There is still to be added to this enumeration of the ingredients of this astonishing compound, the subtle and mysterious agencies of light, and of the electric, galvanic, and magnetic fluids; and last, as if to baffle all inquiry, and render analysis impossible, there is, in universal operation, a perpetual agitation and commixion of the whole mass.

When we reflect upon this ever-agitated heterogeneous fluid, compounded of the most active elements of life and destruction, constantly combining, -separating, now evident to the most ordinary sense, now escaping the grasp of imagination, we cannot, rationally, be surprised at the little, comparatively, that is really known concerning it. All within the power of the most ardent student is to collect the few facts that are established, to dismiss conjecture and hypothesis, and to apply himself to make additions to our knowledge by carefully observing, and accurately and luminously describing, the processes during which he is permitted to be present.

By invisible, but ever-active agencies, the waters of the deep are raised into the air, whence their distribution follows, as it were by measure and weight, in proportion to the beneficial effects which they are calculated to produce. By gradual, but almost insensible expansions, the equipoised currents of the atmosphere are disturbed, the stormy winds arise, and the waves of the sea are lifted up; and that stagnation of air and water is prevented, which would be fatal to animal existence. But the force which operates is calculated and proportioned; the very agent which causes the disturbance, bears with it its own check, and the storm, as it vents its force, is itself setting the bounds of its own fury.'

It is evident, from a slight view of these "complicated and beautiful contrivances," that it is hopeless to expect that all the causes of the phenomena of the atmosphere will ever be entirely attained by human science. At present but few are known, and those imperfectly. Amongst the principal ones which most affect the subject of our present inquiry are, undoubtedly, heat and electricity. The first raises and suspends the evaporated waters invisibly in the air, until some more powerful attraction dissolves the union, and the deserted moisture, exposed to view, falls again to the earth, and revisits it in the varied forms of clouds, mist, rain, dew, snow, hail, sleet, and hoar-frost. To electricity may be principally attributed the more splendid phenomena of lightning, the aurora-borealis, and the other igneous meteors. And the effect of these causes, variously combined and infinitely modified by other agents, is felt in those currents of atmospheric air, which are described by a sacred writer, as "going toward the north, and toward the south, as whirling about continually and returning again according to their circuits."

Imperfect as the preceding sketch necessarily is, it is founded on facts which have been collected by some of the most indefatigable observers,

* Daniell.

and faithful historians, of what they saw. While it enables us to judge a little more correctly, perhaps, of some of the causes of those conditions of our atmosphere, which are hourly presenting themselves to our view, or affecting the state of our existence, it cannot but excite emotions of astonishment and of pity, when we see some of the most ignorant of our species boldly pretending to predict the results of the operations, visible and invisible, near and remote, of this vast and complex, and almost unknown apparatus.

*

We shall present a contrast to the dicta of these daring empirics, by an extract from the recent publication of a gentleman, highly distinguished by the services he has rendered meteorology, which contains a few of the most general and best authenticated facts relating to the phenomena of the atmosphere. From these we may safely reason; and a knowledge of them may frequently enable us to detect order and regularity among objects proverbially spoken of as uncertain and irregular.

SOME OF THE MORE PARTICULAR PHENOMENA OF THE

ATMOSPHERE OF THE Earth.

"The mean height of the barometer (i. e. the mean weight or pressure of the atmosphere) at the level of the sea, is the same in every part of the globe.

"The barometer constantly descends in a geometrical progression for equal ascents in the atmosphere, subject to a correction for the decreasing temperature of the elevation.

"The mean temperature of the earth's surface increases gradually from the poles to the equator.

"The mean temperature of the atmosphere decreases from below upwards in a regular gradation.

"The barometer at the level of the sea is but slightly affected by the annual or diurnal fluctuations of temperature; but, in the higher regions of the atinosphere, is, on the contrary, greatly affected by them.

"The heating and cooling of the atmosphere, by the changes of day and night, take place equally throughout its mass.

"The average quantity of vapor in the atmosphere decreases from below upwards, and from the equator to the poles.

"The western coasts of the extra-tropical climates have a much higher mean temperature than the eastern coasts.

"A wind generally sets from the sea to the land during the day, and from the land to the sea during the night, especially in hot climates.

"Between the tropics the fluctuations of the barometer do not much exceed one quarter of an inch, while beyond this space they reach to three inches.

* Daniell. Essays.

"In the temperate climates the rains and the winds are variable. "As we advance towards the Polar Regions, we find the irregularities of the wind increased; and storms and calms repeatedly alternate, without warning or progression.

"In the extra-tropical climates, a fall in the barometer almost always precedes a period of rain, and indicates a change or acceleration of the aërial currents.

"Barometers, situated at great distances from each other, often rise and fall together with great regularity.

"More than two currents may often be traced in the atmosphere at one time, by the motions of clouds, &c.

"The force of the winds does not always decrease as the elevation increases; but, on the contrary, is often found to augment rapidly.

"The variations of the barometer are less in high situations than in those at the level of the sea.

"In Great Britain, upon an average of ten years, westerly winds exceed the easterly in the proportion of 225 to 140; and the northerly winds exceed the southerly, as 192 to 173.

"Northerly winds almost invariably raise the barometer, while southerly winds as constantly depress it.

"The most permanent rains from this climate come from the southern regions.

"The mean height of the barometer varies but little with the changes of the seasons.

"The apparent permanency and stationary aspect of a cloud is often an optical deception, arising from the solution of moisture on one side, and its precipitation on the other.

"The quantity of vapor in the atmosphere in the different seasons of the year (measured on the surface of the earth, and near the level of the sea) follows the progress of the mean temperature.

"The pressure of the aqueous atmosphere, separated from that of the aërial, generally exhibits directly opposite changes to the latter.

"Great falls of the barometer are generally accompanied by a temperature above the mean for the season, and great rises by one below the

same."

The same authority also states, that "The British islands are situated in such a manner as to be subject to all the circumstances which can possibly be supposed to render a climate irregular and variable. Placed nearly in the centre of the temperate zone, where the range of temperature is very great, their atmosphere is subject, on the one side, to the impressions of the largest continent of the world; and, on the other, to those of the vast Atlantic Ocean. Upon their coasts the great stream of aqueous vapor, perpetually arising from the western waters, first receives the influence of the land, whence emanate those condensations and expansions which de

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.

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. 1 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,

66

* 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.