horse-power for every
square of 5 feet, on that side of the earth's surface facing the sun,
supposing the rays to fall perpendicularly." That being so, we can
exactly calculate in horse-power the energy received from light on that
side of the earth facing the sun, at its distance of 92,000,000 miles.
The area of the earth's surface is, roughly, 200,000,000 x 5280 square
feet, and if the energy received is equal to one horse-power for every 5
square feet, then the amount of energy received by the earth on that
side facing the sun would be equal to 200,000,000 x 5280 x 1/2 x 1/5
horse-power. This power, it must be remembered, is ever directed _away
from_ the sun, and upon that side of the planet that faces the solar
orb. So that we have virtually a repulsive force ever directed against
the earth, estimated by Professor Stokes to be equal to the estimated
horse-power.

This assumption of the repulsive power of light brings the phenomena of
light into harmony with that of heat, because we have already seen (Art.
63) that heat is essentially a repulsive motion, as indicated by Davy,
Rumford and others; and, as heat and light both have a common origin,
then light should possess a repulsive power also.

As further proof of this statement, let me again quote from Clerk
Maxwell. In the quotation already given in this Art. we have seen that
the pressure of sunlight on a square foot is equal to 83.4 lb. He adds
the following words to those already quoted: "A flat body exposed to
sunlight would experience this pressure on its illuminated side only,
and would therefore be repelled from the side on which the light falls."

Now if more conclusive proof of the correctness of the argument I am
advancing were required, I do not think it could be given from any
greater authority than that just quoted. Coming from the pen of one of
the most brilliant scientists that the past century has known, I venture
to think the opinion will be received with that due weight which it
demands.

This statement of Clerk Maxwell's has received, however, definite and
experimental proof from Professor Lebedew of Moscow University, and by
Nichols and Hull of America. The former has given, in the _Annalen der
Physik_ for November 1901, the results of his experiments in relation to
the pressure of light. The following are the results: He proved, 1st,
that the incident beam of light exerts pressure both upon an absorbing
and a reflecting body; 2nd, that the