Page:Popular Science Monthly Volume 29.djvu/390

376 of the observer. A single explosion is rare. There are generally two or three of them. Sometimes they are violent enough to shake houses and give the impression of an earthquake, as was the case in Iowa when the meteor of the 12th of February, 1875, fell. They are often heard over a considerable extent of country, as was the case with the Orgueil meteors, the explosions of which were heard three hundred miles away. When we reflect that these detonations take place at heights where the thinly rarefied air affords a very poor medium for the propagation of sound, we become satisfied that they must be extremely violent. Sometimes a trail of vapors is perceived in the regions of the atmosphere which the body has traversed. These phenomena are manifested in the most diverse regions of the globe, at every season and every hour, and frequently in calm and cloudless weather. Storms and whirlwinds, therefore, have nothing to do with them. Their speed as observed by us being only relative, varies according to the correspondence or non-correspondence of the direction of their path with the course which the earth is pursuing.

The outer configuration of meteorites is remarkable for its fragmentary aspect, or for its angular formations and its likeness to irregular polyhedrons, the edges of which have been blunted.

The number of stones brought down in a single meteoric shower is extremely variable. Sometimes only one is found; sometimes many; and, in rare cases, hundreds and thousands. At the instant the stones reach us their velocity is small, compared with that which the body of which they are fragments had previous to the explosion. If they are of considerable size, they will perhaps bury themselves at a slight depth under a yielding soil, and remain there unperceived. After all the light they give and the noise they make in their flight, the minuteness of the masses which we find upon the surface of the ground is sometimes surprising. The largest one ever found—at St. Catherine, Brazil, 1875—weighed 25,000 kilogrammes; stones of more than 300 kilogrammes, like the one that fell at New Concord on the 1st of May, 1860, are rare, while the weight of 50 kilogrammes is seldom exceeded. Often whole meteorites weigh only a few grammes, or are of the size of a hen's egg, a walnut, or a hazel-nut; and masses of still smaller ones have been observed when they fell upon a bed of snow, as at Hersle, near Upsala, Sweden, in 1869, when many of the stones weighed only a few decigrammes, and one of them as little as six centigrammes. These little grains, it should be remarked, were not fragments broken off by the shock of larger pieces against the ground; but each one was a complete meteorite, enveloped in a crust of half-melted matter. That so small meteors had not been noticed before is explained by the difficulty of distinguishing them from the particles composing the general surface, among which they are lost.

When the meteors of the same shower are numerous, they are generally distributed at various points within an elongated oval area, the