_Gravitation Theory of the Origin and Source of the Sun’s Heat._—There are two forms in which this theory has been presented: the first, the meteoric theory, propounded by Dr. Meyer, of Heilbronn; and the second, the contraction theory, advocated by Helmholtz. It is found that 83·4 foot-pounds of heat per second are incident upon a square foot of the earth’s surface exposed to the perpendicular rays of the sun. The amount radiated from a square foot of the sun’s surface is to that incident on a square foot of the earth’s surface as the square of the sun’s distance to the square of his radius, or as 46,400 to 1. Consequently 3,869,000 foot-pounds of heat are radiated off every square foot of the sun’s surface per second—an amount equal to about 7,000 horse power. The total amount radiated from the whole surface of the sun per annum is 8,340 × 10^{30} foot-pounds. To maintain the present rate of radiation, it would require the combustion of about 1,500 lbs. of coal per hour on every square foot of the sun’s surface; and were the sun composed of that material, it would be all consumed in less than 5,000 years. The opinion that the sun’s heat is maintained by combustion cannot be entertained for a single moment. A pound of coal falling into the sun from an infinite distance would produce by its concussion more than 6,000 times the amount of heat that would be generated by its combustion. It is well known that the velocity with which a body falling from an infinite distance would reach the sun would be equal to that which would be generated by a constant force equal to the weight of the body at the sun’s surface operating through a space equal to the sun’s radius. One pound would at the sun’s surface weigh about 28 pounds. Taking the sun’s radius at 441,000 miles,[201] the energy of a pound of matter falling into the sun from infinite space would equal that of a 28-pound weight descending upon the earth from an elevation of 441,000 miles, supposing the force of gravity to be as great at that elevation as it is at the earth’s surface. It would amount to upwards of 65,000,000,000 foot-pounds. A better idea of this enormous amount of energy exerted by a one-pound weight falling into the sun will be conveyed by stating that it would be sufficient to raise 1,000 tons to a height of 5½ miles. It would project the _Warrior_, fully equipped with guns, stores, and ammunition, over the top of Ben Nevis.