5 o6 POPULAR SCIENCE MONTHLY. 



property," as, for example, upon all personal property; for if the 

 removal of the burden operates uniformly on all interested, or own- 

 ing such property, then there can be no primary exemption. 



THE GKEAT BOMBARDMENT. 



By CHARLES F. HOLDER. 



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 frag- 

 ments. 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 apjDalling 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 shoot- 

 ing star, the meteor of popular parlance, is the result. 



A simple experiment, made by Joule and Thomson, well illus- 

 trates the possibility of this rise in temperature by atmospheric fric- 

 tion. 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 dispropor- 

 tionate 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 surround- 

 ing the atmospheric envelope of the earth. 



