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flect and reverse the grand system of equipoised currents. They are also within the frigoric effects of the immense barriers and fields of ice, which, when the shifting position of the sun advances the tropical climate towards the northern pole, counteract its energy, and present a condensing surface of enormous extent to the increasing elasticity of the aqueous atmosphere.” Amidst all the uncertainty and seeming confusion arising from this complication, general principles may still be recognised; and, it is believed, the more they are studied, the more obvious they will appear.

EVAPORATION.

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The formation and never-failing supply of the condensible elastic vapor, which has been described to be one of the constituents of the atmosphere, is provided for by that law of nature, which has endued water, under all circumstances, even when congealed into ice, with the power of emitting vapor or steam, in a quantity proportioned to its temperature. The presence of water over the globe may be said to be universal; for even in the fraction which is estimated to be land, it is so profusely distributed as to maintain a perpetual exhalation. 'Pasturage, corn-fields, or forests support a continual evaporation, augmented only by the dryness of the air, and the rapidity of its sudden contacts. Even ploughed land will supply as much moisture to the exhaling fluid as an equal sheet of water. It is only when the ground is quite parched, that it ultimately retains its latent store.”* As this property of water clings to it in all its metamorphoses, and attends it in all its localities, it follows that the process of evaporation is constant and universal. It is generally also invisible, but there are times when it may be seen, and we can inspect, at the level of the earth, some of those operations which usually take place in the higher regions of the atmosphere. In the calm evening of a fine summer's day, the rudiments of future clouds often present themselves to our sight, in the first part of their flight; and though they disappear, it is no proof of the suspension, or even of the diminution, of the process, which proceeds as powerfully and effectually during the most brilliant aërial transparency as in the thickest mist.

By means of the visibility which cold imposes upon aqueous vapor, we can often satisfactorily trace its upward progress in the clouds of fine weather. "During the heat of the day it rises from the surface of the land and waters, and reaches its point of condensation in greater or less quantities at different altitudes. Partial clouds are formed in different parallel planes, which always maintain their relative distances. The denser forms of the lower strata, as they float along with the wind, show that the greater abundance of precipitation has been at the first point of deposition, while the feathery shapes and lighter texture of the upper attest a rarer atmosphere. These clouds do not increase beyond a certain point,

* Leslie.

and often remain stationary in quantity and figure for many hours; but, as the heat declines, they gradually melt away, till at length, when the sun has sunk below the horizon, the ether is unspotted and transparent. The stars shine through the night with undimmed lustre, and the sun rises in the morning in his brightest splendor. The clouds again begin to form, increase to a certain limit, and vanish with the evening shades. This gradation of changes, which we see so often repeated in our finest seasons, may at first seem contrary to the true principles; and the precipitations, which occur with an increase of temperature, and disappear with its decline, may, without reflection, be regarded as diametrically opposed to correct theory. But a little consideration will show that such conclusions would be untrue. The vapor rises, and is condensed; but in its descent falls into a warmer air, where it again is evaporated, and becomes invisible; and as the quantity of evaporation from the surface of the earth is exactly equal to supply this process above, the cloud neither augments nor decreases. When the sun declines, the ground cools more rapidly than the air; evaporation decreases, but the dissolution of the cloud continues. The supply at length totally ceases, and the cloud subsides completely away. The morning sun revives the exhalations of the earth, the process of their condensation and consequent cloud-like form commences, and they again undergo the same series of changes."

Descending again to the operations on the earth's surface, we may observe that the ascent of the aqueous vapor is modified by the relative differences of the temperatures of the exhaling fluid and the ambient air. Two of these it may be useful to describe. They are,

1. When the temperature of the fluid is above that of the air; 2. When the temperature of the fluid is below that of the air.

In the first case, the evaporation is proportionate to the difference of temperature. The gaseous fluid in contact with the surface of the warmer water becomes lighter by receiving portions of the excess of heat, and, rising up, carries with it in its ascent the entangled vapor, which has been cooled down by the low temperature of the air into the form of steam. This is the visible evaporation referred to above.

In the second case, though the water is colder than the air, it still, from the law before mentioned, emits vapor from its surface, but invisibly, as there is no condensing disposition now in the air. The vapor, therefore, neither having the power to displace the gaseous fluid, nor heat to cause a circulation of it, can only pass by filtering through its interstices—a most beautiful and astonishing instance of the extreme divisibility of matter.

The force of aqueous vapor disengaged at different temperatures has been determined with great accuracy, and the amount of evaporation has been demonstrated to be, other things being equal, always in proportion to this force.

* Daniell.

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It may be easily imagined, that as the interstices of the gaseous fluid can contain but a certain quantity of elastic vapor, there would naturally be a limit to evaporation. This is actually the case. It very often happens that the interstices are found to be full, and can hold no more, and that then evaporation ceases; sometimes, indeed, they may be said to run over, and it is then we see the excess in the shape of steam, or mist, or cloud. The capacity of these interstices of the gaseous fluid becomes larger or smaller in proportion to the temperature of their particles, and the effect of their contraction or expansion is precisely similar to the grasp or relaxation of the hand on a piece of imbibing sponge. At a low temperature, or when the grasp is tightest, a certain quantity can only enter. On the contrary, at a high temperature, or when the sponge is permitted to expand to the utmost, its capacity is increased, and a large volume may be contained.

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The total quantity of aqueous elastic vapor which can enter between the interstices of the gaseous fluid, or which the latter can hold suspended, depends upon temperature, but this quantity is invariably at the same temperature. A volume of air may contain less than this quantity, but never When it has this exact quantity, it will remain transparent, and is said to be saturated, or at its point of saturation. It is then as damp as it can be; any attempt to insert more vapor will fail, and the rejected vapor will become visible in the form of steam. If we lower the temperature, the aërial interstices will contract, and some of the contained vapor will be squeezed out in the same form. We may increase the temperature to any extent without any visible change, but we render the air drier in proportion to the degree to which we ascend, and in the same degree capable of receiving and supporting an additional quantity of humidity. Atmospheric pressure also affects the amount of the quantity suspended, by opposing the diffusion and retarding the formation of the vapor. From the aqueous fluid being so abundantly spread over the face of the earth, there can be no doubt that the permanently elastic or gaseous atmosphere would very speedily be saturated with its vapor, did not some cause prevent its universal diffusion. This never-failing cause is inequality of temperature, which excites, or diminishes, or suspends, in the way we have described, the process of evaporation.

The absolute quantity of moisture that air is capable of containing, may be conceived from the following statement of Mr. Leslie :-" Air, at the freezing point, is capable of holding a portion of moisture equal to the 160th part of its own weight; at the temperature of 59°, the 80th part; at that of 86°, the 40th part; at 113°, the 20th part; and at that of 140°, the 10th part; so that the air has its dryness doubled at each rise of temperature, answering to 27° of Fahrenheit. While the temperature, therefore, advances uniformly in arithmetical progression, the dissolving power, which

this communicates to the air, mounts with the accelerating rapidity of a geometrical series."

By the improved instruments and accurate observations of this gentleman and others, the total quantity of moisture which could be suspended at one time in the air can be correctly estimated. It has been stated by him, that, at 68° Fahrenheit, a cubic mass of air, measuring 40 inches every way, can retain 252 grains of water. But if a larger scale be preferred, the same numbers will express in pounds troy the quantity of water required to saturate a perfectly dry mass of air constituting a cube of twenty yards in dimension. If the greatest amount possible of the aqueous element were to be suspended in the atmosphere, and this were to pass from a state of absolute dampness into that of extreme dryness, and discharge the whole of its watery store, it would form a sheet of somewhat less than five inches in depth. To furnish the usual supply of rain, the air must, therefore, undergo very frequent changes, equal to that of from dryness to humidity in the course of the year.

The average amount of evaporation in the neighbourhood of London per annum, calculated by Mr. Daniell's hygrometer, a most elegant and perfect instrument for ascertaining the humidity of the atmosphere, is 23,974 inches. The average weight of the quantity of water raised by this process, from a circular surface of six inches diameter, 0.31 gr. per minute. The results of actual measurement by Mr. Howard accord most satisfactorily with this method of estimating the amount of evaporation, and prove most incontestably the accuracy of the calculations upon which it is founded.

The rate at which this process proceeds near London, during the several months of the year, is estimated by Mr. Daniell, and recorded in the British Almanac, as follows:

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The smallest quantity of water is, therefore, lifted into the atmosphere during the month of January, and the greatest in June. The mean quantity held in solution in a cubic foot of air, is 3.789 gr.

The rate of exhalation from the surface of the ground is scarcely of less consequence than the fall of rain, and a knowledge of it might often direct the most important operations. Mr. Leslie invented an instrument for measuring the quantity of moisture exhaled from a humid surface in a given time. This he called the Atmometer, and he has estimated that the daily exhalation from a sheltered surface of water would, at the mean dryness of winter, lower it 0.018 inches and at the mean of summer 0.048 inches.

And he gives the following instance of its use: Suppose a pool for the supply of a navigable canal exposed a surface equal to ten English acres, and that the atmometer sunk 80 parts during the lapse of 24 hours, the quantity exhaled in that time would be 2904 cubic feet, or about 81 tons, equal to 1700 imp. gall. per acre.

The dissipation of moisture is much accelerated by the agency of sweeping winds, the effect being sometimes augmented 5 or even 10 times. In general, this augmentation is proportional to the swiftness of the wind, the action of still air itself being reckoned equal to that produced by a celerity of eight miles each hour.

CLOUDS, FOGS, and Mists.

The presence of the ocean of vapor, which we have described as constantly ascending from the earth, and constituting part of the atmosphere, is, as has also been observed, not always evident to the sight; in its elastic state it is always invisible, and, therefore, it is only in some of its changes that the eye can detect it. By one of the most remarkable of these, those masses of visible aqueous vapor are formed, which, floating in the sky, or drifting through it with the wind, at different elevations, with every variety of color and form, are called clouds; or which, recumbent on the surface of the land or of the water, and spread over greater or smaller portions of them, are denominated fogs, or mists, according to their intensity. In all cases, their composition is similar, and consists of the moisture deposited by a body of air, in minute globules.

Their formation, in every position, is a consequence of decrease of temperatures in some parts of the atmosphere where a certain proportion of aqueous elastic vapor is present; but in those where the latter condition may be wanting, it is evident that the developement of cloud will not follow the decrement of temperature. Nothing is more common than the fact of the necessary conditions existing in some of the atmospheric strata, and at the same time being absent in others; and thus we can understand the causes of the alternate beds of clouds and clear air, which often diversify the sky in serene weather. We can hence also comprehend how, in stormy weather, a solitary cloud sometimes appears to stand stationary over a mountain-top, while myriads of other clouds drift past it on the gale. An observer on the summit feels the multitudinous dew-drops of the seemingly fixed cloud sweeping by with great velocity, and discovers the stationary aspect which it exhibited below to be altogether an illusion. The fact is, the inferior invisible beds of air are relatively warmer and more moist. They dash against the sloping side of the mountain, and are reflected up to the plane of condensation in the atmosphere, where they give out their excess of water in the form of clouds. Above the cooling influence of the mountain-top the temperature of the air may not be depressed to the same point, and hence it continues clear.

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