I want to call the attention of the reader to another fact, and that is, that light always proceeds in straight lines from the sun (Art. 76), and therefore if there be any mechanical action in light at all, that action must be one which is always directed from the sun in straight lines. Now experience universally teaches us, that if a body is pushed, and pushed with such a force as has been indicated, then that body not only moves, but moves in the direction that the supposed horses would push. I have already shown (Art. 76) that the path of light is that of a straight line corresponding to the path of the attractive force of gravity; therefore these horses must ever push in a direction _from_ the sun along the same path that the sun's attractive power takes. In other words, the mechanical action of these supposed horses will be a repulsive one, that repulsion being due to the dynamical action of the light waves upon the body that they come into contact with. If this is correct, then not only is heat a repulsive motion, as stated in Art. 63, but light is equally the possessor of a repulsive motion, because its action is ever directed from the sun. We might continue to follow the supposed horses as they continued their course through space, and we should find that their energy decreased inversely as the square of the distance, partly because the further they proceeded into space the larger the area would be they would have to cover, and therefore their energy would be decreased proportionately. Professor Stokes, in the same work[19] already referred to, in continuation of the same idea, states: "At the distance of the earth the energy received would correspond to about one 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 × 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 × 5280 × 1/2 × 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.