It was thought for a time that the tail was flung away from the Sun by stupendous repelling electrical forces. That electricity plays its part in the formation of the fairy plume is conceivable, and even probable; but recently the physicist has discovered a new source of repellent energy which very plausibly explains the mystery of a Comet’s tail. This new source of energy is nothing less than the pressure or push of the Sun’s light. Solar gravitation is a force more powerful than we can realize. If it were possible for us to live on the Sun, we would find ourselves pulled down so violently that our body would weigh two tons. Our clothing alone would weigh more than one hundred pounds. Running would be a very difficult athletic feat. Light-pressure must indeed be powerful if it can conquer so relentless a force. Because we have never seen objects torn from our hands by the pressure of light, it may be inferred that this newly discovered force affects only bodies that are invisibly small. With the aid of instruments that feel what our hands can never feel and see what our eyes can never see, the modern physicist has critically analyzed the radiation that beats upon the earth from the distant Sun. Light really does sway infinitely small particles, as was first experimentally proved by the Russian Lebedev. Two American astronomers, Nichols and Hull, improved upon his method. They cast the solar effulgence into mighty mathematical scales and found that the earth sustains a light-load of no less than 75,000 tons. Most city-bred people are familiar with the so-called “Sun Motors”—little mills with black and white wings, enclosed in airtight vessels, which spin around in “perpetual motion” under the effect of “Sun Pressure.” It remained for the broad mind of a Swedish physicist, Svante Arrhenius, to apply the principle of light-pressure cosmically. He explained, very simply, that because a Comet’s tail is composed of a very fine dust it can easily be driven away from the Sun by radiation pressure. To understand how it is possible for so immaterial a thing as a sunbeam to produce so huge an effect, we have only to take a very simple example. Assume that you have before you a block of wood weighing one pound. The block exposes a certain amount of surface to the Sun’s light. Saw the block in half, and you increase the amount of that surface. Divide each half again into half, and the exposed surface is further augmented. If this process of subdivision is carried on far enough, the block will be reduced to sawdust.