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Page:Popular Science Monthly Volume 54.djvu/524

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property," as, for example, upon all personal property; for if the removal of the burden operates uniformly on all interested, or owning such property, then there can be no primary exemption.



A THIN stratum of air, an invisible armor of great tenuity, lies between man and the menace of possible annihilation.

The regions of space beyond our planet are filled with flying fragments. Some meet the earth in its onward rush; others, having attained inconceivable velocity, overtake and crash into the whirling sphere with loud detonation and ominous glare, finding destruction in its molecular armor, or perhaps ricocheting from it again into the unknown. Some come singly, vagrant fragments from the infinity of space; others fall in showers like golden rain; all constituting a bombardment appalling in its magnitude. It has been estimated that every twenty-four hours the earth or its atmosphere is struck by four hundred million missiles of iron or stone, ranging from an ounce up to tons in weight. Every month there rushes upon the flying globe at least twelve billion iron and stone fragments, which, with lurid accompaniment, crash into the circumambient atmosphere. Owing to the resistance offered by the air, few of these solid shots strike the earth. They move out of space with a possible velocity of thirty or forty miles per second, and, like moths, plunge into the revolving globe, lured to their destruction by its fatal attraction. The moment they enter our atmosphere they ignite; the air is piled up and compressed ahead of them with inconceivable force, the resultant friction producing an immediate rise in temperature, and the shooting star, the meteor of popular parlance, is the result.

A simple experiment, made by Joule and Thomson, well illustrates the possibility of this rise in temperature by atmospheric friction. If a wire is whirled through the air at a rate of one hundred and seventy-five feet per second, a rise of one degree, centigrade, will be noticed. If the revolutions are increased to three hundred and seventy-two feet per second, the elevation will be 5.3° C. If the temperature increases as the square of the velocity, a rate of speed

    Note.—The meteors shown in the two ideal pictures are, of course, entirely disproportionate in size to the earth and stars. If seen by an observer above the earth, we might imagine an envelope of light around the globe from the continuous ignition of the 150,000,000,000 or more meteors which it is estimated strike the earth every year; in which case, the striking meteors would be represented in the illustrations as a thin light line surrounding the atmospheric envelope of the earth.