Meteorite

METEORITE, a mass of matter from outer space, which has fallen upon the earth's surface. These masses are made up usually of stony matter with varying amounts of metallic iron containing nickel; more rarely of nickeliferous iron alone; and much more rarely of stony matter with little or no metal.

Before coming in contact with the earth, these bodies have been travelling through space with planetary velocities of many miles a second. It is not surprising, therefore, that their arrival in the earth's atmosphere is heralded by very startling phenomena of light and sound. Owing to the resistance of the air, the meteorite becomes incandescent and is then seen as a scintillating ball of fire, sometimes with an apparent diameter greater than that of the moon. The fireball leaves behind it a trail of luminous matter, like a gigantic shooting-star of which the duration of flight has been much prolonged. The period of incandescence, however, is still only a matter of seconds for, as a result probably of the sudden condensation of the air in front of the moving mass and the accumulating pressure, the meteorite soon loses its planetary speed and eventually bursts into fragments. As a consequence partly of this shattering, but mainly of the sudden explosive shock and rise of temperature given to the air by the rapid passage of the meteorite, a short time after the disappearance of the fireball, loud detonations like thunder are heard, and these are generally followed by weird sounds which have been likened to the bellow ing of oxen, the roaring of a fire in a chimney, the tearing of calico, etc. Owing to the rapid reduction in speed of the meteorite, the fragments reach the ground like ordinary falling bodies with velocities not greater than a few hundred feet a second. They, therefore, penetrate the soil to a depth of only a few feet, and in one particular fall, which took place in 1869 at Hessle in Sweden, stones which fell upon ice only a few inches thick rebounded from the surface. Moreover, in spite of the fact that the original mass had been rendered incandescent, the time of flight is too short for any real penetration of heat beneath the surface. Accordingly, the stones, when they reach the ground, are generally only slightly warm to the touch, and the sole evidence of the intense heat to which they had been subjected is a black fused crust which covers them (occasionally only partially), but is rarely more than a milli metre thick. The characteristic feature of most stony meteorites is this black crust, contrasting so remarkably with the white to grey interior scattered through which can usually be seen bright specks of metallic iron and often curious rounded bodies known as chondrules. In these respects meteoric stones are very different

from the rounded nodules of sulphide of iron which weather out of the chalk on the S.E. coast of England and are often called "thunderbolts" and mistaken for meteorites. The term "thunder bolt" indeed as applied to meteorites is a misnomer since they have no connection with thunderstorms, and reports of the fall of "thunderbolts" during storms really refer to lightning-strokes.

It is only under favourable conditions that such phenomena have been observed in their entirety. At the actual place of fall often only detonations have been heard, although the fireball may have been seen at places miles away, and when on March 9, 1923, a small stone of about 3 lb. fell a few yards from a labourer at Ashdon, Essex, only the whizzing of its flight was heard.

Occurrence of Falls.

That the advent of these bodies is an event of somewhat rare occurrence is evident from the fact that the number of falls of which specimens have been preserved is only about i,000, and that of these no more than 15 have taken place in the British Isles. The number of new falls recorded for the whole world in any one year is generally less than ten, though doubtless others occur in sparsely populated regions and escape human observation. One of the earliest falls to be recorded took place about 644 B.C., strange to say, in China, for until quite recently no meteorite in that country appears to have been pre served. Other ancient falls of stones, some of which were made objects of worship, are recorded by Plutarch and Pliny. The stone referred to in the Acts as the image of Diana of the Ephe sians "which fell down from Jupiter" was probably a meteoric stone, as is also doubtless the sacred stone built into the Kaaba at Mecca. The earliest known meteoric stone still preserved and of which details were placed on record is that weighing about 26o lb. which, after a loud crash like thunder, fell at Ensisheim in Alsace about noon on Nov. 16, 1492, and was seen by a child to strike the ground, where it buried itself to a depth of 5 ft. In later years several other falls were as definitely recorded, including that of a 56 lb. stone (now in the Natural History Museum) which fell within ro yd. of a labourer at Wold Cottage in Yorkshire in 1795. Nevertheless it was not until after the publication of the detailed report made by the French physicist Biot on the marvel lous fall of about 2,000 stones which took place at L'Aigle in France on April 26, 1803, that the fact of solid bodies falling from outer space was finally accepted by scientists.