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like these; they have a tendency to soften, improve, and purify the heart and what is man without that link of affection which binds him to the world, and makes him feel that there are those whose happiness is in his keeping; that he has a trust worth all the world beside, and which all should feel who reflect on the proper objects of life? Feelings like these are at variance with that cold and selfish indifference which marks the progress of thousands through life-thousands who have never experienced the sweet throbs of a heart full of tenderness and affection, but whose lives, untaught in the school of kindness and benevolence, are totally void of interest, and whose absence is unlamented. The very situation of the seaman is calculated, more than most others, by a thousand dangers that continually surround him by night and by day-in every storm, every rock, and every shoal, by which he is assailed, or near which he is doomed to pass-to teach him his constant dependence upon a Will superior to human will, and upon an Eye that can penetrate far beyond the human eye. But whilst he is swiftly wafted to some foreign land, a thousand hopes and fears respecting his own dear home are continually rushing upon his mind, and he is led to sympathise in anticipations for afflictions that may never come, and participate in joys that may never be realised. This state of trial, to which the feelings are constantly subject, will produce those virtues and that peculiar disinterestedness which is said, with truth, to be a distinguishing characteristic of the seaman.

The benefit derived by nations, societies, and individuals, from his efforts, surpass all human calculation. Whether we go back to the earlier ages, and estimate the advantages of the discovery of entire continents, or consider (at this era of the world) the knowledge, wealth, and power derived from commerce, we shall feel our obligation and our dependence upon each other for the growth of improvement, and for the happiness of this enlightened age. That interchange of good wishes between nations, as between individuals, which is the peculiar trait of the wise and benevolent, is made more certain and more secure by the facility with which it is effected. The productions of every climate are brought within our reach; the manners, habits, and customs of every continent and every island are placed before our eyes; the mechanic, the philosopher, and the statesman equally acknowledge the happy result of that interchange of ideas which so materially adds to the increase of knowledge.

Whilst abroad, surrounded by fleets of other nations, his heart beats high, and he breathes with greater effort as he catches a glance at the flag of his own country; and a native of that country, however far removed and insignificant, becomes at once a brother. Mingling in a throng where every face is new, and every sympathy closed against him, he finds his resources of enjoyment wholly within himself; it is then he feels the absence of his friends and the once delightful joys of home. But this intercourse with the world, however, enlarges his mind, gives a freer scope to his imagination, a more correct knowledge of human nature, and teaches a deference for the opinion of others he learns how much man can suffer, and how much enjoy.

As the land on which he recently trod, and apparently so large and so boundless, recedes from his sight, and his vision is bounded by the ocean and the sky, he is led to contemplate more frequently the wisdom and the power of that Being who created and sustains the thousands of worlds with which this universe is fitted up, as well as the insignificance and the uncertain destiny of man. If he be permitted to escape the miseries of shipwreck, loss of health, and famine, and again reach the termination of his trials and privations, how will his heart bound within him as he passes the threshold of his own dear home, where all crowd around to welcome him with smiles of affection! How many anxious questions are upon every tongue-what afflictions, what changes, what lost, and what gained! and the events of a year pass before him in one short hour. His anxieties are allayed, and again happy in the bosom of his family, or surrounded by his friends, his past sufferings are as a tale told and for ever forgotten.

THE SKYLARK.

"Bird of the wilderness,

Blithesome and cumberless,

Light be thy matin o'er moorland and lea! Emblem of happiness!

Bless'd is thy dwelling-place!

O to abide in the desert with thee!

Wild is thy lay and loud,

Far in the downy cloud ;

Love gives its energy, love gave it birth. Where, on thy dewy wing,

Where art thou journeying? Thy lay is in heaven, thy love is on earth.

O'er fell and fountain sheen,

O'er moor and mountain green,
O'er the red streamer that heralds the day !
Over the cloudlet dim,

Over the rainbow's rim,
Musical cherub, hie, hie thee away!

Then when the gloaming comes, Low in the heather-blooms, Sweet will thy welcome and bed of love be! Emblem of happiness!

Bless'd be thy dwelling-place!

O to abide in the desert with thee!"

Ettrick Shepherd.

OUR LITERARY LETTER-BOX.

OUR readers may perhaps recollect, that the first communication, on opening the Letter-Box, was one from Bath, signed "Peter Grievous." We expected that it would have called forth some observation from other correspondents; but no notice was taken of it, except a good-natured, but very prosing, communication, in which the writer wanted to preach; talked about a Bank of Faith, resignation, &c. &c. in such a manner as to show that he com. pletely misapprehended our object in printing, and, we presume, Peter's We have now, however, object in writing, the particular letter alluded toreceived the following from Clonmell:

"Clonmell.

"Sir,-As you have commended 'Peter Grievous's' candid and goodhumoured exposition of his case to all your readers, in the hope that some of them would suggest some matter for your future consideration; and as I have not seen his case attended to by any of your correspondents, allow me, an humble individual, to remark, that the profession of an artist, industriously followed, could not fail to better his condition, and might probably (with his present income, which is sufficient to 'keep the wolves from the door, without shaking a muscle or stirring a limb ') enable him to become the 'perfect personification of a gentleman.'

"With the abilities which shine through Peter's description, and his time being at his own disposal, his little smattering of the fine arts' might, from

six months' application, enable him to produce good bold pencil-sketches from nature, which in their locality would sell quickly at a moderate price; and than expensive highly-finished drawings. In search of landscapes, his home

from the facility with which they can be executed, might be more profitable

circuit would become agreeable and even up the Rhine' might not (in time) be an 'unprofitable spec.'

“I submit this in the hope that it may be of some assistance in opening a 'brighter prospect' for 'Peter,' and remain your constant reader,

"J. H. R."

TO THE EDITOR OF THE LONDON SATURDAY JOURNAL. "SIR,-We have already enjoyed the advantages of 'cheap postage,' and it is evident that the increase of letters which have been sent through the post has been a great public accommodation; and so far as the revenue has deen sustained, has produced no correspondent inconvenience or expense in the working of the plan the same apparatus being employed as before.

“A great principle having been thus employed to an important end, I wish

to know, Mr. Editor, whether the same might not be employed in obtaining cheap travelling-the importance of which, in a country like this, needs no comment. The great fact insisted on, when cheap postage was first recommended, was, that the expense of conveying a letter was inappreciable for long or short distances; and that, by a great reduction in the charge of conveyance, such an increase of letters might take place as would in time leave the revenue but little injured.

"The charge made for the conveyance of passengers by railroad in the firstclass carriages is usually about 3d. per mile-a rate pretty uniform with that charged for outside coach-passengers on turnpike-roads; but the railroads carry goods at about canal price; the coaches on turnpike-roads never under about 1d. per lb. for 100 miles, which is not much less than the charge for passengers, if reckoned by weight-and as the power of horses is limited, and the use of that power expensive, this charge does not appear excessive, nor do I imagine it could much be reduced. By the railway, a passenger and his luggage, which together may average about 2 cwt., is charged for 100 miles about 2 s., while 2 cwt. of goods are carried for about from 2s. 6d. to 4s. 6d., according to the competition existing by canal; and such goods are conveyed at a speed nearly equal to that of I wish to know whether these passengers. railway charges bear a just proportion to the actual cost of conveyance, and whether five or seven times as many passengers might be conveyed for nearly the same expense as the present number, if the carriages were constructed with reference to it? Each carriage holding six persons weighs about four tonsthus carrying only about 12 cwt. Could not carriages be constructed which would bear a large additional number of passengers and their luggage, without requiring much additional strength ?-If so, the expense of engine power would be but little increased, and the fares might be greatly reduced.

"It may be objected, that the proprietors of railways would not risk such a reduction of the fares, at an uncertainty of correspondent traffic. Admitting this would be a valid objection, might not the income be secured by selling tickets for the year (whether transferable or not, I will not determine) at so much for a given number, thus giving individuals the option of travelling on the line several times for the same sum? By some such plan (I do not enter into details, satisfied they would present no real difficulty), I think, the proprietors of railways might be secured from loss, and even materially increase their dividends, to the great advantage of the public and themselves.

"I am desirous of knowing if the principle of cheap postage could be so applied; for, if the actual expense of conveying a passenger be very small, why not (if a certain amount of traffic can be secured) allow the public to have more trips for the same money? Let another Rowland Hill start to establish cheap railway fares.'

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A GLASGOW READER.-Tobacco-smoking, like dram-drinking, is, on the whole, a mere idle and nasty habit, and, as generally practised, is too often associated with low and dissipated tastes. This opinion is given with reference to the strong and often filthy stuff used as tobacco in this country. But smoking, like bathing, is an Eastern luxury, of which John Bull, with his beerguzzling and cloudy tendencies, has but a dim idea. To persons whose minds are much exposed to excitement, nothing can be more soothing and grateful than to inhale a mild and fragrant tobacco-it is a tranquillising sedative, and, to sedentary persons especially, frequently gently stimulates a languid stomach, and aids in quieting nervous irritation. But the tobacco used generally in Britain is a two-edged sword, acting as a stupifying narcotic, and creating unnatural excitement, by irritating the stomach and provoking thirst. temperance and abstinence societies should direct their efforts as much against British tobacco as against British gin or whiskey.

The

Y. Z., PLYMOUTH (the signature, we presume, of a lady) asks about the words "mama" and "papa," whether they are "merely fashionable terms, or have any reference in their origin to father and mother?" By looking into a dictionary-say Richardson's-under the head "mama," she will find:"Without doubt, the word is formed by Nature herself, since all infants of all nations begin to speak with this word, as the most easy of pronunciation; being in fact formed solely by the compression of the lips." A similar observation may be made on "papa," and as both terms, slightly modified, are nearly universal, we may consider their origin to be a common one. Under "Papa," Richardson gives us, Greek, Пarras; French, Pape (Pope); Italian, Spanish, and Latin, Papa; Persian, Ba-ba; Arabic, Baaba, &c. Our correspondent may easily see the origin of mammalia, mammalogy, the term for the division and description of the noblest animals, including man; and of the Papacy, the Papal States, &c. "Father" and "mother," like "mama" and "papa," are words widely diffused, as Persian Pader, Italian Padre, French Père, Dutch Vader, German Vater, Latin Mater (maternal), Italian Madre, German Mutter, &c.

G. L. H., KIRRIEMUIR, asks about a good Dictionary of the English Language; and as we are quoting from one, we will recommend it to him"A New Dictionary of the English Language, by Charles Richardson. London: Pickering." It is admirable, and will give our correspondent all he wants.

"SIR,-I am anxious to avail myself of your Letter-Box,' on a subject which, I trust, you will not deem frivolous or unimportant. To be as economical as possible of time and space, I will comprise it in the few following queries:

"1. What, out of the many and various plans that have been propounded for facilitating composition and improving style, do you consider the most practicable, and the best adapted to the case of an individual whose style is rambling and verbose, and yet whose time is so much occupied with pressing and professional avocations as to render a severe and regular discipline in composi tion almost impracticable?

"2. Or, which of our writers, ancient or modern, do you think the safest model in style and systematic arrangement?

"I am afraid you will consider the above queries as vague, crude, or commonplace; but I assure you they are submitted to you in all honest sincerity, by one who, being in a situation of professional responsibility, is anxious to remedy, as soon as possible, the deficiencies of which he is deeply conscious. "Doncaster." "ADOLESCENS.”

We know a very

We hardly know how to answer this correspondent. worthy and a very clever man, who has again and again appeared before the public as an author, and has again and again met with failures which would have damped the courage of less ardent minds, whose fault-and probably cause of failure-is the one hinted at by Adolescens. A mind of very considerable knowledge (though not of originality) and an ardent enthusiasm are buried under a "rambling and verbose" style, and rendered literally useless. Yet no advice can cure this man, who is otherwise a very amiable person.

There can be no doubt that style is generally a characteristic of mind, and that, therefore, when a person writes in a "rambling and verbose" manner, the fountain and the stream must have something in common. We will throw "word-mongers" out of the question-those wise rhetoricians whose thoughts are so much exhausted on their manner that they have none to spare for their matter-and consider the case of a really intelligent person who wishes to put matter into his sentences. Let him, if he wishes to cure the defect of a "rambling and verbose" style, first, thoroughly understand what he is going to write about; secondly, aim at saying all he has got to say in a clear, unaffected, and direct manner; and, thirdly, try to occupy as small a space as possible. A writer whose mind is full of all sorts of floating ideas, and who, in his anxiety to exhaust his theme, drags in all sorts of allusions, references, quotations, metaphors, similes, &c. must, almost of necessity, be "rambling and verbose," because he overlays his matter with words. If he had to defend himself from some serious accusation, and were only allowed a given space within which to make his defence, it is very probable that his rambling verbosity would vanish, and that he would make his statement with a lucidity and directness which would perhaps astonish himself.

Our correspondent doubtless knows that Dr. Johnson recommends those who wish to write the English language, to give their days and nights to the volumes of Addison. Dr. Johnson's authority is higher than we pigmies of the present day are sometimes disposed to admit; and Addison stands foremost amongst our English classics. But, with all deference, we should say—study no model whatever, for it may make the writer a poor mannerist; read, for the purpose of filling the mind with knowledge, and the imagination with images; and when you write, avoid imitation as far as possible, eschew all attempts at spinning fine phrases, and resist all tendency to affectation and circumlocution. Then, if what is written has not at least the merit of being clear, distinct, and direct and marked by a certain individuality, the writer may rest assured that wordjingling will as little make an author, as rolling pebbles in one's mouth will make an orator.

All Letters intended to be answered in the LITErary Letter-Box are to be addressed to "THE EDITOR of the LONDON SATURDAY JOURNAL," and delivered FREE, at 113, Fleet-street.

The VOLUMES of the LONDON SATURDAY JOURNAL may be had as follows:VOLUME I., containing Nos. 1 to 26, price 5s. 6d. in cloth. VOLUME II., containing Nos. 27 to 52, price 5s. 6d. in cloth. VOLUMES I, and II. bound together, containing the Numbers for 1839, price 10s. 6d. in cloth.

BACK NUMBERS and PARTS, to complete Sets, may always be obtained.

London: WILLIAM SMITH, 113, Fleet Street. Edinburgh: FRASEK and Co. Dublin: CURRY and Co-Printed and Stereotyped by Bradbury and Evans, Whitefriars.

LONDON SATURDAY JOURNAL

No. 78.]

PUBLISHED BY WILLIAM SMITH, 113, FLEET STREET.

SATURDAY, JUNE 27, 1840.

HOW DEW IS FORMED.

[PRICE TWOPENce.

taken by itself incontrovertible, is made the basis of a spacious superstructure of error. Without following up the views enterDew must be familiar to every one as those small translucent glo- tained by the Stagyrite, and even abandoning the premises which bules of water which, particularly in the morning, are found glitter- he had assumed, the Romans made a retrograde movement, and ing like innumerable gems all over the face of nature. Dew has in relapsed into the rude opinions of remote and superstitious times. all ages been an object of admiration and interest; and whilst The philosophers and poets of that great people uniformly reprepoets have found in it a natural image of purity and beauty by sent dew as falling from the heavens, as if it were a species of nocwhich to compare other objects, popular superstition in ancient turnal rain. The theory of the descent of dew long prevailed in times ascribed to it a celestial origin, and many little supernatural the schools of philosophy, and maintained its ground undisputed virtues. That it fell from heaven, was considered certain; and amid the mutations to which the other departments of science that it was an essence distilled from the heavenly bodies, was con- were subjected. Some of the views of the alchemists have sidered probable from its bright and sparkling nature; thus re- already been alluded to. These vague but often ingenious ensembling in lucid clearness "the lights that live along the sky." | thusiasts further asserted, that when the dew evanishes before the In the writings of the mythologists it is recognised as the daughter beams of the sun, it reascends to its native region in the highest of Jove and the Moon; and Plutarch says that it is most abundant heavens; and other wild opinions were entertained by them, at full moon, by which assertion he evidently implies that the moon which, however, it is unnecessary to state in this place. Nor is it has some mysterious influence over its production. Amongst the at all requisite that the fanciful speculations of Van Helmont virtues which, in the infancy of knowledge, were supposed to (which involve the subject of the production of watery fluids reside in it, popular persuasion ascribed to it the property of during the night) should be noticed. Towards the end of the cleansing from impurity, removing spots and stains, and restoring seventeenth century, a fact was discovered which, had it been reato the features, which time had impaired, the glowing charms of soned upon in the true spirit of philosophical investigation, might youthful beauty. Nay, like the celebrated elixir of times less have exploded the popular doctrine, and led to the discovery of the remote from our own, it was supposed to lengthen the term of true theory. It was remarked that a bell-glass which had been human existence; and Ammianus Marcellinus, a Roman historian placed over a plant during the night was found in the morning to of the fourth century, asserts that the greater longevity of moun- be covered with dew in the inside, whilst externally little or no taineers, as compared with the duration of life amongst the inha- moisture appeared to adhere to it. It is evident that the humidity bitants of the plains, is to be ascribed to the frequent aspersion of which formed these globules must either have risen out of the dew on their gelid bodies. It was likewise supposed to counter- ground, exuded from the plant, or existed in the small portion of act corpulency; and ladies collected the precious liquid by expos- confined air, for it could not possibly have passed through the ing clothes or woollen fleeces to the humid atmosphere of the night glass as a deposition of the superincumbent atmosphere. But for the purpose of collecting it, that by repeated applications of with the simple ascertainment of the fact the investigation seems this powerful antagonist to obesity, the symmetry and proportions to have terminated, no generalisation appearing to have taken of their fine forms might be preserved. Such being the popular place. belief in ancient times, it is not to be wondered at that those philosophical visionaries, the alchemists (who grasped eagerly at every floating superstition which held out the slightest hope of their realising their favourite dreams), believed that it was endued with supernatural powers, and employed it as an agent in all their operations; and some of them even had the audacity to affirm that, like aqua regia, it was capable of dissolving gold itself.

After those vague chimeras, which are characteristic of the infancy of nations and of science, had begun to dissipate, and give place to more profound and correct views of the phenomena of nature, juster views began to be entertained regarding dew. Amongst the Greeks, Aristotle, one of the master-spirits of antiquity, defines dew as "humidity detached in minute particles from the clear chill atmosphere." He further states that "dew is only formed beneath a calm and cloudless sky, but never in windy weather." Other observations are made respecting the circumstances most favourable to the production of dew; and, upon the whole, Aristotle's views may be called an approximation to the truth; but in informing us that dew is the humidity of the atmosphere, and only following the position up with remarks on the state of the sky best adapted to its production, he stops short at the most interesting part of the subject. This is one amongst many instances to be found in the writings of the ancient philosophers, in which disjointed fragments, as it were, of truth are given, but no grand, complete, and thoroughly established theory propounded. Often a single fact or observation, founded in truth, and

VOL. III.

In the year 1733, a considerable advance towards the truth was made by Gersten, a German professor, who propounded an opinion opposite to that of the descent of dew. This philosopher remarked, that plates of copper exposed during the night had only their under surfaces wet with dew, and found that plants exhale in various proportions the moisture which constitutes the aqueous deposit. The dissertation which embodied these views called other investigators to the field of inductive science; and a series of facts relative to the deposition of dew on plates of glass at different heights, and also the quantities deposited, ascertained by employing towels and cloths, were collected by a French philosopher called M. du Fay. A very curious discovery was soon afterwards made by Muschenbroeck of Utrecht, which was, that dew forms in very different proportions on different substances, and that not only the texture but the colour of the substance materially influences the deposition of humidity. For instance, a plate of polished metal will remain dry, whilst glass will be covered with the liquid; and a bit of red morocco leather will during a night acquire twice as much aqueous deposit as a piece of the same size but of a blue or black colour.

The experiments of Muschenbroeck were successfully repeated by M. du Fay. Electricity had about this period begun to attract universal attention, and it was customary to ascribe to the operation of this powerful and mysterious agent many effects which were otherwise difficult to be accounted for. That it should exercise some influence over the deposition of dew was by no means

Bradbury and Evans, Printers, Whitefriars.

DD

question, and the results were briefly as follows:-A metallic leaf placed upon a plate of glass not only prevented dew from forming on the covered part of the glass, but for a small space all round it, and also on the opposite side of the plate, whilst every other part of it was perfectly wet. A piece of tin-foil being pasted on the inside of a window, and another similar piece on the outside, when dew began to form first upon the inside of the pane, the tinfoil on the same side appeared wetter than the rest of the pane, all except that part of it opposite the exterior coating, which invariably appeared to be drier than the rest. When the dewing commenced upon the outside, the principle of formation was exactly reversed. In every case it was found that a piece of glass placed over the metallic leaf so as entirely to cover it, completely destroyed its effects on the metal. In every case the formation of dew depended upon the nature of the extreme surfaces of bodies.

These interesting, but somewhat perplexing, facts for some time constituted a problem which philosophers were inadequate to solve. Now, however, every difficulty has been satisfactorily explained, and the theory of the formation of dew is as well understood as any other part of natural philosophy. 1st. The whole depends upon the facility with which bodies part with the heat which they contain. 2d. Bodies only acquire dew according to the rapidity with which they cool when exposed to the free aspect of the heavens. Now, bright metals, although good conductors of heat, do not part with it so readily on exposure to air; whereas with glass, as well as with a great variety of other substances, the very reverse is the case: hence, whilst the glass rapidly gave forth its heat, became cool, and consequently attracted dew, that portion of the pane which was covered with metallic leaf was prevented from giving forth its heat; and thus dew could not

an improbable notion; and accordingly M. du Fay, who had signalised himself in this department of science, prepared to put the matter to the test of experiment. He grounded his reasoning on the fact, that since some bodies have the power of imbibing, or rather receiving, a greater portion of dew than others, it might depend upon these bodies being electrics in the one case, and conductors in the other. In order, therefore, to compare the humifying action of vitreous with that of resinous substances, he took two basins of the same dimensions, one of tin, and the other of glass; the former he thickly coated with shell-lac-a resinous body-and exposed it along with the one made of glass during a fine clear night. The result was, that the glass basin contained twice as much aqueous deposit as that which was coated. A very curious fact also came under his notice during these investigations, namely, that a watch-glass placed on a porcelain saucer will receive five or six times more dew than one similarly placed on a silver saucer. That the metallic body in this case prevented the deposition of dew is evident; and it was farther proved by an experiment in which one end of a slip of glass was made to rest on a brass plate, and the other on one of glass. The end which reposed on the former was found quite dry, whilst the opposite extremity was profusely wetted. The next philosopher of eminence who made observations on this subject was Professor Leslie. He was greatly assisted in his experiment by the hygrometer, an instrument which, in the year 1798, he had brought nearly to a state of perfection. The term hygrometer is compounded of two Greek words, and signifies a measurer of moisture; hence its usefulness in determining the state of humidity of the atmosphere at any given period of the evening. The results of Leslie's investigations were confirmatory of the principle discovered by Du Fay,that dew always began to form at the surface of the earth, and continued to mount upwards as the night advanced. The follow-collect upon it, according to Leslie's theory of pulsations of air. ing explication of the phenomena of dew by Leslie explains a phenomenon which must often have been observed, viz., the formation of a fog or low cloud. "In fine calm weather, after the rays of the declining sun have ceased to warm the surface of the ground, the descent of the higher mass of air gradually chills the undermost stratum, and disposes it to dampness, till their continued intermixture produces a fog or low cloud. Such fogs are, towards the evening, often observed gathering in narrow vales, or along the course of sluggish rivers, and generally hovering within a few inches of the surface. But in all situations these watery deposits, either to a greater or a less degree, occur in the same disposition of the atmosphere. The minute suspended globules, attaching themselves to the projecting points of the herbage, form dew in mild weather, or shoot into hoar frost when cold predominates. They collect most readily on glass, but seem to be repelled by a bright surface of metal." The provision of nature for compensating the daily vicissitudes of climate is exceedingly beautiful. It is evident that the sun's rays striking the surface of the earth during the day will be absorbed by it, and thus the stratum of air contiguous to the mass of the globe will be retained in a state of comparative warmth and dryness, at least in clear and calm weather. The more elevated strata, on the other hand, will be much colder, as is known to be the fact by those who ascend high mountains, or rise to great heights in balloons. When the sun sinks below the horizon, and exercises no longer any calorific influence over the earth, the lower or warm strata of air, by virtue of their lightness, bulk for bulk, compared with those higher up, ascend, their place being supplied by the colder strata from the upper regions. This interchange goes on perpetually during night and day; and thus the descent of chill air explains the formation of dew in low situations first, and its progressive deposition at higher elevations as the cold increases. This is the theory of Professor Leslie, but another explanation of the phenomena will be found further on.

The remarkable discovery of Du Fay: already noticed-namely, that the presence of a piece of metal will prevent dew from accumulating even on glass, which otherwise would have been profusely bedewed-remained to be explained. A set of very curious experiments were undertaken by M. Benedict Prevost to determine the

The impression received on a plate of polished metal scarcely amounts to the tenth part of what is communicated to a surface of glass, wood, cloth, paper, earth, or grass. Various circumstances, which will be afterwards noticed, tend materially to modify the distribution of heat and the formation of dew on the surfaces of bodies.

The

Our knowledge of the principles upon which dew is formed, was greatly advanced by the researches and experiments of Dr. Wells; and amongst other facts of importance we are indebted to him for the very important one-that bodies become colder than the neighbouring air before they are dewed. It is a rare circumstance that the simplest truths of science are the soonest discovered; and the remark holds good in the present instance, for various investigators, amongst whom were the learned Dr. Wilson of Glasgow, and Mr. Six of Canterbury, had propounded the doctrine that the formation of dew was always accompanied by the production of cold. Thus cold was supposed to be the effect instead of the cause of aqueous globules collecting on the surfaces of bodies. learned and ingenious Dr. Wells, however, set philosophers right upon this subject, having prosecuted his researches with assiduity and ardour for upwards of two years at a friend's villa in the neighbourhood of London, in spite of professional demands upon his time, and the precarious state of his health. After a protracted drought, when the air was very still and the sky serene, and about half an hour before sunset, Dr. Wells exposed to the sky parcels of wool and swan-down, which had been previously weighed. They were placed upon a smooth, unpainted, and perfectly dry fir table. Twelve minutes after sunset the wool was found to be fourteen degrees colder than the air, and had, as yet, acquired no additional weight. The swan-down, of which there was a much greater quantity, had not become so cold as the wool by one degree, and remained likewise without any increase of weight. Twenty minutes more brought the wool down half a degree lower than the surrounding atmosphere, and still there was no dew found upon it. The grass was, at the same time, fifteen degrees colder than the air four feet above the ground. By an induction of facts at once ample and conclusive, the learned experimenter established the proposition stated, that the coldness of

bodies invariably precedes the formation of dew upon them, and this, in favourable circumstances, continued to increase somewhat progressively during the whole night, so that from midnight to sunrise the deposition was even greater than from sunset to midnight.

That substances having a free exposure imbibe dew to a greater extent than those which have a covering over them, was proved by Dr. Wells in the following manner :-He took two parcels of wool of ten grains each, and placed one of them under a sheet of pasteboard bent into the shape of a penthouse, and the other upon a grassplat fully exposed to the sky. In the morning, the sheltered wool was found to have increased only two grains in weight, whilst that which remained uncovered had imbibed sixteen grains of dew. He further found that parcels of wool, each weighing ten grains, being teased out into flattened balls of two and a half inches diameter, and laid on a grassplat, on a gravel-walk, and on fresh garden-mould, acquired, during a clear calm night, respectively sixteen, nine, and eight grains of humidity. Repose of a body seems necessary to its acquiring its utmost degree of coldness and its full deposit of dew; so that a moving body will not become so humified as one at rest. Hence those who are exposed to nightair effect two purposes by walking smartly-they not only keep their bodies warm, but prevent the chilling dews from settling profusely upon their clothes.

We have seen in the experiments of Dr. Wells, that a covering placed at a little distance above the body will prevent the dew from settling upon it, to the same extent as would have been the case had no such screen between it and the sky been interposed. Hence we find that, in cloudy nights, little or no dew is formed. A very great change takes place in a short time in the temperature of the air, if the night, having been previously clear, becomes cloudy, or having been cloudy, becomes clear. In the former case, the thermometer rises considerably, and the deposition of dew is checked; and in the second instance the thermometer falls considerably, and dew begins rapidly to form. In every case, however, the quantity of dew deposited depends upon the state of the atmosphere with regard to moisture. Sometimes there is little humidity in the air, at other times a great deal. Dew is more abundant in the spring and autumn than at other seasons, and of course is always very copious when the atmosphere is inclined to humidity. When the air is in a proper state, a very few degrees of difference of temperature between the grass and the atmosphere is sufficient to determine the formation of dew. At other times, a difference to the extent of thirty degrees may exist without any deposition taking place. Insulated bodies or prominent points, such as the spicular extremities of vegetables, become sooner covered with dew than others, because they are sooner deprived of heat; and the effects of an open exposition may be seen in the fields every morning. There the leaves of trees and shrubs will often be found to have remained dry, or nearly so, during the night; whilst the blades of grass are seen densely covered with shining globules. It is a fact mentioned by Aristotle, that dew does not form in windy weather; and Dr. Wells informs us, that if the night, from being clear and serene, becomes dark and stormy, the dew which had already been deposited will disappear.

"I had often," says Dr. Wells, "smiled in the pride of halfknowledge at the means frequently employed by gardeners to protect tender plants from cold, as it appeared impossible to me 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 I had before deemed useless. Being desirous, however, of acquiring some precise information on this subject, I fixed perpendicularly in the earth of a grassplat four small sticks, and over their upper extremities, which were six inches above the grass, and formed the corners 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 small sticks; and there was no substance to radiate downwards to the latter grass, except the cambric handkerchief." On examination, the sheltered grass was found nearly of the same temperature as the air, whilst the unsheltered was five degrees colder; and on another occasion, the grass destitute of any covering was found eleven degrees colder than the air, whilst that over which the handkerchief was spread was only three degrees colder. From these experiments we see the propriety of sheltering, even with a very thin covering, those plants and vegetables to whose healthy growth warmth is necessary. Snow acts in winter as a protector from cold; but to have the full advantage of any artificial covering, it should be placed a little above the subjacent body. In tropical climates, the deposition of dew on animal substances hastens their decay; and as this is apt to happen only on clear nights, it was anciently supposed that moonlight was a promoter of animal decomposition.

In reference, first, to the cooling of bodies under an open sky, and, second, to the prevention of this by the interposition of clouds or other covering above the bodies, there are two theories by which phenomena have been explained. By the one, the coldness induced on the ground is said to result from the radiation of heat into free space; and the prevention of this is supposed to arise from the clouds radiating heat back to the earth. By the other, it is supposed that the pulses of cold air accumulated by that vertical play already described, in which the chill strata of air descend and the warm strata rise, is the cause of the temperature falling, and the consequent formation of dew. Clouds, or any other interposing screen, will disturb this process; but we confess, although Leslie be the author of this theory, that, taken alone, it does not explain phenomena in the same simple and explicit manner as the other. However, there seems no reason for doubting that both causes may operate, and that, under certain circumstances, the effects produced by the latter may predominate over those produced by the former. What is called the doctrine of latent heat has also been brought forward to explain the increase of heat at the earth's surface on a cloudy night. That the condensation of transparent vapour into cloud, a denser body, causes an extrication of heat, there can be no doubt; but that this would continue during a whole night, is highly improbable. But the thermometric observations of Dr. Wells render the opinion utterly untenable. He found that the sudden intervention of a cloud gave rise to as sudden an elevation of temperature, and sudden clearness to an equally rapid fall in the mercury of the thermometer. Moreover, we find the atmosphere itself, at moderate elevations, of a pretty uniform temperature; whilst bodies at the surface of the ground undergo rapid alternations of heat and cold. This fact alone is fatal to the theory derived from the doctrine of latent heat.

The importance of dew in the economy of nature cannot be But in fully appreciated in a moist northern climate like ours. tropical regions, where the seasons are divided into wet and drythat is, months in which a great quantity of rain falls, and months in which there is almost none at all,-during the protracted drought vegetation is necessarily parched, and stands much in need of moisture; so that the deposition of vapour in the form of dew is of incalculable benefit to it. Upon the same principle as that on which dew is formed, ice may be procured in warm climates-the East Indies, for instance,-when the temperature of the atmosphere is above the freezing point. This is effected by simply exposing water in shallow vessels to the aspect of a calm and cloudless sky. Of all substances, water is the one which radiates caloric best, and accordingly, from rapidly parting with its heat, congelation upon the surface immediately takes place.

DECORUM.

No man can be good enough to enable him to neglect the rules of prudence; nor will Virtue herself look beautiful, unless she be bedecked with the outward ornaments of decency and decorum.—Fielding.

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