The reason why meteoric impact would suffice to warm the sun to his present temperature if the meteoric showers were heavy, and to warm him far beyond his present temperature if for a few days very heavy meteoric showers fell upon him, is simply that his attraction upon matter approaching him from without is capable of generating a tremendous velocity. We know that when a cannon-ball strikes a metal target, with a velocity perhaps of some 400 yards per second, great heat is excited, and there is a momentary flash of light. If the velocity were doubled, the quantity of heat would be doubled also. Conceive, then, the tremendous heat which would be excited if a cannon-ball could be caused to strike a target with a velocity exceeding that just named some 1500 times! The ball and target would both be vaporised by the shock, if--which, however, could never happen--the target resisted the blow and brought the ball to rest. Now matter which reaches the sun from without, under the influence of his tremendous attraction, strikes his globe with a velocity 1500 times greater than that of a cannon ball striking a target at a distance of two or three hundred yards. The heat excited is, therefore, very intense; and if meteors were showering at all times and in dense flights upon the sun's surface, we should require no other explanation of the sun's heat. But it appears that meteoric systems are neither so numerous nor so rich as to account for the sun's uniform emission of heat, though occasional meteoric showers upon the sun may be heavy enough to increase appreciably the amount of heat he emits. It would seem, from experiments which have been made by Professor Piazzi Smyth, of the Edinburgh Observatory, and later by the Astronomer Royal at Greenwich, that from time to time the sun's emission of heat really is greater than usual. It seems not at all improbable that the increase is due to the occasional fall of large masses of meteors in great numbers upon the sun.