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Page:Popular Science Monthly Volume 7.djvu/499

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DISTRIBUTION OF ATMOSPHERIC MOISTURE.

charge used on this occasion was sixteen pounds, lowered ten feet below five-feet ice; its effect was the breaking up of a space of 400 yards square, besides splitting the ice in several directions. The last charge would be equivalent to two pounds of "cotton gunpowder," but the results with the latter explosive would, in all probability, be far more effective.

The work of an exploring expedition in the arctic regions for the period of twelve months has now been detailed. No unforeseen accident, no detention in the ice, in fact no casualty of any description has been taken into consideration, but every thing has progressed under the most exceptionally favorable circumstances. That the same will be the case with the Arctic Expedition of 1875 is too much to expect, but that it will be successful in exploring a large area of unknown land may be confidently hoped and anticipated.—Geographical Magazine.

 

DISTRIBUTION OF ATMOSPHERIC MOISTURE.

ALL over the earth, the more largely where its beams reach the surface with the least diminution of heat, the sun is continually engaged in evaporating moisture from all exposed surfaces of water; this remains suspended in the atmosphere, and is carried about by the winds in the form of impalpable vapor or of clouds, till the point of saturation is reached, and the moisture falls again to the earth's surface in the form of rain, or snow, or hail. Air becomes lighter, and consequently expands and ascends, when it grows hotter, and becomes heavier and falls with cold. The hotter it is the more moisture it is able to hold in solution. Between the equator and the poles there is a difference of 80° of average annual temperature. In the torrid zone the light, warm, vapor-laden air is ascending continually to the upper regions of the atmosphere, and there flowing outward north and south toward the poles, and the cold, heavy air from the polar regions comes rushing along the surface to fill its place. As the seasons change, the line of the greatest heat in the world gradually moves its position. At the equinoxes of spring and autumn it runs along the actual equator, or near it. In winter it lies south of the earth's equator, about midway between the equator and tropic of Capricorn. Not more than half as much of the tropic of Capricorn as of the tropic of Cancer runs over land, and this makes a material difference, because the more sea the more the intense heat is deadened and absorbed. In summer the great continental area traversed by the tropic of Cancer, a long line of which is removed from the ameliorating influence of the sea, becomes excessively heated, and from the great African Sahara, through Nubia and Arabia to the north of India, runs a tract of intense heat, in which the July average in the shade rises to 90°.