Page:Popular Science Monthly Volume 37.djvu/385

Rh due to the fact that there are two distinct classes of meteorites, irons and stones, the characteristics of which make it difficult to assign a like origin to both. It is probable, however, that they all belong to our solar system; that they are revolving round the sun in some different plane from the ecliptic, and that the earth is constantly meeting them in its yearly journey. When they come-into contact with our atmosphere, although they are moving with planetary velocity—sometimes at the rate of forty-five miles a second, more than twice as fast as the earth moves in its orbit—their motion is rapidly reduced, owing to the resistance of the air, so that in most cases they come to the ground like a spent cannonball. Their passage through the air is only of a few seconds' duration, yet the rapid reduction of velocity determines a great heating effect, so that the meteorite, a moment before intensely cold, is immediately fused on the surface, forming a coating varying from a fiftieth to a hundredth of an inch in thickness, and this crust is one of the first characteristics by which a meteorite is recognized. Moreover, the material burns away unevenly, forming pittings or thumb-marks, resembling the marks left by the fingers on a mass of putty—a character also observed on large grains of partially burned powder picked up after the discharge of large guns. The meteorite from Cynthiana, Ky., in the Harvard collection, shows similar marks though more in furrows, made by a flow of the melted surface from the front to the back of the mass during its passage.

The unequal heating of meteorites by the atmosphere causes pieces to crack off, and sometimes the whole mass explodes. In addition, the air rushing in to fill the space behind the rapidly moving body, causes a sound variously compared to claps of thunder, firing of musketry, the tearing of calico, and the like—a noise frequently heard after the passage of the meteorite, owing to the circumstance that the sound travels so much more slowly than the mass itself. Furthermore, the high temperature of the surface causes the mass to glow with a brilliant light, making it appear like a ball of fire, and visible at distances depending on its height above the horizon, sometimes over an area of one thousand miles. Thus a meteorite was seen in 1876 to pass over the States of Kansas, Nebraska, Iowa, Missouri, Wisconsin, Illinois, Michigan, Kentucky, Indiana, Ohio, West Virginia, and Pennsylvania; and explosions were heard like cannonading even to the distance of one hundred and fifty miles from its course. Over Illinois it was seen to break in pieces like a rocket, and over Indiana and Ohio the pieces were computed to cover an area forty miles long and five miles broad. At Rochester, Fulton County, Indiana, during the meteorite's passage, a farmer heard the thud of something striking the ground near his house, and in the morning found a