correct than men of that class in general, of whom we chiefly complain in that they cannot or will not perform that which they fancy to be their duty, without reflecting on their brethren, who also perform what they know to be theirs, with at least equal zeal and sincerity.

The evangelical ministers have picked up certain notions of preaching, which we deem particularly erroneous, and they pertinaciously appeal to their own prejudices as the just and sole criterion of gospel preaching. They never seem to distinguish between the first period of the apostolic age, between preaching anong Jews and Heathens, when the Scriptures of the New Testament were not yet written, or not yet collected, and the very different circumstances in which we are now placed, as members of a Church regularly constituted upon the foundation of the Apostles and Prophets, Jesus Christ himself being the head corner stone. On this foundation, which, on all hands it is acknowledged, that our Church has securely laid, we have an important moral superstructure to raise, in attending to which we are most erroneously accused of neglecting the foundation on which in effect we proceed, and without which it is at once acknowledged that all our doings were indeed nothing worth.

The evangelical ministers in general never get beyond the foundation, and never dream that there is the slightest difference between preaching to unconverted heathens or persons in the first initiation, and those who know and acknowledge the whole system of the gospel. Preaching with them is every thing, the sum and substance of the gospel, the mode by which it is communicated, and the means by which it is upheld. It is not a little remarkable, at the same time, that their preaching is almost altogether elementary and doctrinal; and that they accuse, even to the extent of violence and calumny, such of their brethren as proceeding on the acknowledged basis of gratuitous redemption, give all diligence to add to their faith virtue; and to virtue knowledge; and to knowledge temperance; and to temperance patience; and to patience godliness; and to godliness brotherly kindness; and to brotherly kindness charity. (2 Pet. i. 5, 6, 7.)

We mean not to apply these observations in their utmost latitude to Mr. Venn. He appears to have been not only a good man, but to have possessed much more moderation than the ge nerality of evangelical preachers. Still we perceive the general symptoms of his class, and undefined predilection for doctrinal

and