mensions of the glass, the heat proceeding from the external atmosphere. The portion of the glass, therefore, covered by the tin foil, becomes colder than any other part of the pane, and the tin foil itself receives the same temperature, which is not reduced by the effect of the radiation of objects in the room, because the tin foil itself is a good reflector of heat, and a bad absorber. Hence the tin foil presents a colder surface to the atmosphere of the room than any other part of the surface of the pane, and, consequently, receives a more abundant deposition of ice. If a body, which is a good radiator of heat, be exposed in a situation where other good radiators are not present, it will have a tendency to fall in its temperature below the temperature of the surrounding medium; because, in this case, while it loses heat by its own radiation, its absorbing power is not satisfied by a corresponding supply of heat from other objects. A clear sky, in the absence of the sun, has scarcely any sensible radiation of heat: if, therefore, a good radiator be exposed to the aspect of an unclouded firmament at night, it will lose heat considerably by its own radiation, and will receive no corresponding portion from the radiation of the firmament to repair this loss, and its temperature consequently will fall. A curious experiment made by Dufay affords a striking illustration of this fact. He exposed a glass cup, placed in a silver basin, to the atmosphere during a cold night, and he found in the morning a copious deposition of moisture on the glass, while the silver vessel remained perfectly dry. He next reversed the experiment, and exposed a silver cup in a glass basin. The result was the same: the glass was still covered with moisture, and the metal free from it. Now metal is a bad radi ator of heat, and, consequently, has a tendency to preserve its temperature. Glass is a much better radiator, and has, therefore a tendency to lose its temperature. These vessels being exposed to the aspect of a clear sky, received no considerable rays of heat to supply the loss sustained by their radiation. This loss in the metal was inconsiderable; and, therefore, it maintained its temperature nearly or altogether equal to that of the air; the glass, however, radiating more abundantly, and absorbing little, suffers a depression of temperature. The glass, therefore, presented a cold surface to the air contiguous to it, and reduced the temperature of that air, until it attained that temperature at which it was below a state of saturation with respect to the vapour with which it was charged; a deposition of vapour, therefore took place on the glass." This observation of Dufay was attended with no practical result, but was considered as merely an interesting experiment, until Dr. Well's attention was drawn to it; and he perceived in it a foundation for explaining the mode in which dew is formed. The vulgar notion relative to the formation of dew is, that moisture is deposited from the air in consequence of the coldness of night, which was the opinion held by Aristotle, who also remarks that it seldom appears but on clear nights. Now the fallacy of this mode of explanation will be sufficientiy apparent if we consider that the cause of the deposition of moisture on any substance, must arise from a difference between its temperature and that of the adjacent air: when a glass of cold water is brought into a room, it must have been often remarked that its surface is covered immediately with moisture, but if the water be warm, no such deposition takes place. This fact alone is sufficient to prove that there could be no dew unless those substances on which it is deposited were colder than the external air : now as they are freely exposed, how does it arise that they become colder than the adjacent air? Dr. Wells explained this most satisfactorily by a series of observations and experiments, by which he proved the formation of dew to arise from the radiation of heat from the surface of the earth. He observed that dew is deposited only during clear nights, when there are no clouds interposed to prevent the radiation of the heat into free space, or to reflect back the heat given off from the surface of the earth. On exposing substances whose radiating powers were different, to a clear and cloudless sky, he found that quantities of dew were deposited on each of them proportional to their several dispositions to give heat: when he exposed wool, wood, glass, metal, &c. he found that the metal acquired the least dew, which accords with what we have stated relative to the radiating power of metallic surfaces, and also serves to explain the experiment of Dufay, just mentioned, wherein the glass vessel was found covered with dew, while the silver was quite free from moisture. The reason why it seldom freezes on cloudy nights, and that generally a clear moonlight, or bright starry night was formerly thought productive of cold, is therefore apparent from this admirable theory of Dr. Wells, who also observed that the temperature of the earth was sensibly raised by the interposition of clouds during a clear night, and immediately lowered on their passing away from that portion of the heavens over the place chosen as the subject of examination. We shall conclude this part of our subject by a quotation from Dr. Wells' Essay relative to the radiation of heat from the surface of the earth. "I had often smiled in the pride of half knowledge at the means employed by gardeners to protect plants from cold, as it appeared to me impossible that a thin mat, or any such flimsy covering 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 became, 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 I had before deemed useless. Being desirous of acquiring some precise information on this subject, I fixed perpendicularly in the earth of a grass plot, four small sticks, and over their upper extremities, which were six inches above the grass, and formed the sides of a square, whose sides were two feet long, I drew tightly a very thin cambric handkerchief. In this disposition of things, therefore, nothing existed to prevent the free passage of air from the exposed grass to that which was sheltered, except the four sticks, and there was no substance to radiate downwards except the cambric handkerchief." On examination of the grass thus sheltered it was found to have exactly the same temperature as the adjacent air, while the ground unsheltered was found to be considerably colder, having given off its heat, which was not reflected back by any awning, as the night chosen was clear and cloudless. Hence we see that the true object of covering tender plants during cold weather is not to prevent their suffering from the cold of the adjacent air, but to prevent the loss of heat by radiation. We also can now understand the reason why plants will be effectually protected by snow, which prevents their attaining a lower temperature than freezing water, by protecting them from the effects of radiation. We shall conclude this short sketch of some of the phenomena of radiation by explaining the process of procuring ice in Bengal, in which upwards of three hundred persons are constantly employed. We wish to observe that a different solution of the process was formerly given by Dr. Black; but as it is now understood to depend on the same principle as the formation of dew, and has been satisfactorily accounted for by Dr. Wells, by the theory of radiation-we think it necessary to do more than merely advert to the former erroneous explanation. We shall extract Dr. Lardner's account of the mode of its formation. "A position is selected where the ground is not exposed to the radiation of surrounding objects: a quantity of dry straw being strewed on the ground, water is placed in flat unvarnished earthen pans, so as to expose an extensive surface to the heavens; the straw being a bad conductor of heat, intercepts all supply of heat which the water might receive from the ground; and the porous nature of the pans allowing a portion of the water to penetrate them, produces a rapid evaporation, by which a considerable quantity of the heat of the water is carried off in the latent state with the vapour. At the same time, the surface of the water radiates heat upwards, while it receives no corresponding supply from any other radiator above it. Thus heat is dismissed by evaporation and radiation; and, at the same time, there is no corresponding supply received either from the earth below, or from the heavens above. The temperature of the water contained in the pans is thus gradually diminished, and at length attains the freezing point. In the morning the water is found frozen in the pans; it is then collected and placed in caves surrounded with straw, which being a bad conductor of heat, prevents any communication of heat from without by which the ice might be liquefied. In this way ice may be preserved during the hottest seasons, for the purposes of use or luxury.* We have now considered a few of the subjects contained in Dr. Lardner's most useful volume and regret that the nature of our publication has prevented our entering as minutely as we could wish into the several parts of it: we have been necessarily obliged to pass over very superficially even those branches of our subject of which we undertook the consideration, and to omit all notice of some of the most important subjects connected with this branch of science. We have not entered upon the subjects of specific heat which led to some of the most important improvements in the steam engine, nor evaporation, ebullition, or liquefaction: those who wish for information of these most useful and important topics we refer to the several chapters of Dr. Lardner's work, where they will find them explained clearly and perspicuously, without any sacrifice of scientific accuracy, or that simplicity which is the object sought to be attained in all the volumes of the Cabinet Cyclopedia. If we might indulge the hope that the imperfect sketch here given of a part of this subject was acceptable to our readers we would, at some future period, perhaps when the " Dog Star rages," consider that highly interesting phenomena afforded by the cooling processes of nature. We must now conclude these few remarks on one of nature's most active and necessary agents; one on which the very form of existence depends, as we find that heat regulates the state of all bodies, as the most solid may be rendered fluid or aeriform by great additions of heat, and vice versa, that all aeriform matter, by the abstraction of heat may be rendered liquid or solid, being thus led" to regard heat as one of the great maintaining powers of the universe; and to attach to all its laws and relations a degree of importance which may justly entitle them to the most assiduous enquiry." • That the process of procuring ice at Bengal, does not depend solely on the cold produced by evaporation, as was supposed by Dr. Black, and has been assumed as the true explanation of Mr. Lunn in his treatise in the Encyclopædia Metropolitana, but on the reduction of temperature produced by radiation, will be quite evident, if we consider merely the facts detailed relative to the process; it is mentioned by Mr. Williams in his account of the formation of ice, that the nights on which it is procured, are clear and calm, and that the straw on which the earthenware pans are placed must be quite dry; now wind would encrease the evaporation, and wetting the straw would also diminish the temperature, if evaporation were the cause of the congelation; it is consequently quite evident, that though evaporation may assist the process, yet that it is not at all a principal cause of the production of the ice. FAMILIAR EPISTLES FROM LONDON. No. I. ADVANTAGES OF MAIL COACH TRAVELLING-RETROSPECTION AND SENTIMENTALITY-IMPROVEMENTS IN LONDON-DIFFICULTY OF GETTING OUT OF LONDON -TAKING LODGINGS-POLITICS-THE QUARTERLY REVIEW-THE REFORMED HOUSE. My dear O'Brien I arrived here about three weeks ago by the mail. It was not my will that consented to this mode of travelling; but remittances had not come, and when one cannot have one's own vehicular convenience and post horses, the next resource for a gentleman, who wishes to keep moving, is his Majesty's mail. There is a despatch about it, and precision, and consequence, and high prices, which most favourably distinguish it from its cheap and nasty competitors on the road, with their heavy luggage outside, and heavy vulgarity within. I was accidentally forced to make use of one of them about six months ago, and found myself jammed in between three gross looking persons with horribly fat knees, who had boiled ham and biscuits in their pockets, talked radicalism until dusk, and then drew on red night-caps, and began the most abominable snoring. I felt exceedingly tempted to cut their throats, but was deterred by considerations of cleanliness. I made up my mind, however, that for the future no consideration or necessity, short of reaching some old gentleman or lady already in extremis, who was likely to leave me an estate, should induce me to embark in a coach that was not the King's, or my own, for the time being. Although years and the world have pretty well worn away the excessive tendency to the pensive, or tearful, or dhrimmindhru frame of mind, which in my early youth made me waste my precious time upon bad poetry and worse flute-playing; yet I confess to you that when I approached London once more, the vivid recollections it brought to my mind and heart of byegone times was more than my stoicism was able to master, and though I clenched my teeth, and muttered psha with my lips, it would not do, and I shed tears. Five hours before I would have deemed this utterly impossible, but there is nothing of which we know so little, till the occasion comes, as our own feelings Do you remember O'Brien?-to be sure you do; that glorious summer evening, when you and I, and poor George, made our first entry into this mighty city. What excitement of spirits-what wonder and expectation we felt, and what bursts of joyous gaiety from him, the youngest and liveliest of the three, who now withers in the grave! but I'll not think of this. The mail from S passes by the end of the road where old Lady C. and Ellen lived, and where we have so often walked together, and spent happier evenings than I shall ever spend again. I have visited the old lady's grave, and I have seen Ellen, aye Ellen herself, and her husband! They have a monstrous fine house and a whole retinue of servants, but no children, for which I felt-God forgive me, something like gladness, or gratification, or I know not what. Either there is a lurking fiendishness in our nature, or I am a bad specimen of humanity settle it how you will. I was at all events glad to get out of the house again, for when I saw that face, though it is not what it was, and heard that voice which is less altered, though not to my hearing the same, my heart was wrung, and I could with difficulty maintain the steady cold composure, which I would have rather than have lost. died on the spot tain it, and got me away to the Regent's Park to walk and think. "Ye winged hours that o'er us past, But this is folly; I'll begin again by There are wonderful changes, and what is still more wonderful in these days, great improvements in the geography of our "ancient neighbourhood" since we were here five years ago. When I walked forth from the Salopian in the morning, and looked up for the old Golden Cross Inn, where we used to go to bed to feast the fleas, and listen to the rattling of coaches, and do without sleep; lo! it was clean gone-not a vestige of it there, more than if it had never been. An immense space now laid open behind the statue of Charles, with a fine sweep right and left to the Strand and Pall Mall. The houses of St. Martin's Lane, from the Church down to the Strand, are swept away, and a fine new range has been built, terminating with the beautiful portico of the Church. By the bye a great dispute has lately arisen about this portico, which a certain modest architect who designed the London University (so called) says is not beautiful at all, except in the eyes of the vulgar. This assertion is equally idiotic and impudent, and the man who has made it is laughed at for his pains. This new range forms the right hand boundary of the space I have just told you of, when you look from Whitehall, and the Union Club-house and the College of Physicians form the left; so you may judge how wide it is, In depth it extends northward to the King's Mews, which they say is to be pulled down, and a National Gallery for paintings and sculpture built on the side. All the vile neighbourhood lying between Chandos-street and the Strand has been completely swept away, and new streets made, forming various openings into the Strand, which itself has been widened from Charing-cross to Bedford-street, and new houses built on the North side. All these valuable and beautiful improvements were designed, and nearly completed under the Tory Government. The Whigs would have been afraid to have attempted them, because all the money they have cost, and it is no trifle, has been given to bricklayers, and carpenters, and labourers. The Whigs want so much for their own hangers-on, that they cannot ask for money to be employed in this way. But I have forgotten to tell you of the other improvements to the westward in the same neighbourhood.— When you were here, Regent-street was the "New-street," and came down, as you will recollect, directly upon the front of Carlton House. That fine House with its beautiful portico and screen towards Pall Mall, has been carried away, every stick and stone of it, and the line of Regent-street now continues right forward to St. James's Park, to which you descend by a flight of steps; a plain lofty pillar has been raised to the memory of the Duke of York; and on each side, ranges of magnificent houses, with plots of ornamental ground between them, and the back of the houses in Pall Mall occupy the old site of the gardens of Carlton House. Descending into the Park, still more improvements present themselves. The interior, which you may remember was a huge field, occupied generally by sorry-looking cows, a sluggish canal in the centre, and a shabby wooden paling for the circumference, is turned into an ornamental planted enclosure. Well-cut walks lead round a fine piece of water formed by widening and deepening the old canal, and round the whole there is an open iron railing. Here scores of people come to walk on week days, and thousands on a Sunday. I am sorry that the quiet and the shade of the walk under the wall of the Carlton House garden is lost, and monstrous |