The solar heat is due to two principal causes—the condensation of the original nebula, and the fall of meteorites. According to the best established calculations of thermodynamics, the former has produced a quantity of heat eighteen million times greater than that which the sun radiates yearly, supposing the original nebula was cold, which there is no reason to believe was the case. It is, therefore, certain that the solar temperature produced by this condensation far exceeded the above. If condensation continues, the radiation of heat may go on for centuries without loss. The heat emitted every second is equal to that which would result from the combustion of eleven quadrillions six hundred thousand milliards of tons of coal burning at once! The earth intercepts only one five hundredth millionth part of the radiant heat, and this one five hundredth millionth suffices to maintain all terrestrial life. Of sixty-seven millions of light and heat rays which the sun radiates into space, only one is received and utilized by the planets. [Illustration] Well! to maintain this source of heat it is only necessary that the rate of condensation should be such that the sun’s diameter should decrease seventy-seven meters a year, or one kilometer in thirteen years. This contraction is so gradual that it would be wholly imperceptible. Nine thousand five hundred years would be required to reduce the diameter by one single second of arc. Even if the sun be actually in a gaseous state, its temperature, so far from growing less, or even remaining stationary, would increase by the very fact of contraction; for if on the one hand the temperature of a gaseous body falls when it condenses, on the other hand the heat generated by contraction is more than sufficient to prevent a fall in temperature, and the amount of heat increases until a liquid state is reached. The sun seems to have reached this stage. The condensation of the sun, whose density is only one-fourth that of the earth, may thus of itself maintain for centuries, at least for ten million years, the light and heat of this brilliant star. But we have just spoken of a second source of heat: the fall of meteorites. One hundred and forty-six million meteorites fall upon the earth yearly. A vastly greater number fall into the sun, because of its greater attraction. If their mass equals about the one hundredth part of the mass of the earth, their fall would suffice to maintain the temperature,—not by their combustion, for if the sun itself was being consumed it would not have lasted more than six thousand years, but by the sudden transformation of the energy of motion into heat, the velocity of impact being 650,000 meters per second, so great is the solar attraction.