A History of Science - v.3 (Williams)

"The original cause of motion in the atmosphere is

the influence of the sun heating the surface of the earth exposed to that luminary. We have not supposed

that surface to have been of one uniform shape and similar substance; from whence it has followed that

the annual propers of the sun, perhaps also the diurnal propers, would produce a regular condensation of rain in certain regions, and the evaporation of humidity in

others; and this would have a regular progress in certain determined seasons, and would not vary. But

nothing can be more distant from this supposition, that is the natural constitution of the earth; for the globe is composed of sea and land, in no regular shape or mixture, while the surface of the land is also irregular with respect to its elevations and depressions, and various with regard to the humidity and dryness of

that part which is exposed to heat as the cause of evaporation. Hence a source of the most valuable motions in the fluid atmosphere with aqueous vapor, more or less, so far as other natural operations will admit; and hence a source of the most irregular commixture of the several parts of this elastic fluid, whether saturated or not with aqueous vapor.

"According to the theory, nothing is required for the production of rain besides the mixture of portions of the atmosphere with humidity, and of mixing the

parts that are in different degrees of heat. But we

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have seen the causes of saturating every portion of the atmosphere with humidity and of mixing the

parts which are in different degrees of heat. Consequently, over all the surface of the globe there should

happen occasionally rain and evaporation, more or

less; and also, in every place, those vicissitudes should be observed to take place with some tendency to regularity, which, however, may be so disturbed as to be

hardly distinguishable upon many occasions. Variable winds and variable rains should be found in proportion as each place is situated in an irregular mixture

of land and water; whereas regular winds should be found in proportion to the uniformity of the surface; and regular rains in proportion to the regular changes of those winds by which the mixture of the atmosphere necessary to the rain may be produced. But as it will

be acknowledged that this is the case in almost all this earth where rain appears according to the conditions

here specified, the theory is found to be thus in conformity with nature, and natural appearances are thus

explained by the theory."[1]

The next ambitious attempt to explain the phenomena of aqueous meteors was made by Luke Howard, in

his remarkable paper on clouds, published in the Philosophical Magazine in 1803--the paper in which

the names cirrus, cumulus, stratus, etc., afterwards so universally adopted, were first proposed. In this paper

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Howard acknowledges his indebtedness to Dalton for

the theory of evaporation; yet he still clings to the idea that the vapor, though independent of the air, is combined with particles of caloric. He holds that clouds

are composed of vapor that has previously risen from the earth, combating the opinions of those who believe that they are formed by the union of hydrogen and oxygen existing independently in the air; though he

agrees with these theorists that electricity has entered largely into the modus operandi of cloud formation. He opposes the opinion of Deluc and De Saussure that

clouds are composed of particles of water in the form

of hollow vesicles (miniature balloons, in short, perhaps filled with hydrogen), which untenable opinion

was a revival of the theory as to the formation of all vapor which Dr. Halley had advocated early in the eighteenth century.

Of particular interest are Howard's views as to the formation of dew, which he explains as caused by the particles of caloric forsaking the vapor to enter the cool body, leaving the water on the surface. This comes as

near the truth, perhaps, as could be expected while the old idea as to the materiality of heat held sway. Howard believed, however, that dew is usually formed in

the air at some height, and that it settles to the surface, opposing the opinion, which had gained vogue in France

and in America (where Noah Webster prominently advocated

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it), that dew ascends from the earth.

The complete solution of the problem of dew formation-- which really involved also the entire question of precipitation of watery vapor in any form--was made

by Dr. W. C. Wells, a man of American birth, whose life, however, after boyhood, was spent in Scotland (where as a young man he enjoyed the friendship of David Hume) and in London. Inspired, no doubt,

by the researches of Mack, Hutton, and their confreres of that Edinburgh school, Wells made observations on evaporation and precipitation as early as 1784, but

other things claimed his attention; and though he asserts that the subject was often in his mind, he did not

take it up again in earnest until about 1812.

Meantime the observations on heat of Rumford and

Davy and Leslie had cleared the way for a proper

interpretation of the facts--about the facts themselves there had long been practical unanimity of opinion.

Dr. Black, with his latent-heat observations, had really given the clew to all subsequent discussions of the subject of precipitation of vapor; and from this time on it had been known that heat is taken up when water evaporates, and given out again when it condenses.

Dr. Darwin had shown in 1788, in a paper before the Royal Society, that air gives off heat on contracting and takes it up on expanding; and Dalton, in his essay of 1793, had explained this phenomenon as due

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to the condensation and vaporization of the water contained in the air.

But some curious and puzzling observations which Professor Patrick Wilson, professor of astronomy in the University of Glasgow, had communicated to the Royal Society of Edinburgh in 1784, and some similar

ones made by Mr. Six, of Canterbury, a few years later, had remained unexplained. Both these gentlemen

observed that the air is cooler where dew is forming than the air a few feet higher, and they inferred that the dew in forming had taken up heat, in apparent violation of established physical principles.

It remained for Wells, in his memorable paper of 1816, to show that these observers had simply placed the cart before the horse. He made it clear that the

air is not cooler because the dew is formed, but that the dew is formed because the air is cooler--having become

so through radiation of heat from the solids on which the dew forms. The dew itself, in forming, gives out

its latent heat, and so tends to equalize the temperature.

Wells's paper is so admirable an illustration of the lucid presentation of clearly conceived experiments and logical conclusions that we should do it injustice not to present it entire. The author's mention of the

observations of Six and Wilson gives added value to his

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own presentation.

Dr. Wells's Essay on Dew

"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 hoar-frost, 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 bad 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 on clear and dewy nights he

always found the mercury lower in a thermometer laid upon the ground in a meadow in his neighborhood 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 five degrees 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, already formed in the atmosphere, had descended, and partly from the evaporation of moisture from the ground, on which his thermometer had been placed. The conjecture of Mr.

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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 that country in a clear and calm night, I laid a thermometer upon grass wet with

dew, and suspended a second in the air, two feet above the other. An hour afterwards the thermometer on

the grass was found to be eight degrees lower, by Fahrenheit's division, than the one in the air. Similar results having been obtained from several similar experiments, made during the same autumn, I determined in the next spring to prosecute the subject with

some degree of steadiness, and with that view went frequently to the house of one of my friends who lives 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

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"1. I observed one morning, in winter, that the insides

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was occasioned by the weather's being cloudy or fair, provided the heat of the internal air exceeded that of the external equally in both of those states of the atmosphere.

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"The remote reason of these differences did not immediately present itself. I soon, however, saw that

the closed shutter shielded the glass which it covered from the heat that was radiated to the windows by

the walls and furniture of the room, and thus kept it nearer to the temperature of the external air than

those parts could be which, from being uncovered, received the heat emitted to them by the bodies just

mentioned.

"In making these experiments, I seldom observed

the inside of any pane to be more than a little damped, though it might be from eight to twelve degrees colder than the general mass of the air in the room; while, in the open air, I had often found a great dew to form on substances only three or four degrees colder than the atmosphere. This at first surprised me; but the cause now seems plain. The air of the chamber had once

been a portion of the external atmosphere, and had

afterwards been heated, when it could receive little accessories to its original moisture. It constantly required

being cooled considerably before it was even

brought back to its former nearness to repletion with water; whereas the whole external air is commonly, at night, nearly replete with moisture, and therefore readily precipitates dew on bodies only a little colder than itself.

"When the air of a room is warmer than the external

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