"Following up these reflections we should infer that while this immeasurable stream of ponderable matter rolls unceasingly towards the sun, it must augment in density as it approaches the center of convergence. And here the conjecture naturally rises that that weak nebulous light, of vast dimensions, which embraces the sun--the Zodiacal light--may owe its existence to these crowded meteors. However this may be, it is at least proved that the luminous phenomenon arises from matter which circulates in obedience to planetary laws; the entire mass constituting the Zodiacal light must be constantly approaching, and incessantly raining its substance down upon, the sun. "We observe the fall of an apple and investigate the law which rules its motion. In the place of the earth we set the sun, and in place of the apple we set the earth, and thus possess ourselves of the key to the mechanics of the heavens. We now know the connection between hight of fall, velocity, and heat at the surface of the earth. In the place of the earth let us set the sun, with 300,000 times the earth's mass, and instead of a fall of a few feet, let us take cosmical elevations; we thus obtain a means of generating heat which transcends all terrestrial power. "It is easy to calculate both the maximum and the minimum velocity imparted by the sun's attraction to asteroids circulating round him; the maximum is generated when the body approaches the sun from an infinite distance as the entire pull of the sun being then expended upon it; the minimum is that velocity which would barely enable the body to revolve round the sun close to his surface. The final velocity of the former, just before striking the sun, would be 390 miles a second, that of the latter 276 miles a second. The asteroid on striking the sun with the former velocity, would develop more than 3,000 times the heat generated by the combustion of an equal asteroid of solid coal; while the shock, in the latter case, would generate heat equal to that of the combustion of upward of 4,000 such asteroids. It matters not whether the substances falling into the sun be combustible or not; their being combustible would not add sensibly to the tremendous heat produced by their mechanical collision.