nces observed of such meteors may be brought under the
following heads: 1. Their general appearance. 2. Their path. 3. Their
shape or figure. 4. Their light and colour. 5. Their height. 6. The
noise with which they are accompanied. 7. Their fire. 8. Duration, 9.
Their velocity. Under these different heads meteors have been
investigated by the scrutinizing of philosophy, and many superstitious
notions, long entertained concerning them, entirely exploded. Meteoric
phenomena, it has been demonstrated, all proceed from one common
cause--irregularity in the density of the atmosphere. When the
atmospheric fluid is homogenous and of equal density, the rays of light
pass without obstruction or alteration in their shape or direction; but
when they enter from a rarer into a denser medium, they are refracted or
bent out of their course; and this with greater or less effect according
to the different degrees of density in the media, or the deviation of
the ray from the perpendicular. If the second medium be very dense in
proportion, the ray will be both refracted and reflected; and the object
from which it proceeds, will assume a variety of grotesque and
extraordinary shapes, and it will sometimes appear as in a reflection
from a concave mirror, dilated in size, and changed in situation.

The following striking effects are known to proceed from this simple
cause.

The first is the mirage, seen in the desert of Africa. M. Monge, a
member of the National Institute, accompanied the French army into
Egypt. In the desert, between Alexandria and Cairo, the mirage of the
blue sky was inverted, and so mingled with the sand below, as to impart
to the desolate and arid wilderness an appearance of the most rich and
beautiful country. They saw, in all directions, green islands,
surrounded with extensive lakes of pure and transparent water. Nothing
could be conceived more lovely and picturesque than this landscape. On
the tranquil surface of the lakes, the trees and houses, with which the
islands were covered, were strongly reflected with vivid hues, and the
party hastened forward to enjoy the cool refreshments of shade and
stream, which these populous villages preferred to them. When they
arrived, the lake, on whose bosom they floated, the trees, among whose
foliage they were embowered, and the people who stood on the shore
inviting their approach, had all vanished, and nothing remained but an
uniform and irksome desert of sand and sky, with