All moving power has its origin in the rays of the sun. While
Stephenson’s iron tubular railway-bridge over the Menai Straits, 400
feet long, bends but half an inch under the heaviest pressure of a
train, it will bend up an inch and a half from its usual horizontal
line when the sun shines on it for some hours. The Bunker-Hill
monument, near Boston, U.S., is higher in the evening than in the
morning of a sunny day; the little sunbeams enter the pores of the
stone like so many wedges, lifting it up.
In winter, the Earth is nearer the Sun by about 1/30 than in summer;
but the rays strike the northern hemisphere more obliquely in winter
than the other half year.
M. Pouillet has estimated, with singular ingenuity, from a series of
observations made by himself, that the whole quantity of heat which the
Earth receives annually from the Sun is such as would be sufficient to
melt a stratum of ice covering the entire globe forty-six feet deep.
By the action of the sun’s rays upon the earth, vegetables, animals,
and man, are in their turn supported; the rays become likewise, as
it were, a store of heat, and “the sources of those great deposits
of dynamical efficiency which are laid up for human use in our coal
strata” (_Herschel_).
A remarkable instance of the power of the sun’s rays is recorded at
Stonehouse Point, Devon, in the year 1828. To lay the foundation of a
sea-wall the workmen had to descend in a diving-bell, which was fitted
with convex glasses in the upper part, by which, on several occasions
in clear weather, the sun’s rays were so concentrated as to burn the
labourers’ clothes when opposed to the focal point, and this when the
bell was twenty-five feet under the surface of the water!