lision has converted
the motion of the ball into intense heat, or when we jump from the table
to the floor, the temperature of the body is slightly raised,--the
degree of heat produced in both cases being ascertainable by the
application of Joule's law.

The principle thus demonstrated has given a new interest and a vast
impulse to the science of Thermotics. It is the fundamental and
organizing conception of Professor Tyndall's work, and in his last
chapter he carries out its application to the planetary system. The
experiments of Herschel and Pouillet upon the amount of solar heat
received upon the earth's surface form the starting-point of the
computations. The total amount of heat received by the earth from the
sun would be sufficient to boil three hundred cubic miles of ice-cold
water per hour, and yet the earth arrests but 1/2,300,000,000 of the
entire thermal force which the sun emits. The entire solar radiation
each hour would accordingly be sufficient to boil 700,000,000,000 cubic
miles of ice-cold water! Speculation has hardly dared venture upon the
source of this stupendous amount of energy, but the mechanical
equivalent of heat opens a new aspect of the question. All the celestial
motions are vast potential stores of heat, and if checked or arrested,
the heat would at once become manifest. Could we imagine brakes applied
to the surface of the sun and planets, so as to arrest, by friction,
their motions upon their axes, the heat thus produced would be
sufficient to maintain the solar emission for a period of one hundred
and sixteen years. As the earth is eight thousand miles in diameter,
five and a half times heavier than water, and moves through its orbit at
the rate of sixty-eight thousand miles an hour, a sudden arrest of its
motion would generate a heat equal to the combustion of fourteen globes
of anthracite coal as large as itself. Should it fall into the sun, the
shock would produce a heat equal to the combustion of five thousand four
hundred earth-globes of solid coal,--sufficient to maintain the solar
radiation nearly a hundred years. Should all the planets thus come to
rest in the sun, it would cover his emission for a period of forty-five
thousand five hundred and eighty-nine years. It has been maintained that
the solar heat is actually produced in this way by the constant
collision upon his surface of meteoric bodies, but for the particulars
of this hypothesis we must refer to the book itself.

P