timetre of
the earth's surface are competent (apart from atmospheric absorption) to
raise the temperature of 1.7633 grammes of water one degree Centigrade
per minute. This number (1.7633) he called the "solar constant"; and the
unit of heat chosen is known as the "calorie." Hence it was computed
that the total amount of solar heat received during a year would suffice
to melt a layer of ice covering the entire earth to a depth of 30.89
metres, or 100 feet; while the heat emitted would melt, at the sun's
surface, a stratum 11.80 metres thick each minute. A careful series of
observations showed that nearly half the heat incident upon our
atmosphere is stopped in its passage through it.

Herschel got somewhat larger figures, though he assigned only a third as
the spoil of the air. Taking a mean between his own and Pouillet's, he
calculated that the ordinary expenditure of the sun per minute would
have power to melt a cylinder of ice 184 feet in diameter, reaching from
his surface to that of Alpha Centauri; or, putting it otherwise,
that an ice-rod 45.3 miles across, continually darted into the sun with
the velocity of light, would scarcely consume, in dissolving, the
thermal supplies now poured abroad into space.[700] It is nearly certain
that this estimate should be increased by about two-thirds in order to
bring it up to the truth.

Nothing would, at first sight, appear simpler than to pass from a
knowledge of solar emission--a strictly measurable quantity--to a
knowledge of the solar temperature; this being defined as the
temperature to which a surface thickly coated with lamp-black (that is,
of standard radiating power) should be raised to enable it to send us,
from the sun's distance, the amount of heat actually received from the
sun. Sir John Herschel showed that heat-rays at the sun's surface must
be 92,000 times as dense as when they reach the earth; but it by no
means follows that either the surface emitting, or a body absorbing
those heat-rays must be 92,000 times hotter than a body exposed here to
the full power of the sun. The reason is, that the rate of
emission--consequently the rate of absorption, which is its
correlative--increases very much faster than the temperature. In other
words, a body radiates or cools at a continually accelerated pace as it
becomes more and more intensely heated above its surroundings.

Newton, however, took it for granted that radiation and temperature
advance _pari passu_--that