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two of the compass each way; on the south of the equator they proceed from the south-east. The origin of them is this :—the powerful heat of the torrid zone rarefies, or makes lighter the air of that region ; the air in consequence of this rarefaction rises, and to supply its place, a colder atmosphere from each of the temperate zones moves towards the equator. But these north and south winds pass from regions, where the rotatory motion of the earth's surface is less, to those where it is greater. Unable at once to acquire this new velocity, they are left behind, and instead of being north and south winds as they would be if the earth's surface did not turn round, they become north-east and south-east winds, The monsoons belong to the class of periodical winds. They blow half the year from one quarter, and the other half from the opposite direction : when they shift, variable winds and violent storms prevail for a time, which render it dangerous to put to sea. The monsoons, of course, suffer partial changes in particular places, owing to the form and position of the lands, and to other circumstances, but it will be sufficient to give their general directions. From April to October, a south-east wind prevails north of the equator; southward of this a south-east wind : from October to April, a north-east wind north of the equator ; and a north-west between the equator and 10° of south latitude.
The land and sea breezes, which are common on the coasts and islands situated between the tropics, are another kind of periodical winds. During the day, the air, over the land, is strongly heated by the sun, and a cool breeze sets in from the sea; but, in the night, the atmosphere over the land gets cooled, while the sea, and consequently the air over it, retains a temperature nearly even at all times; accordingly, after sunset, a landbreeze blows off the shore. The sea-breeze generally sets in about ten in the forenoon, and lasts till six in the evening; at seven, the land-breeze begins and continues till eight in the morning, when it dies away. These alternate breezes are, perhaps, felt more powerfully on the coast of Malabar than anywhere; their effect there extends to a distance of twenty leagues from the land.
DEW. Dew, or the moisture insensibly deposited from the atmosphere on the surface of the ground, is a wellknown phenomenon. It was long supposed that its precipitation was owing to the cooling of the atmosphere towards evening, which prevented it from retaining so great a quantity of watery vapour in solution, as during the heat of the day. But it has been recently proved, that the deposition of dew is produced by the cooling of the surface of the earth, which takes place previously to the cooling of the atmosphere. The earth is an excellent radiator of caloric, whilst the atmosphere does not possess that property in any sensible degree. Towards evening, therefore, when the solar heat declines, and after sunset, when it entirely ceases, the earth rapidly cools by radiating towards the skies; whilst the air has no means of contact with the cooled surface of the earth to which it communicates its caloric. Its solyent power being thus reduced, it is unable to retain so large a portion of watery vapour, and deposits those pearly drops called dew. This view of the matter explains the reason why dew falls more copiously in calm than in stormy weather, and in a clear than in a cloudly atmosphere. Accumulations of moisture in the atmosphere not only prevents the free radiation of the earth towards the upper regions, but themselves radiate towards the earth; whereas, in clear nights, the radiation of the earth passes without obstacle through the atmosphere to the distant regions of space, whence it receives no caloric in exchange. The same principle enables us
to explain the reason why a bottle of wine taken fresh from the cellar (in summer particularly) will soon be covered with dew. The bottle being colder than the surrounding air, absorbs caloric from it; the moisture therefore, which that air contained, becomes visible, and forms the dew which is deposited on the bottle. In like manner, in a warm room or in a close carriage, the inside of the windows is covered with vapour, because the windows being colder than the breath, deprive it of part of its caloric, and by this means convert it into watery vapour. Bodies attract dew in proportion as they are good radiators of caloric ; as it is this quality which reduces their temperature below that of the atmosphere. Hence we find, that little or no dew is deposited on rocks, sands, or water, while grass and living vegetables, to which it is so highly beneficial, attract it in abundance : a remarkable instance of the wise and bountiful dispensations of Providence. The same benevolent design we may observe also, in the abundance of dew in summer and in hot climates, in which its cooling effects are so much required. The more caloric the earth receives during the day, the more it will radiate afterwards; and, consequently, the more rapidly its temperature will be reduced in the evening, in comparison with that of the atmosphere. In the West Indies, accordingly, where the intense heat of the day is strongly contrasted with the coolness of the evening, the dew is prodigiously abundant. When dew is frozen the moment it falls, it gets the name of hoar-frost. Such are the principal circumstances which are supposed to concur in the formation of aqueous meteors. Their beneficial influence upon the earth is a point more easy to determine. We observe all nature languish when the atmosphere retains for too long a time the moisture arising from the earth. Plants fade and droop; animals feel their strength failing them; man himself, breathing nothing but dust, can with difficulty procure shelter from the sultry heat, by which his frame is parched and overpowered. But scarcely have the waters of heaven descended from the clouds, when all living beings begin to revive; the fields resume their green attire,—the flowers their lively tints,—animals the sportive freedom of their motions, and the elements of the air their healthful equilibrium. Snow itself, whose very name alarms the natives of the tropics, is productive of real advantages in the economy of nature; it secures the roots of plants against the effects of intense cold; it serves to moisten gently those lands, from which, owing to their local situation, the rain is too soon carried off : and it paves for the inhabitant of the north commodious and agreeable roads, along which he gaily skims in his light and nimble sledge. Hail alone, of all the aqueous meteors, never appears but as a harbinger of distress. Birds and quadrupeds instinctively conceal themselves as soon as they have any presentiment of its coming. Man can neither foresee its approach, nor arrest its ravages; he has been able to ward off the thunderbolts of the sky, but he sees the hail destroy his corn, break his fruit trees, and shatter the very house where he dwells, without being able to prevent it.
LESSON IX. ON THE MECHANICAL PROPERTIES OF AIR. We shall now examine the second class of fluids, distinguished by the name of aëriform, or elastic fluids, the principal of which is the air we breathe, which surrounds the earth, and is called the atmosphere. There is a great variety of elastic fluids, but they differ only in their chemical, not in their mechanical properties; and it is the latter we are to examine. There is no attraction of cohesion between the particles of elastic fluids, so that the expansive power of heat has no adversary to contend with but gravity ; any increase of tem
perature, therefore, expands elastic fluids prodigiously, and a diminution proportionally condenses them. The most essential point in which air differs from other fluids is by its spring or elasticity ; that is to say, its power of increasing or diminishing in bulk, according as it is less or more compressed—a power of which liquids are almost wholly deprived.
The atmosphere is thought to extend to about the distance of forty-five miles from the earth ; and its gravity is such, that a man of middling stature is computed to sustain the weight of about fourteen tons. Such a weight would crush him to atoms were it not that air is also contained within our bodies, the spring or elasticity of which counterbalances the weight of the external air, and renders us insensible of its pressure. Besides this, the equality of pressure on every part of the body enables us more easily to support it; when thus diffused we can bear even a much greater weight without any considerable inconvenience. In bathing we support the weight and pressure of the water, in addition to that of the atmosphere; but this pressure being equally distributed over the body, we are scarcely sensible of it; whilst, if the shoulders, the head, or any particular part of the frame were loaded with the addi. tional weight of a hundred pounds we should feel severe fatigue. On the other hand, if the air within a man met with no external pressure to restrain its elasticity, it would distend his body, and at length bursting the parts which confine it, put a period to his existence. The weight of the atmosphere, therefore, so far from being an evil, is essential to our existence. When a person is cupped, the swelling of the part under the cup is produced by taking away the pressure of the atmosphere; in consequence of which the internal air distends the part.
A column of air reaching to the top of the atmosphere, and whose base is a square inch, weighs fifteen pounds when the air is heaviest. The rule that fluids