Another theory, better supported, has been put forward by the physicists Mayer, Watterston, and Thompson; it explains the maintenance of the solar heat by a constant fall of meteors on the surface of the solar star. A multitude of corpuscles gravitate round the sun, and approach him with sufficient nearness to be attracted by his surface, and fall upon it. These are _asteroids_, which turn in whirling swarms around the sun. A shower of corpuscles, of meteorolites, may be always falling on his surface. Their fall would cause a great development of caloric, in consequence of the transformation of their enormous speed into heat, and this caloric would suffice, according to the authors of this theory, for the maintenance of solar radiation. Let us quote Professor Tyndall on this point: "It is easy to calculate both the maximum and the minimum velocity imparted by the sun's attraction to an asteroid circulating round him. The maximum is generated when the body approaches from an infinite distance, the _entire pull_ of the sun being then exerted upon it. The minimum is that velocity which would barely enable the body to revolve round the sun close to its 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 9000 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 upwards of 4000 such asteroids. It matters not therefore 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. Here then we have an agency competent to restore his lost energy to the sun, and to maintain a temperature at his surface which transcends all terrestrial combustion. "The very quality of the solar rays--their incomparably penetrative power--enables us to infer that the temperature of their origin must be enormous; but in the fall of asteroids we find the means of producing such a temperature."--P. 423. The fall of these asteroids on the surface of the sun would be followed by an increase in the bulk of that star, and there has been no such increase since the earliest period of its observation. Also, the augmentation of the sun's bulk by these foreign bodies, would have produced an accelerant motion in the orbits of all the stars, which, however slight, would be distinctly perceptible; whereas, for the 2000 years of celestial observation, whose records we possess, unbroken and perfect regularity in the progression of the stars of our solar world is registered.