We may conclude our survey of the solar system with a word about
"shooting stars," or meteors, and comets. There are few now who do not
know that the streak of fire which suddenly lights the sky overhead at
night means that a piece of stone or iron has entered our atmosphere
from outer space, and has been burned up by friction. It was travelling
at, perhaps, twenty or thirty miles a second. At seventy or eighty miles
above our heads it began to glow, as at that height the air is thick
enough to offer serious friction and raise it to a white heat. By the
time the meteor reached about twenty miles or so from the earth's
surface it was entirely dissipated, as a rule in fiery vapour.