^{page 63 note *} Herschel's, Astronomy, Edition 1833.—See last Ed., § (399).Google Scholar

^{page 63 note †} Joule “On the Generation of Heat in the Galvanic Circuit,” communicated to the Royal Society of London, Dec. 17, 1840, and published, Phil. Mag., Oct. 1841. “On the Heat evolved during the Electrolysis of Water,” Literary and Phil. Soc. of Manchester, 1843, Vol. vii., Part 3., Second Series. “On the Calorific Effects of Magneto-Electricity, and the Mechanical Value of Heat,” communicated to the British Association, August 1843, and published, Phil. Mag., Sept. 1843. “On the Changes of Temperature produced by the Rarefaction and Condensation of Air,” communicated to the Royal Society, June 1844, and published, Phil. Mag., May 1845. Joule and Scoresby “On the Powers of Electromagnetism, Steam, and Horses,” Phil. Mag., June 1846.

^{page 63 note ‡} Once every 20 years or so.—See Table of Mechanical Energies of the Solar System, appended.

^{page 63 note §} “Actual energy,” as Mr Rankine has called it.

^{page 64 note *} This assertion is founded on the supposition that conduction is the only means by which heat could reach the Sun's surface from the interior, and perhaps requires limitation. For it might be supposed that, as the Sun is no doubt a melted mass, the brightness of his surface is constantly refreshed by incandescent fluid rushing from below to take the place of matter falling upon the surface after becoming somewhat cooled and consequently denser—a process which might go on for many years without any sensible loss of brightness. If we consider, however, the whole annual emission at the present actual rate, we find, even if the Sun's thermal capacity were as great as that of an equal mass of water, that his mean temperature would be lowered by about 3° cent. in two years We may, I think, safely conclude that primitive heat within the Sun is not a sufficient source for the emission which has continued without sensible (if any) abatement for 6000 years.—(May 4 1854.)

^{page 65 note *} To make the argument perfectly conclusive, it would have to be assumed that meteors not only are, but have been, always falling to the earth for some immense period of time. The conclusion, however, appears sufficiently probable with the facts we know.

^{page 65 note †} See Philosophical Magazine, May 1848, for reference to a lecture in Manchester, on the 28th April 1847, in which Mr Joule said, that “the velocity of a meteoric stone is checked by the atmosphere and its vis viva converted into heat, which at last becomes so intense, as to melt the body and dissipate it in fragments too small probably to be noticed in their fall to the ground, in most cases.”

^{page 66 note *} Mémoire sur la Chaleur Solaire, &c, Paris 1838; See Comptes Rendus, July 1838; or Pouillet, Traité de Physique, vol. ii.

^{page 66 note †} This is rather more than double the estimate Mr Waterston has given. The velocity of impact which he has taken is 545 miles per second, in the calculation of which, unless I am mistaken there must be some error.

^{page 67 note *} For a demonstration that it is not possible, see Addition No. 1.

^{page 68 note *} The star which Mr Hind discovered in April 1848, and which only remained visible for a few weeks, during which period it varied considerably in appearance and brightness, but was always of a “ruddy” colour, may have not experienced meteoric impact enough to make, its surface more than red hot.

^{page 69 note *} On this account gunpowder would not do.

^{page 70 note *} These would rise and be regularly diffused into space.

^{page 70 note *} Both the elements that enter into combination are of course included in the weight of the burning matter.

^{page 76 note *} That a comet may escape with only a slight loss by evaporation, if the resistance is not too great to allow it to escape at all is easily understood, when we consider that it cannot be for more than a few hours exposed to very intense heat (not more than two or three hours within a distance equal to the Sun's radius from his surface). If it consist of a cloud of solid meteors, the smallest fragment may be wholly evaporated immediately; but all whose dimensions exceed some very moderate limit of few feet would, unless kept back by the resisting medium and made to circulate round the Sun until evaporated, get away with only a little boiled off from their surfaces.

(See Encyc. Brit., 8th edit. vol. iv., p. 87.) The discrepancies are probably due to proper motions of the spots, which, from the explanation given above in Addition iii., may be expected to be very considerable.