s, that there is no difficulty in appreciating the
age of which it is the characteristic. The most superficial outline
enables us to recognize at once its resemblance to that period of Greek
life to which I have referred. To bring its features into relief, I
shall devote a few pages to a cursory review of the progress of some of
the departments of science, selecting for the purpose topics of general
interest.

First, then, as respects the atmosphere, and the phenomena connected
with it.

[Sidenote: The atmosphere.] From observations on the twilight, the
elasticity of aerial bodies, and the condensing action of cold, the
conclusion previously arrived at by Alhazen was established, that the
atmosphere does not extend unlimitedly into space. Its height is
considered to be about forty-five miles. From its compressibility, the
greater part of it is within a much smaller limit; were it of uniform
density, it would not extend more than 29,000 feet. Hence, comparing it
with the dimensions of the earth, it is an insignificant aerial shell,
in thickness not the eightieth part of the distance to the earth's
centre, and its immensity altogether an illusion. It bears about the
same proportion to the earth, that the down upon a peach bears to the
peach itself.

A foundation for the mechanical theory of the atmosphere was laid as
soon as just ideas respecting liquid pressures, as formerly taught by
Archimedes, were restored, the conditions of vertical and oblique
pressures investigated, the demonstration of equality of pressures in
all directions given, and the proof furnished that the force of a liquid
on the bottom of a vessel may be very much greater than its weight.

[Sidenote: Its mechanical relations.] Such of these conclusions as were
applicable were soon transferred to the case of aerial bodies. The
weight of the atmosphere was demonstrated, its pressure illustrated and
measured; then came the dispute about the action of pumps, and the
overthrow of the Aristotelian doctrine of the horror of a vacuum.
Coincidently occurred the invention of the barometer, and the proof of
its true theory, both on a steeple in Paris and on a mountain in
Auvergne. The invention of the air-pump, and its beautiful illustrations
of the properties of the atmosphere, extended in a singular manner the
taste for natural philosophy.

[Sidenote: Its chemical relations.] The mechanics of the air was soon
followed by its chemistry. From remote ages it