situated along their track at various points, and thus a knowledge has
been obtained of their height above the ground during their flight and
of the length of their visible courses. They generally appear at an
elevation of eighty or a hundred miles, and are seldom visible after
having descended to within five miles of the ground, unless the
observer happens to be near the striking-point, when he may actually
witness the fall. Frequently they burst while high in the air and
their fragments are scattered like shrapnel over the surface of the
ground, sometimes covering an area of several square miles, but of
course not thickly; different fragments of the same meteorite may
reach the ground at points several miles apart. The observed length of
their courses in the atmosphere varies from fifty to five hundred
miles. If they continued a long time in flight after entering the air,
even the largest of them would probably be consumed to the last scrap,
but their fiery career is so short on account of their great speed
that the heat does not have time to penetrate very deeply, and some
that have been picked up immediately after their fall have been found
cold as ice within. Their size after reaching the ground is variable
within wide limits; some are known which weigh several tons, but the
great majority weigh only a few pounds and many only a few ounces.

Meteorites are of two kinds: stony meteorites and iron meteorites. The
former outnumber the latter twenty to one; but many stone meteorites
contain grains of iron. Nickel is commonly found in iron meteorites,
so that it might be said that that redoubtable alloy nickel-steel is
of cosmical invention. Some twenty-five chemical elements have been
found in meteorites, including carbon and the ``sun-metal,'' helium.
The presence of the latter is certainly highly suggestive in
connection with the question of the origin of meteorites. The iron
meteorites, besides metallic iron and nickel, of which they are almost