Слике страница
[merged small][merged small][merged small][merged small][ocr errors]

every month of the year. As a rapid radiation is their proximate cause, tender plants may be preserved from their influence, by interposing a slight screen of mat, or other substance; and this was known in the practice of horticulturists, long before the principle was explained. The same principle may be applied artificially, to the cooling of bodies that radiate well;—thus in Bengal, by supporting vessels containing water, on non-conducting feet, ice may be obtained, during nights when the atmospheric temperature has not fallen below 50°.

n d fa

h }

Terrestrial radiation is retarded by the same causes that retard that of the sun :thus, the radiation is greater in high than in low latitudes, in the upper regions of the air, than it is near the surface. In the view we have taken of the causes of the winds, we have noticed no other circumstances, than the variation of temperature on the surface of the sphere, and in the atmosphere itself, combined with the varying velocity of rotation of different points in different latitudes. Upon this hypothesis, we have seen, that the pressure in two latitudes, would be nearly constant, and the winds in the open ocean regular. In higher latitudes, the variation of temperature being greater and less regular, the winds would become variable, and creating waves in the air, cause variations in the pressure; variations would also arise, from deviations in the atmosphere itself, from the law of a regular decrease of temperature, from the equator to the poles. These causes increasing with the increase of the latitude,—the oscillations of the barometer, which, at the equator, are very small, become greater, as we proceed from it towards the frigid zone, in which they reach their maximum. Careful observations have however shown, that, even in equatorial regions, slight oscillations do occur. This was first remarked by Lamanon, the naturalist, attached to the unfortunate expedition of La Peyrouse. Observations made at Calcutta, by Dr. Balfour, established the same fact; and it has been still farther confirmed by the researches of Humboldt. The period of these oscillations, is semidiurnal. Captain Sabine, in his several voyages, for the purpose chiefly of observations on the pendulum, has found time to attend to this interesting subject; and has added to our stock of evidence and knowledge in respect to it.

The variations of the barometer, in temperate climates, are so great, and so difficult to reduce to any fixed law, that it becomes difficult to determine, whether these horary oscillations extend to extra-tropical regions. By collating, and comparing together, numerous observations made at different hours, and determining the mean altitude for each of the different times of observation, we may obtain an average; and thus elicit the law of regular movement, even when most concealed from our view. In the latitude of 45° 47' N., it was found, by M. Ramond, that ten


days were sufficient to neutralize the irregular oscillations, and exhibit the periodic motions. These coincide accurately in period and direction, with those of the equator; but were less in amount. Thus, as remarked by Captain Sabine, no doubt can remain, that “the amount of atmospheric tides diminishes progressively, from the equator to the tropics; and continues at a diminishing rate, as far as the fifty-second degree of latitude.”

For these oscillations, our author proceeds to assign an explanation, which may be thus briefly stated :—The alternations of day and night, produce a change in the temperature of the surface of the earth,—this is communicated to the atmosphere; and, in so doing, a motion is produced in the particles that compose the two great aerial currents. During the action of the sun, (the heated surface being below,) the warm particles will ascend, and be replaced by the colder particles from above. In the absence of the sun, this interchange ceases. Now, although in a general point of view, this interchange will equally affect both aerial currents, and pervade the whole length of the atmospheric column,-yet, upon a closer examination, we shall see, that the operation is not instantaneous; and that the stratum in contact with the heating surface, must have its temperature disproportionately augmented. In the absence of the sun, the lower stratum will be disproportionately cooled. In the first case, the barometer would fall; and in the second, rise. Our author next shows, that, when these effects are considered as extending to a whole meridian, although the barometer would fall at the equator, if that point alone were supposed to be heated,-it would rise in all other latitudes, in consequence of the lower incoming currents being checked; but, as at each intermediate place, the barometer would have a tendency to fall, from the action of the sun upon its surface, the actual effect would be the difference of these two changes; and hence the fall could be no where so great as at the equator. The nearer we approach the pole, the more the revulsive action will accumulate ; until, at some given point, the tendencies to a rise and to a fall of the barometer, will balance each other, and no oscillations occur. Beyond this, the revulsive action will be in excess; and a fall in the barometer, at the equator, will correspond in point of time, to a rise in these higher latitudes. To reduce this theory to the test of experiment, Mr. Daniell compares observations made at Mellville Island, by Captain Parry, with those of Major Long, of the United States' army; which, by a fortunate coincidence, were regularly taken during his residence in winter quarters, at “ Engineer Cantonment,” during the same months of the same year. The result of this comparison, is strictly in accordance with the theory; and furnishes a very strong, if not complete proof of its truth.

These oscillations of the barometer were formerly ascribed to waves raised in the atmosphere, by the attraction of the sun and moon, in the same way as tides are raised in the ocean; but a strict mathematical investigation has shown, that any such cause would be insufficient to produce the effects observed ; and besides, the oscillations are not found to correspond, as the aquatic waves do, with the position of the moon, but with the diurnal motion of the earth. Such indeed was the extent once ascribed to the action of the sun and moon upon the atmosphere, that it was attempted to explain the phenomena of the trade winds by means of it. This attempt, however, although supported by the great mathematical skill of Dalembert, * was unsuccessful; his argument having been overthrown by Laplace,t who has shown that this theory is insufficient to account for these phenomena. Still, however, we cannot doubt the lunar influence upon the atmosphere; but it is of less amount than any of the other causes of motion we have assigned.

Another action that must affect our atmosphere, is that of Electricity. That this is sufficient to produce changes of an important character, is obvious; but we have not yet sufficient data to enable us to assign the laws of its action. One phenomenon, however, we think, can only be explained, by introducing the agency of Electricity; and this is hail. Hail is water congealed in a granular form ; it frequently falls in the hottest weather, and occasionally even in warm climates. Ordinary precipitation, at a low temperature, will not account for it; this indeed is the cause of snow; and the two are so different in appearance, that we cannot consider them as proceeding from a similar combination of circumstances. The only manner in which we can imitate the granular form of hail, is by freezing water in a Torrecellian Vacuum ; and hence we may infer, that it is formed under low pressure; yet, as vapour is confined almost wholly to the lower half of the atmosphere, this essential circumstance cannot exist in those regions where aqueous matter is usually found. The best explanation we have yet met of this phenomenon, is one for many years taught in his public lectures, by a late distinguished professor of one of our colleges. I

The decomposition of animal and vegetable matter on the surface of the earth, of stagnant waters, and even of the ocean itself, is constantly giving out hydrogen and its compounds; these being lighter than atmospheric air, consequently rise, and speedily reach a lofty position in the atmosphere; from the property pos

• Dalembert, Reflexions sur les causes des Vents. † Méchanique Céleste.

# John Kemp, LL. D., Professor of Natural Philosophy and Mathematics, in Columbia College, New-York,

sessed by the gases, of mixing with each other, in spite of the difference of their specific gravities, these gases do not remain separate after they have risen, but are mingled with the surrounding atmospheric air; in this way an explosive compound is formed, that may, by an electric discharge, be inflamed; such discharges, we are aware, are frequent in all parts of the atmosphere, and the product of such combustion is water; as water occupies a space far less than the gases by whose condensation it is formed, a void will be caused, towards which the neighbouring columns of air will rapidly rush, and expanding, will have such an increased capacity for heat, as to absorb all that is generated in the combustion of the hydrogen; the water formed in a lofty region, when a low temperature prevails, will freeze; and, congealing in a rare medium, will assume the granular form characteristic of hail. We cite this beautiful and plausible theory, as an act of justice to one, who, content with the meed of usefulness, shunned the public gaze; whose exertions in the cause of science are almost forgotten, and whose talents and learning were, we fear, hardly appreciated, even by those who had the good fortune ta receive his instructions.

We shall close this article by an extract from our author, in which he sums up the joint and consistent conclusions drawn from his theory, and from an examination of the phenomena themselves.

“There are two distinct atmospheres, mechanically mixed, surrounding the earth; whose relations to heat are different, and whose states of equilibrium, considering them as enveloping a sphere of unequal temperature, are incompatible with each other. The first is a permanently elastic fluid, expansible in an arithmetic progression by equal increments of beat, decreasing in density and temperature according to fixed ratios, as it recedes from the surface, and whose equipoise under such circumstances would be maintained by a regular system of antagonist currents. The second is an elastic fluid condensible by cold with the evolution of caloric; increasing in force in geometrical progression with equal increments of temperature ; permeating the former and moving through its interstices as a spring of water Hows through a sand rock. When in a state of motion, this intestine filtration is retarded by the inertia of the gaseous medium; but in a state of rest, the particles press only on those of their own kind. The densi. ty and temperature of this fluid have likewise a tendency to decrease as its distance from the surface augments; but by a less rapid rate than that of the former, Its equipoise would be maintained by the adaptation of the upper parts of the medium, in which it moves, to the progression of its temperature, and by a current flowing from the botter parts of the globe to the colder. Constant evaporation on the line of greatest heat and unceasing precipitation, at every other situation, would be the necessary accompaniments of this balance. Now the conditions of these two states of equilibrium, to which, by the laws of hydrostatics, each fluid must be perpetually pressing, are essentially opposed to each other. The vapour or condensible elastic fluid is forced to ascend a medium whose heat decreases much more rapidly than its own natural rate ; and it is therefore condensed and precipitated in the upper regions. Its latent caloric is evolved by the condensation, and communicated to the air ; und it thus tends to equalize the temperature of the medium in which it moves, and to constrain it to its own law. This process must evidently disturb the equilibrium of the permanently elastic fluid, by interfering with that definite state of temperature and density which is essential to its


[ocr errors]

maintenance. The system of currents is unequally affected by the unequal expansion ; and the irregularity is extended by their influence much beyond the sphere of the primary disturbance. The decrease of this elasticity above, is ac. companied by an extremely important reaction upon the body of vapour itself; being forced to accommodate itself to the circumstances of the medium in which it moves, its own law of density can only be maintained by a corresponding decrease of force below the point of condensation ; so that the temperature of the air, at the surface of the globe, is far from the term of saturation ; and the current of vapour which moves from the hottest to the coldest points, penetrates from the equator to the poles, without producing that condensation in mass, which would otherwise cloud the whole depth of the atmosphere with precipitating moisture. The clouds are thereby confined to parallel horizontal planes, with intermediate clear spaces, and thus arranged are offered to the influence of the sun, which dissipates their accumulations, and greatly extends the expansive power of the elastic vapour. The power of each Auid being in proportion to its own elasticity, that of the vapour compared with the air, can never, at most, exceed 1.30: so that the general character of the mixed atmosphere is derive ed from the latter ; which in its irresistible motions must hurry the former along with it. The influence, however, of the vapour upon the air, although slower in its action, is sure in its effects, and the gradual and silent processes of evaporation and precipitation govern the boisterous power of the winds. By the irresistible force of expansion unequally applied, they give rise to undulations in the elastic fluid ; the returning waves dissipate the local influence, and the accumulated effect is annihilated, again to be re-produced.

“ In tracing the harmonious results of such discordant processes, it is impossible pot to pause, to offer up a bumble tribute of admiration of the designs of a beneficent Providence, thus imperfectly developed, in a department of creation where they have been supposed to be the most obscure. By an invisible, but ever-active agency, 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 bene. ficial 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.

"The complicated and beautiful contrivances by which the waters are collected above the firmament,” and are at the same time “divided from the waters which are below the firmament,” are inferior to none of those adaptations of InFINITE WISDOM which are perpetually striking the inquiring mind in the animal and vegetable kingdoms. Had it not been for this nice adjustment of conflicting elements, the clouds and concrete vapours of the sky would have reached from the surface of the earth to the remotest heavens ; and the vivifying rays of the sun would never have been able to penetrate through the dense mists of perpe tual precipitation,”

ART. II. - The Constitutional History of England, from the

Accession of Henry VII. to the Death of George II. By HENRY HALLAM. Two volumes, quarto. London, 1827.

There is something imposing in the title of this work.

The history of the Constitution of any country, implies a view of the origin and formation of that Constitution, of the alterations

« ПретходнаНастави »