NOTES. Xlll 



power of its own iuto space, would indicate a medium temperature between 

 that of the celestial spaces ( 132 F.) and that of the earth's surface below 

 it (82 F. at the equator, -3.5 F. in the Polar Sea). Under the equator, 

 then, it would stand, on the average, at 25 F., and in the Polar Sea at 

 68 F. The presence of the atmosphere tends to prevent the thermometer 

 so exposed from attaining these extreme low temperatures first, by imparting 

 heat by conduction ; secondly, by impeding radiation outwards" (Sir John 

 Herschel, in the Edinburgh Review, Vol. Ixxxvii. 1848, p. 223). " Si la 

 chaleur des espaces planetaires n'existoit point, notre atmosphere eprouverait 

 un refroidissement, dont on ne peut fixer la limite. Probablement, la vie des 

 plantes et des animaux seroit impossible a la surface du globe, ou releguee 

 dans une etroite zone de cette surface" (Saigey, Physique du Globe, p. 77). 



( 7S ) p. 39. Traite de la Comete de 1743, avec une Addition sur la Force 

 de la Lumiere et sa Propagation dans 1'Ether, et sur la Distance des Etoiles 

 fixes, par Loys de Cheseaux (1744). On the transparency of space, see 

 Gibers, in Bode's Jahrbuch fur 1826, S. 110121 ; Struve, Etudes d'Astr. 

 stellaire, 1847, p. 8393, and Note 95. Compare also Sir John Herschel, 

 Outlines of Astr. 798 ; and Kosmos, Bd. i. S.158 (English edit. vol. i. p. 142). 



( 79 ) p. 40. Halley on the Infinity of the Sphere of Fixed Stars, in the 

 Phil. Trans. Vol. xxxi. for the year 1720, p. 2226. 



(8) p. 40. Kosmos, Bd. i. S. 92 (English Edition, Vol. i. p. 80). 



( 81 ) p. 40. "Throughout by far the larger portion of the extent of the 

 Milky Way in both hemispheres, the general blackness of the ground of the 



heavens on which its stars are projected," &c "In those regions where 



that zone is clearly resolved into stars well separated, and seen projected on a 



black ground, and where we look out beyond them into space" (Sir John 



Herschel, Outlines of Astronomy, p. 537 and 539). 



( 82 ) p. 40. Kosmos, Bd, i. S. 89, 113, and 393, Note 23 (English edition, 

 p. 77, 99, and Note 53) ; Laplace, Essai philosophique sur les Probabilites, 

 1825, p. 133 ; Arago, in the Annuaire du Bureau des Long, pour 1832,p. 188, 

 pour 1836, p. 216; John Herschel, Outlines of Astronomy, 577. 



(M) p. 40. The vibratory motion of the effluxes at the head of some 

 coitnets such as was observed in the comet of 1744, and by Bessel in Halley's 

 comet between the 12th and 22d October, 1835 (Schumacher, Astron. Nachr. 

 No. 300 302, S. 185 232) "may, indeed, in particular individuals of this 

 class of bodies, influence the translatory motion and the rotation, and even 

 leaius to infer polar forces (S. 201 and 229) different from the ordinary attract- 

 ing power of the Sun ;" but the acceleration of the three and a half yearly period 



