21. Further, the surface of a meteorite is generally covered with pittings, which have been compared in form to thumb-marks: stones from the Supuhee, Futtehpur, and Pane 4h. Knyahinya falls present good examples of this character. It is remarkable that pittings bearing a close resemblance to those of meteorites have been observed on the large partially burned grains of gunpowder, which have been Pane 4h. picked up near the muzzle after the firing of the 35-ton and 80-ton guns at Woolwich. The pitting of the gunpowder grains is attributed to unequal combustion, but that of meteorites seems to be due not so much to inequality of combustibility as to that of conductivity, fusibility and frangibility of the matter at the surface.
Fragmentary form of meteorites.
22. As picked up, complete and covered with crust, meteorites are not spherical, nor have they any definite shape: in fact, they are always irregular angular fragments, such as would be obtained on breaking up a rock presenting no regularity of structure.
In the case of the Butsura fall of May 12, 1861,[12] fragments of the stone were picked up three or four miles apart, and, wonderful to say, it was possible to reconstruct Pane 4h. with much certainty the portion of the meteorite to which they once belonged: a model of the reconstructed portion is Pane 4a. shown in the case. Two of the fragments, in other respects fitting perfectly together, are even on the faces of the junction now coated with a black crust, showing that one disruption took place when the meteorite had a high velocity; two other fragments found some miles apart fitted perfectly, and were neither of them incrusted at the surface of fracture, thus indicating another disruption at a time when the velocity of the meteorite had been so far reduced that the material of the new faces was not blackened through the generation of heat. Sometimes, as in the case of the meteorite of Orgueil, the fragments reach the ground before the detonation is heard, proving that the fracture has taken place at a part of the course where the velocity of the meteorite was considerably greater than that of the sound-vibrations (1100 feet a second).
The detonations.
23. The sudden condensation of air in front of the meteorite, the consequent generation of heat and expansion of the outer shell, have been held to account not only for the break-up of the meteorite into fragments, but partly also for the crash like that of thunder which is a usual accompaniment of the fall. Others have referred this noise solely to the sudden rush of air into the space traversed by the meteorite in the early part of the course. It has, however, now been discovered that the mere flight of a projectile through the air with a velocity exceeding that of sound (1100 feet a second) is itself sufficient to cause a loud detonation; neither explosion, like that of a bomb-shell, nor simple fracture of the meteorite by reason of pressure or sudden heat, is a necessary preliminary to the production of the loud noise. It is found, in fact, that when a projectile is fired with high initial velocity, say 2350 feet a second, an observer near the path of the projectile begins to distinguish two detonations as soon as his distance from the cannon reaches 500 feet; the first of them, a sharp one, appears to come from that part of the projectile's path which is nearest to the observer, and travels with the velocity of the projectile; the later and duller one appears to come from the cannon itself, and travels with the velocity of sound. If the projectile is intercepted near the cannon, only a single detonation is heard by an observer in the same position as before, and it travels at the rate of 1100 feet a second. If the initial velocity of the projectile is less than that of sound, only a single detonation is heard, and it starts from the cannon.
The rolling sound, which follows the detonation of a meteorite, is due, as in the case of thunder, to echoes from the ground and the clouds.
The detonations due to the different members of a swarm of meteorites will combine to form a single detonation unless they are separated by perceptible intervals of time.
The sounds heard after the loud detonations.
24. After the detonation, sounds are generally heard which have been variously likened to the flapping of the wings of wild geese, the bellowing of oxen, Turkish music, the roaring of a fire in a chimney, the noise of a carriage on the pavement, and the tearing of calico: these sounds are probably due to the whirling and oscillation of the fragments while traversing the air, with small velocity, near the observers, and correspond to the hiss or hum observed in the case of a projectile travelling with a velocity less than that of sound.