Kieff in 1890;[746] and Knut Ångström, taking account of the arrestive power of carbonic acid, inferred enormous atmospheric absorption, and a solar constant of four calories.[747] In other words, the sun's heat reaching the outskirts of our atmosphere is capable of doing without cessation the work of an engine of four-horse power for each square yard of the earth's surface. Thus, modern inquiries tend to render more and more evident the vastness of the thermal stores contained in the great central reservoir of our system, while bringing into fair agreement the estimates of its probable temperature. This is in great measure due to the acquisition of a workable formula by which to connect temperature with radiation. Stefan's rule of a fourth-power relation, if not actually a law of nature, is a colourable imitation of one; and its employment has afforded a practical certainty that the sun's temperature, so far as it is definable, neither exceeds 12,000° C., nor falls short of 6,500° C. FOOTNOTES: [Footnote 698: _Principia_, p. 498 (1st ed.).] [Footnote 699: _Comptes Rendus_, t. vii., p. 24.] [Footnote 700: _Results of Astr. Observations_, p. 446.] [Footnote 701: "Est enim calor solis ut radiorum densitas, hoc est, reciproce ut quadratum distantiæ locorum a sole."--_Principia_, p. 508 (3d ed., 1726).] [Footnote 702: _Jour. de Physique_, t. lxxv., p. 215.] [Footnote 703: _Ann. de Chimie_, t. vii., 1817, p. 365.] [Footnote 704: _Phil. Mag._, vol. xxiii. (4th ser.), p. 505.] [Footnote 705: _Nuovo Cimento_, t. xvi., p. 294.] [Footnote 706: _Comptes Rendus_, t. lxv., p. 526.] [Footnote 707: The direct result of 5-1/3 million degrees was doubled in allowance for absorption in the sun's own atmosphere. _Comptes Rendus_, t. lxxiv., p. 26.] [Footnote 708: _Ibid._, p. 31.] [Footnote 709: _Nature_, vols. iv., p. 204; v., p. 505.] [Footnote 710: _Ibid._, vol. xxx., p. 467.] [Footnote 711: _Ann. de Chim._, t. x. (5th ser.), p. 361.] [Footnote 712: _Comptes Rendus_, t. xcvi., p. 254.] [Footnote 713: _Phil. Mag._, vol. viii., p. 324, 1879.] [Footnote 714: _Ibid._, p. 325.] [Footnote 715: _Sitzungsberichte_, Wien, Bd. lxxix., ii., p. 391.] [Footnote 716: _Wiedemann's Annalen_, Bd. xxii., p. 291; _Scheiner, Strahlung und Temperatur der Sonne_, p. 27.] [Footnote 717: _Comptes Rendus_, March 28, 1892; _Astr. and Astrophysics_, vol. xi., p. 517.] [Footnote 718: _Phil. Trans._, vol. clxxxv., p. 361.] [Footnote 719: _Proc. Roy. Society_, December 12, 1901.] [Footnote 720: Scheiner, _Temp. der Sonne_, p. 36.] [Footnote 721: _Astroph. Jour._, vol. ii., p. 318.] [Footnote 722: _Astr. Nach._, Nos. 1,815-16.] [Footnote 723: _L'Astronomie_, September, 1884, p. 334.] [Footnote 724: _Amer. Jour. of Science_, vol. i. (3rd ser.), p. 653.] [Footnote 725: Young, _The Sun_, p. 310.] [Footnote 726: _Op._, t. vi., p. 198.] [Footnote 727: _Rosa Ursina_, lib. iv., p. 618.] [Footnote 728: _Méc. Cél._, liv. x., p. 323.]