found that the solar energy diminished, according
to a certain law, as the thickness of the air
crossed by the sunbeams increased; and from
this law M. Pouillet was enabled to infer what
the absorption would be if the rays were directed
downwards to his instrument from the zenith.
It is twenty-five per cent.

Taking into account the whole terrestrial
hemisphere turned towards the sun, the amount
intercepted by the atmospheric envelope is four-
tenths of the entire radiation in the direction of
the earth. Thus, were the atmosphere removed,
the illuminated hemisphere of the earth would
receive nearly twice the amount of heat from
the sun that now reaches it. The total amount
of solar heat received by the earth in a year, if
distributed uniformly over the earth's surface,
would be sufficient to liquefy a layer of ice one
hundred feet thick, and covering the whole
earth. It would also heat an ocean of fresh
water sixty-six miles deep, from the temperature
of melting ice to the temperature of ebullition.

The heat emitted by the sun, if used to melt
a stratum of ice applied to the sun's surface,
would liquefy the ice at the rate of two
thousand four hundred feet per hour. It would
boil, per hour, seven hundred thousand millions
of cubic miles of ice-cold water. Expressed in
another form, the heat given out by the sun,
per hour, is equal to that which would be
generated by the combustion of a layer of solid coal ten
feet thick, entirely surrounding the sun; hence,
the heat emitted in a year is equal to that which
would be produced by the combustion of a layer
of coal seventeen miles in thickness.

This, then, is the sun's expenditure which
has been going on for ages, without our being
able, in historic times, to detect the loss. When
the tolling of a bell is heard at a distance, the
sound of each stroke soon sinks, the sonorous
vibrations are quickly wasted, and renewed
strokes are necessary to maintain the sound.
Like the bell, "the sun rings in olden wise,"
but how is its tone sustained? How is the
perennial loss made good? We are apt to
overlook the wonderful in the common.
Possibly to many of us—and even to some of the
most enlightened among us—the sun appears
as a fire, differing from our terrestrial fires only
in the magnitude and intensity of its combustion.
But what is the burning matter which can thus
maintain itself? All that we know of cosmical
phenomena declares our brotherhood with the
sun—affirms that the same constituents enter
into the composition of his mass as are already
known to terrestrial chemistry. But no earthly
substance with which we are acquainted—no
substance which the fall of meteors has landed
on the earth, would be at all competent to
maintain the sun's combustion. The chemical energy
of such substances would be too weak, and
their dissipation too speedy. Were the sun a
block of burning coal, and were it supplied with
oxygen sufficient for the observed emission, it
would be utterly consumed in five thousand
years. On the other hand, to imagine it a body
originally endowed with a store of heat—a hot
globe now cooling—necessitates the ascription
to it of qualities wholly different from those
possessed by terrestrial matter. If we knew
the specific heat of the sun, we could calculate
its rate of cooling. Assuming the specific heat
to be the same as that of water—the terrestrial
substance which possesses the highest specific
heat—at its present rate of emission, the entire
mass of the sun would cool down fifteen
thousand degrees of Fahrenheit in five thousand
years. In short, if the sun be formed of matter
like our own, some means must exist of restoring
to it its wasted power.

The facts are so extraordinary, that—as the
professor well observes—the soberest hypothesis
regarding them must appear wild. Now,
however bold it may appear at first sight, the
meteoric theory of the sun deserves our careful
consideration. Kepler's celebrated statement
that "there are more comets in the heavens
than fish in the ocean," implies that a small
portion only of the total number of comets
belonging to our system are seen from the earth.
But besides comets, and planets, and moons, a
numerous class of bodies belong to our system,
which, from their smallness, might be regarded
as cosmical atoms. Like the planets and the
comets, these smaller asteroids obey the law of
gravity, and revolve in elliptic orbits round the
sun. It is they which, when they come within
the earth's atmosphere, are fired by friction, and
appear to us as meteors and falling stars.

On a bright night, twenty minutes rarely pass
at any part of the earth's surface without the
appearance of at least one meteor. Twice a
year (on the 12th of August and the 14th of
November) they appear in enormous numbers.
During nine hours in Boston, when they were
described as falling as thick as snow-flakes, two
hundred and forty thousand meteors were
observed. The number falling in a year might,
perhaps, be estimated at hundreds or
thousands of millions, and even these would constitute
but a small portion of the total crowd of
asteroids that circulate round the sun. From
the phenomena of light and heat, and by direct
observations on Encke's comet, we learn lhat
the universe is filled by a resisting medium
(the ether), through the friction of which all
the masses of our system are drawn gradually
towards the sun. And though the larger planets
show, in historic times, no diminution of their
periods of revolution, it may be otherwise with
the smaller bodies. In the time required for the
mean distance of the earth to alter a single yard, a
small asteroid may have approached thousands
of miles nearer to the sun.

Following up these reflections, we should be
led to the conclusion, that while an immeasurable
stream of ponderable meteoric matter moves
unceasingly towards the sun, it must augment
in density as it approaches its centre of
convergence. And here the conjecture naturally
rises, whether that vast nebulous mass, the
zodiacal light, which embraces the sun, may not
be a crowd of meteors. It is at least proved that
this luminous phenomenon arises from matter