ience_, vol. xxxiv., p. 348.]

[Footnote 680: _Berlin Abhandlungen_, 1889.]

[Footnote 681: _Amer. Jour. of Science_, vol. xli., p. 243. See
Appendix, Table II.]

[Footnote 682: _Astrophy. Jour._, vol. ix., p. 219; Fowler, _Knowledge_,
vol. xxiii., p. 11.]

[Footnote 683: _Amer. Jour, of Science_, vol. xiv., p. 89; _Nature_,
vol. xvi., p. 364; _Month. Not._, vol. xxxix., p. 440.]

[Footnote 684: _Month. Not._, vol. xxxviii., p. 473; Trowbridge and
Hutchins, _Amer. Jour. of Science_, vol. xxxiv., p. 263.]

[Footnote 685: Scheiner, _Die Spectralanalyse_, p. 180.]

[Footnote 686: _Comptes Rendus_, t. lxvii., p. 1123.]

[Footnote 687: Rev. A. L. Cortie, _Month. Not._, vol. li., p. 18.]

[Footnote 688: Young, _The Sun_, p. 135; Hale, _Astr. and Astrophysics_,
vol. xi., p. 312 Buss, _Jour. Brit. Astr. Ass._, vol. ix., p. 253.]

[Footnote 689: _Phil. Trans._, vol. clxx., p. 46.]

[Footnote 690: _Comptes Rendus_, t. xcvii., p. 555; t. ci., p. 1145.]

[Footnote 691: Liveing and Dewar, _Astr. and Astrophysics_, vol. xi., p.
705.]

[Footnote 692: _Comptes Rendus_, t. lx., p. 213; t. lxiii., p. 289.]

[Footnote 693: _Ibid._, t. cviii., p. 1035.]

[Footnote 694: _Ibid._, t. cxi., p. 431.]

[Footnote 695: _Astroph. Jour._, vols. iv., p. 317; vi., p. 426.]

[Footnote 696: _Trans. Roy. Soc. Edin._, vol. xxxii., p. 452.]

[Footnote 697: _Comptes Rendus_, t. cxi., p. 941; Huggins, _Proc. Roy.
Soc._, vol. xlvi., p. 168.]

CHAPTER V

_TEMPERATURE OF THE SUN_

Newton was the first who attempted to measure the quantity of heat
received by the earth from the sun. His object in making the experiment
was to ascertain the temperature encountered by the comet of 1680 at its
passage through perihelion. He found it, by multiplying the observed
heating effects of direct sunshine according to the familiar rule of the
"inverse squares of the distances," to be about 2,000 times that of
red-hot iron.[698]

Determinations of the sun's thermal power, made with some scientific
exactness, date, however, from 1837. A few days previous to the
beginning of that year, Herschel began observing at the Cape of Good
Hope with an "actinometer," and obtained results agreeing quite
satisfactorily with those derived by Pouillet from experiments made in
France some months later with a "pyrheliometer."[699] Pouillet found
that the vertical rays of the sun falling on each square cen