An Essay on Dew and Several Appearances Connected with It

Front matter

AN

ON

,

Dedication

To JAMES DUNSMURE, Esquire,

Merchant in London.

Introduction

ESSAY ON DEW,

&c.

INTRODUCTION.

I was led, in the autumn of 1784, by the event of a rude experiment, to think it probable, that the formation of Dew is attended with the production of cold. In 1788, a paper on Hoarfrost, by Mr. Patrick Wilson of Glasgow, was published in the first volume of the Transactions of the Royal Society of Edinburgh, by which it appeared, that this opinion had been entertained by that gentleman, before it had occurred to myself. In the course of the same year, Mr. Six of Canterbury mentioned in a paper communicated to the Royal Society, that, in clear nights, he always found the mercury lower in a thermometer laid upon the ground, in a meadow in his neighbourhood, than it was in a similar thermometer suspended in the air, six feet above the former; and that, upon one night, the difference amounted to 5° of Fahrenheit's scale. Mr. Six, however, did not suppose, agreeably to the opinion of Mr. Wilson and myself, that the cold was occasioned by the formation of dew, but imagined, that it proceeded, partly from the low temperature of the air, through which the dew had descended, and partly from the evaporation of moisture from the ground, on which his thermometer had been placed. The conjecture of Mr. Wilson, and the observations of Mr. Six, together with many facts, which I afterwards learned in the course of reading, strengthened my opinion; but I made no attempt, before the autumn of 1811, to ascertain by experiment, if it were just, though it had, in the mean time, almost daily occurred to my thoughts. Happening, in that season, to be in the country, on a clear and calm night, I laid a thermometer upon dewy grass, and suspended a second, in the air, two feet above the other. An hour afterwards, the thermometer on the grass was found to be 8° lower, by Fahrenheit's division, than the one in the air. Similar results having been obtained from several similar experiments, during the same autumn, I determined, in the next spring, to prosecute the subject with some degree of steadiness, and with this view went frequently to the house of one of my friends, who lived in Surrey. At the end of two months, I fancied that I had collected information worthy of being published; but fortunately, while preparing an account of it, I met, by accident, with a small posthumous work by Mr. Six, printed at Canterbury in 1794, in which are related differences observed on dewy nights, between thermometers placed upon grass, and others in the air, that are much greater than those mentioned in the paper presented by him to the Royal Society in 1788. In this work, too, the cold of the grass is attributed, agreeably to the opinion of Mr. Wilson, altogether to the dew deposited upon it. The value of my own observations appearing to me now much diminished, though they embraced many points left untouched by Mr. Six, I gave up my intention of making them known. Shortly after, however, upon considering the subject more closely, I began to suspect, that Mr. Wilson, Mr. Six, and myself, had all committed an error, in regarding the cold, which accompanies dew, as an effect of the formation of that fluid. I, therefore, resumed my experiments, and having, by means of them, I think, not only established the justness of my suspicion, but ascertained also the real cause both of dew, and of several other natural appearances, which have hitherto received no sufficient explanation, I venture now to lay before the learned an account of some of my labours, and of various conclusions which may be drawn from them, mixed with facts and opinions which have been published by others.

Part I.

Aristotle and many other writers, have remarked, that dew appears only on calm and serene nights. The justness of this observation, however, has not been universally admitted. For Musschenbroek says, that dew forms in Holland, while the surface of the country is covered with a low mist; but as he mentions at the same time, that it is deposited upon all bodies indiscriminately, the moisture, of which he speaks, cannot properly be called dew, as will be more distinctly seen hereafter. Other writers of considerable reputation have also regarded clearness of the atmosphere, as not being requisite for the production of dew, misled, I believe, partly by theory, and partly by observing on misty mornings copious dews, which had been produced during preceding clear nights. Respecting this point I can aver, after much experience, that I never knew dew to be very abundant, except in serene weather. In regard to the necessity of the air being still, I know of no person who rejects it, except Mr. Prieur, a late French author of little consideration, and he asserts, in opposition to the most common observation, that a fresh wind is requisite for the production of dew.

The remark of Aristotle, however, is not to be received in its strictest sense, as I have frequently found a small quantity of dew on grass, both on windy nights, if the sky was clear, or nearly so, and on cloudy nights, if there was no wind; but none has ever been seen by me on nights, that were both cloudy and windy. If, indeed, the clouds were high, and the weather very calm, I have sometimes seen, though the sky was entirely hidden, no very inconsiderable quantity of dew. Again; entire stillness of the atmosphere is so far from being necessary for the formation of this fluid, that its quantity has seemed to me to be increased, by a very gentle motion in the air.

If, in the course of the night, the weather, from being calm and serene, should become windy and cloudy, not only will dew cease to form, but that which had previously formed, will either disappear, or diminish considerably.

In calm weather, if the sky be partially covered with clouds, more dew will appear, than if it were entirely covered, but less than if it were entirely clear.

Dew probably begins, in this country, to form upon grass, in places shaded from the sun, during clear and calm weather, soon after the heat of the atmosphere has declined. My opportunities, however, for making such observations have not been numerous, since, while pursuing this subject, I seldom went into the country, till late in the afternoon; but I have frequently felt grass moist, in dry weather, several hours before sunset. On the other hand, I have scarcely ever known dew to be present in such quantity upon grass, as to exhibit visible drops, before the sun was very near the horizon, or to be very copious, till some time after sunset. It continues likewise to form, in shaded places, after sunrise; but the interval between sunrise, and its ceasing to form, is, according to my observation, which, upon this point, has not been extensive, considerably shorter, than that between its first appearance in the afternoon, and sunset. Contrary, however, to what happens at sunset, if the weather be favourable, more dew forms a little before, and, in shaded places, a little after sunrise, than at any other time. Muss- chenbroek, therefore, errs greatly when he says, that dew does not form after the sun has risen. The preceding observations, on the early appearance of dew in the afternoon, are to be restricted to what happens to grass, or other substances highly attractive of dew, placed on the ground; for it occurs much later on similar substances, which are elevated a few feet above the ground, though upon these it continues to form, as long after the rising of the sun, as upon the former, if they be equally sheltered from the rays of that body.

The formation of dew, after it has once commenced, continues during the whole night, if the weather remain still and serene. Mr. Prieur, indeed, of whom I have already spoken, asserts, that dew forms only in the evening and morning, and that any which forms in the former season always disappears in the course of the night. I can affirm, however, from long experience, that grass, after having been dewed in the evening, is never found dry until after sunrise, unless the weather has, in the mean time, changed. Upon one serene and still night, I placed fresh parcels of wool upon grass every hour, and by weighing each of them, after exposure for an hour, found, that they had all attracted dew.

When dew forms upon a smooth dense body as glass, and it is only by means of such a body, that the process can be accurately observed, the appearances are altogether similar to those, which occur on a like body, when exposed to the steam of water, a little warmer than itself. The exposed surface has first its lustre diminished, by a slight damp uniformly spread over it. As the moisture increases, it gathers into irregularly shaped flat drops, in the beginning very small, but which afterwards enlarge and run into one another, forming streamlets, by means of which it escapes from the body which had received it.

During nights, that are equally clear and calm, dew often appears in very unequal quantities, even after allowance has been made, for any difference in their lengths. One great source of these differences is very obvious. For it being manifest that, whatever theory be adopted concerning the immediate cause of dew, the nearer the atmosphere approaches the state of saturation with water, previously to the operation of that cause, the more copious will the precipitation of moisture be, after this operation has commenced, all the circumstances, which tend to increase the quantity of moisture in the atmosphere, must likewise tend to increase the production of dew. Thus dew, in equally calm and clear nights, is more abundant immediately after rain, than during a long tract of dry weather. It is more abundant, also, throughout Europe, with perhaps a few exceptions, and in some parts of Asia and Africa, during southerly and westerly winds, than during those, which blow from the north and the east. Aristotle says, that Pontus is the only country, in which dew is more copious during a northerly, than a southerly wind. But a similar fact occurs in Egypt; for dew is scarcely ever observed there, excepting during the prevalence of the Etesian winds. Both cases, however, though contrary to the letter, are consonant with the spirit of the rule; as the north wind, in one country, proceeds from the Euxine sea, and, in the other, from the Mediterranean. Another circumstance, of the same kind with the blowing of wind from the south and the west, as shewing that the air contains much moisture, is the lessening the weight of the atmosphere. My experience on this point has not, indeed, been great, as the falling of the mercury in the barometer is very commonly attended with wind or clouds, both unfavourable to the production of dew; but still the greatest dew, I have ever witnessed, occurred while the barometer was sinking. A similar observation is made by Mr. de Luc, who says that rain may be foretold, when dew is uncommonly abundant, in relation to the climate and season.

To the greater or less quantity of moisture in the atmosphere, at the time of the action of the immediate cause of dew, are likewise to be referred several other facts respecting its copiousness, the reason of which is not so apparent, as in the preceding examples. Dew, for instance, is commonly more plentiful in spring and autumn, than in summer; since a greater difference is generally found between the temperatures of the day and the night, in the former seasons of the year, than in the latter. In spring, this circumstance is prevented often from having a considerable effect, by the opposite influence of northerly and easterly winds; but during still and serene nights in autumn, dew is almost always highly abundant.

In the second place; dew is always very copious, on those clear and calm nights, which are followed by misty or foggy mornings, the turbidness of the air in the morning shewing, that it had already, during the preceding night, arrived nearly at the point of being saturated with moisture.

Thirdly; I have observed dew to be unusually plentiful on a clear morning, which had succeeded a cloudy night. For the air having, in the course of the night, lost little or no moisture, was in the morning more charged with watery vapour, than it would have been if the night had also been clear.

Fourthly; heat of the atmosphere, if other circumstances are favourable, which, according to my experience, they seldom are in this country, occasions a great formation of dew. For as the power of the air, to retain watery vapour in a pellucid state, increases much faster than in proportion to its heat, a decrease of its heat, in any small given quantity, during the night, must bring it, if the temperature be high, much nearer to the point of saturation, before it be acted upon by the immediate cause of dew, than if the temperature were low. We read, accordingly, in the writings of those, who have travelled into hot climates, of a copiousness of dew frequently observed by them there, which very much exceeds what occurs, at any time, in this country. But even here, dew, though for the most part scanty in our hottest season, is sometimes very abundant during it, an example of which occurred to me on the night, common to the 29th and 30th of July 1813; for on that night, notwithstanding its shortness, more dew appeared than has ever been observed by me on any other.

In the last place; I always found, when the clearness and stillness of the atmosphere were the same, that more dew was formed between midnight and sunrise, than between sunset and midnight, though the positive quantity of moisture in the air must have been less in the former, than in the latter time, in consequence of a previous precipitation of part of it. The reason, no doubt, is the cold of the atmosphere being greater in the latter, than in the first part of the night.

But there are many circumstances, influencing the quantity of dew, which, though much more open to accurate observation, than those hitherto mentioned, are yet much less easy to be understood.

In my first attempts to compare the quantities of dew, formed during different times, or in different situations, I attended only to the appearance, which it made on bodies having smooth surfaces. But quickly seeing this method to be very imperfect, I next employed wool to collect dew from the atmosphere and found it well adapted for this purpose, as it readily admits amongst its fibres the moisture, which forms on it, and retains what it receives so firmly, that I never but once had occasion to suspect, that it suffered any part of what it had acquired, to pass entirely through it. The wool, which I used, was white, moderately fine, and already imbued with a little moisture, from having been long exposed to the air of a room, in which no fire was kept. This was divided into parcels of 10 grains each. Immediately before exposure, the fibres of every parcel were pulled somewhat asunder, so as to give it the form of a flattened sphere, the greatest diameter of which was about two inches. As in doing this, I went by the judgment of my sight alone, some little inequality must have existed among different parcels, but none, I think, sufficient to affect the accuracy of my conclusions from the experiments, in which they were employed, more especially as my conclusions scarcely ever rested upon single trials.

Previously to mentioning the particular purposes for which these parcels were exposed to the air, I think it right to describe the scene of by far the greater part of my observations. This was a garden in Surrey, distant, by the public road, about four miles from the bridge over the Thames at Blackfriars, but not more than a mile and a quarter from a densely built part of the suburbs, on the south side of that river. The form of the garden was oblong, its extent nearly half an acre, and its surface level. At one end was a dwelling house of moderate size, at the other a range of low buildings; on one side a row of high trees, on the other a low fence, dividing it from another garden. If this fence had been absent, the garden would have been on the latter side entirely open. Within it were some fruit trees, but as it had not been made long, their size was small. Towards one end, there was a grassplat, in length 62 feet, and nearly 16 broad, the herbage of which was kept short by frequent mowing. The rest of the garden was employed for the bearing of culinary vegetables. All of these circumstances, however trifling they may appear, had influence on my expe- riments, and most of them, as will hereafter be seen, must have rendered the results less remarkable, than they would have been, if they had occurred on a wide open plain, considerably distant from a large city. I now proceed to relate the influence, which several differences in the situation, mechanical state, and real nature of bodies, have upon the production of dew.

One general fact relative to situation is, that whatever diminishes the aspect of the sky, with regard to the exposed body, occasions the quantity of dew, which is formed upon it, to be less than it would be, if the exposure to the sky were complete.

I placed, on several clear and still nights, 10 grains of wool upon the middle of a painted board 4½ feet long, 2 feet wide, and 1 inch thick, elevated 4 feet above the grassplat, by means of 4 slender wooden props of equal height, and at the same time attached, loosely, 10 grains of wool to the middle of its underside. The two parcels were consequently only an inch asunder, and were equally exposed to the action of the air. Upon one night, however, I found, that the upper parcel had gained 14 grains in weight, but the lower only 4. On a second night, the quantities of moisture, acquired by like parcels of wool, in the same situations as in the first experiment, were 19 and 6 grains; on a third, 11 and 2; on a fourth, 20 and 4; the smaller quantity being always that, which was gained by the wool attached to the lower side of the board.

I bent a sheet of pasteboard into the shape of a house-roof, making the angle of flexure 90 degrees, and leaving both ends open. This was placed one evening, with its ridge uppermost, upon the same grassplat, in the direction of the wind, as well as this could be ascertained. I then laid 10 grains of wool, on the middle of that part of the grass, which was sheltered by the roof, and the same quantity on another part of the grassplat, fully exposed to the sky. In the morning, the sheltered wool was found to have increased in weight only 2 grains, but that, which had been exposed to the sky, 16 grains.

In these experiments, the aspect of the sky was almost entirely cut off from the situations, in which little dew was formed. In others, where it was less so, the quantity gained was greater. Thus 10 grains of wool placed upon the spot of the grassplat, which was directly under the middle of the raised board, and which enjoyed, therefore, a considerable oblique aspect of the sky, acquired, during one night 7, during a second 9, and during a third 12 grains of moisture, while the quantities gained, during the same times, by equal parcels of wool, laid upon another part of the grassplat, which was entirely exposed to the heavens, were 10, 16, and 20 grains.

As no moisture, falling like rain from the atmosphere, could, on a calm night, have reached the wool in any of the situations, where little dew was formed, it may be thought, that the substances, which sheltered them, prevented, mechanically, the access of that fluid. But this supposition would not explain why some dew was always found in the most sheltered places, and why a considerable quantity occurred upon the grass under the middle of the raised board. But a still stronger proof of the want of justness in this supposition is afforded by the following experiment. I placed, upright on the grassplat, a hollow cylinder of baked clay, the height of which was 2½ feet, and diameter 1 foot. On the grass, surrounded by the cylinder, were laid 10 grains of wool, which, in this situation, as there was not the least wind, would have received as much rain, as a like quantity of wool fully exposed to the sky. But the quantity of moisture, obtained by the wool surrounded by the cylinder, was only a little more that 2 grains, while that acquired by 10 grains of fully exposed wool was 16. This occurred on the night, during which the wool under the bent pasteboard gained only 2 grains of moisture.

Dew, however, will, from various circumstances, form in very different quantities, upon substances, of the same kind, although these should be similarly situated, in regard to the sky.

In the first place it is requisite, for the most abundant formation of dew, that the substance attracting it should rest on a stable horizontal body of some extent. Thus, upon one night, while 10 grains of wool, laid upon the raised board, increased 20 grains in weight, an equal quantity, suspended 5½ feet above the ground, increased only 11 grains. On another night, 10 grains of wool gained on the raised board 19 grains, but the same quantity suspended in the air, on a level with the board, only 13; and on a third, 10 grains of wool acquired, on the same board, grains of weight, during the time in which other 10 grains, hung in the air, at the same height, acquired only ½ a grain.

In the second place, the quantities of dew attracted by equal masses of wool, similarly exposed to the sky, and resting on equally stable and extended bodies, sometimes vary considerably, in consequence of a difference in some of the other circumstances of those bodies. 10 grains of wool, for instance, having been placed upon the grassplat, on a dewy evening, 10 grains upon a gravel walk which bounded the grassplat, and 10 grains upon a bed of bare garden mould, immediately adjoining the gravel walk; in the morning, the wool on the grass was found to have increased 16 grains in weight, but that on the gravel walk only 9, and that on the garden mould only 8. On another night, during the time that 10 grains of wool, laid upon grass, acquired 2½ grains of moisture, the same quantity gained only ½ a grain upon the bed of garden mould, and a like quantity, placed upon the gravel walk, received no accession of weight whatever.

Two objections will probably be made against the accuracy of these experiments. One is, that wool placed on grass may, by a kind of capillary attraction, receive dew previously formed on the grass, in addition to its own. To this I answer, that wool in a china saucer, placed on the grass, acquired very nearly as much weight, as an equal parcel immediately touching the grass. The second objection is, that a part of the increased weight in the wool might arise from its imbibing moisture, as a hygroscopic substance. I do not deny, that some weight was given to the wool in this way; but it may be safely affirmed, that this quantity must have been very small. For on very cloudy nights, apparently the best fitted to increase the weight of hygroscopic substances, wool upon the raised board would, in the course of many hours, acquire little or no weight; and in London, I have never found 10 grains of wool, exposed to the air on the outside of one of my chamber windows, to increase, during a whole night, more than ½ a grain in weight. When this weight was gained, the weather was clear and still; if the weather was cloudy and windy, the wool received either less or no additional weight. This window is so situated, as to be, in great measure, deprived of the aspect of the sky.

It being shewn, that wool, though highly attractive of dew, was prevented, by the mere vicinity of a gravel walk, or bed of garden mould, for only a small part of it actually touched those bodies, from acquiring nearly as much dew, as an equal parcel laid upon grass, it may be readily inferred, that little was formed upon themselves. In confirmation of this conclusion, it may be said, that I never saw dew upon either of them. Another fact of the same kind is, that while returning to London from the scene of my experiments about sunrise, I never observed, if the atmosphere was clear, the public road, or stone pavement on the side of it, to be moistened with dew, though grass within a few feet of it, and painted doors and windows of houses not far from it, were frequently very wet. If, indeed, there was a foggy morning, after a clear and calm night, even the streets of London would sometimes be moist, though they had been dry the day before, and no rain had in the meanwhile fallen. This entire, or almost entire, freedom of certain situations from dew depends, however, much more upon extraneous circumstances, than upon the nature of the substances found there; for river sand, though of the same nature with gravel, when placed upon the raised board, or upon grass, attracted dew copiously.

Another difference in point of situation relates to elevation in the air. A substance, at the distance of several feet above the ground, though in this situation later dewed, than if it touched the earth, would, notwithstanding, if placed upon a stable body of some extent, such as the raised board lately mentioned, acquire more dew during a very still night, than a similar substance lying on grass.

The last difference of this kind, of which I shall speak, occurred among bodies lying on different parts of the raised board. For one, that was placed at the leeward end of it generally acquired more dew, than a similar body at the windward extremity.

Difference in the mechanical state of bodies, though all other circumstances be similar, has likewise an effect on the quantity of dew, which they attract. Thus, more dew is formed upon fine shavings of wood, than upon a thick piece of the same substance. It is chiefly for a similar reason, I believe, that fine raw silk, fine unwrought cotton, and flax, were found by me, to attract somewhat more dew, than the wool I employed, the fibres of which were thicker, than those of the other substances just mentioned.

The only bodies, as far as I know, whose relation to dew can be thought affected by their real nature, in any considerable degree, are the metals, when clear from rust, or soil; for all other substances, after allowance has been made for any difference, which may exist in their mechanical states, seem to attract dew in quantities not very unequal, if they be similarly situated.

This peculiarity of metals consists in their remaining often dry upon their upper surfaces, though equally exposed to the atmosphere with other bodies, which in consequence of such exposure have become wet. Musschenbroek was the first, who distinctly remarked this fact; but Dufay, I believe, published it before him, referring, at the same time, the discovery to its proper author. Both Musschenbroek and Dufay, however, made too large an inference from their experiments; for they asserted, that dew never appears on the upper surface of bright metals, whereas the contrary has since been observed by many persons, and I have myself known dew to form on gold, silver, copper, tin, platina, iron, steel, zinc, and lead. Dew, however, never forms nearly so copiously on metals as upon other substances; commonly, it only sullies the lustre of their surface, and even when it is sufficiently abundant to gather into drops, these are almost always small and distinct. Two other facts of the same kind are; first, that the dew, which has formed upon a metal, will often disappear, while other substances in their neighbourhood remain wet; and secondly, that a metal, which has been purposely moistened, will often become dry, though similarly exposed with bodies which are attracting dew. This inaptitude to attract dew, in metals, is communicated to bodies of the other class, which touch or are near to them. For I have found, that wool laid upon a metal will acquire much less dew, than an equal quantity laid upon grass in the immediate vicinity.

A large metallic surface resists the formation of dew more powerfully than a small one. I conclude also, from various collateral facts, that a considerable difference, in the thickness of two pieces of metal, will be attended with a similar consequence, though I have no observation, which proves this directly.

If a metal be closely attached to a substance of some thickness, which attracts dew powerfully, the attraction of the metal itself for dew, instead of being increased from this circumstance, becomes diminished, provided the metal cover the whole of the upper suface of the lower body. On the other hand, if only a part of this body be covered, the production of dew on the metal is forwarded by the conjunction, and this somewhat in pro- portion, to the quantity of surface in the lower body left uncovered. The justness of the first of these observations is proved by the following experiment. I joined, in the form of a cross, two pieces of very light wood, each 4 inches long, a third of an inch in breadth, and 1 line in thickness. To one side of this cross I fastened, by means of mucilage, a square piece of gilt paper, and then exposed the instrument to the sky, with its metallic side uppermost, on a dewy night, by suspending it, in a horizontal position, a few inches above the ground. A few hours after, the unattached parts of the metalled paper were found covered with minute drops of dew, while those, which adhered to the cross, were dry.