AEROLOGY

On examining and comparing tlie aerolites, tlie first circumstance that strikes us as remarltable is tiieir per feet resemblance to one another in their composition, whatever be their form or magnitude. Their exterior surface is black, as if they had been exposed to the heat of a furnace. Internally they are of a greyish white. Their specific gravity, which is very nearly the same in all of them, varies between 3*352 and 4'281, that of water being taken as unit. Their chemical analysis gives, in almost every instance, the same substances, combined in very nearly the same proportions. They are composed of silex, magnesia, sulphur, iron in the metallic state, nickel, and some traces of chrome. Sometimes they are formed of a spongy or cellular texture, the cavities being filled with a stony substance. They have occasionally been found without nickel. These common and constant characters indicate with the greatest evidence a common origin, and their composition renders it probable that it is to be sought elsewhere than in the earth. Iron is scarcely ever found (if, indeed, it is found at all) in the metallic state in terrestrial substances; volcanic matter contains it only in the state of an oxide. Nickel is also very rare, and never found on the surface of the earth; and chrome is still more rare. The fall of the aerolites is accompanied by meteors, named bolides, or fire balls. They are, in fact, inflamed globes, which appear instantaneously in the atmosphere, and move through it with extreme velocity, sometimes even eqiial to that of the earth in its orbit. The direction of their motion is inclined to the horizon. After shining with great splendour for a lew instants, they explode with a loud noise, and often at a great height, 30 or 40 miles above the surface of the earth. They do not affect any peculiar direction with respect to the motion of the earth, but seem to come from all points of the heavens indifferently. With regard to the hj-pothesis which explains the origin of the aerolites, by supposing them to be propelled from lunar volcanos, it may be remarked, that no improbable amount of mechanical force would be required. As there is no atmosphere about the moon sufficient to offer a sensible resistance to the motion of a solid body, the force required is only that which would be sufficient to overcome the moon's attraction, which is found by calculation to be about four times the force with which a ball is expelled from a cannon with the ordinary charge of gunpowder. A body projected with a velocity of about 7770 feet per second from the lunar surface, would be detached from the moon, and be brought to the earth by terrestrial gravitation. But philosophers seem now disposed to a ign the aerolites a different origin. From the phenomena of comets there is reason to believe that portions of chaotic matter are dispersed in the planetary regions in detached parcels, or perhaps in considerable ma es. The earth in describing its orbit may meet with such ma es directly, or pa so near to them as to carry them along with it by virtue of its attraction. On plunging into the atmosphere with the velocity due to the height from which they have fallen, which is that of their distance from the earth, when they begin to obey its attractive force, an enormous heat is evolved by the rapid and powerful condensation of the air; the matter becomes inflamed, and the aerolite is the product of the combustion. In the same manner shooting stars, and other igneous meteors of frequent occurrence, are explained. The chaotic matter may be entirely consumed long before it reaches the earth, in which case the appearance of the bolide will not be accompanied with the fall of an aerolite. (See Metieor. ) Philosophers were long inclined to disbelieve in the fall of stones through the atmosphere. The fact, however, is now placed beyond all doubt by numerous and well authenticated instances which have occurred in almost all quarters of the world, even within the present century. A very complete list of the falls of stony or earthy matters, with the times and places of their occurrence, and the appearances they exhibited, is published in the Edinburgh Philosophical Journal, from a work by Chladni, in German, in which the whole subject of meteoric stones is ably and fully treated. [s. 31]