38 COSMOS. 



temperature they now possess, and which we designate as 

 76 of a mercury thermometer, had a temperature of about 

 1400 or even many thousand times lower ! 



It still remains for us to consider two hypotheses in rela- 

 tion to the existence of a fluid filling the regions of space, 

 of which one the less firmly-based hypothesis -refers to the 

 limited transparency of the celestial regions ; and the other, 

 founded on direct observation and yielding numerical results, 

 is deduced from the regularly shortened periods of revolution 

 of Encke's comet. Olbers in Bremen, and, as Struve has ob- 

 served, Loys de Cheseaux at Geneva, eighty years earlier* 

 drew attention to the dilemma, that since we could not con- 

 ceive any point in the infinite regions of space unoccupied by 

 a fixed star, i. e., a sun, the entire vault of heaven must ap- 

 pear as luminous as our sun if light were transmitted to us 

 in perfect intensity ; or, if such be not the case, we must as- 

 sume that light experiences a diminution of intensity in its 

 passage through space, this diminution being more excessive 

 than in the inverse ratio of the square of the distance. As 

 we do not observe the whole heavens to be almost uniformly 

 illumined by such a radiance of light (a subject considered 

 by Halleyf in an hypothesis which he subsequently rejected), 

 the regions of space can not, according to Cheseaux, Olbers, 

 and Struve, possess perfect and absolute transparency. The 

 results obtained by Sir William Herschel from gauging the 



p 



set) to the heating influence of the earth's radiation, and the cooling 

 power of its own into space, would indicate a medium temperature be- 

 tween that of the celestial spaces (132 Fahr.) and that of the earth's 

 surface below it, 82 Fahr., at the equator, 3* Fahr., in the Polar Sea. 

 Under the equator, then, it would stand, on the average, at 25 Fahr., 

 and in the Polar Sea at 68 Fahr. The presence of the atmosphere 

 tends to prevent the thermometer so exposed from attaining these ex- 

 treme low temperatures : first, by imparting heat by conduction ; sec- 

 ondly, by impeding radiation outward." Sir John Herschel, in the 

 Edinburgh Review, vol. 87, 1848, p. 222. " Si la chaleur des espaces 

 planetaires n'existait point, notre atmosphere 6prouverait un refroidis- 

 sement, dont on ne peut fixer la limite. Probablement la vie des plantes 

 et des animaux serait impossible a la surface du globe, ou releguee dans 

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



* Traiti de la Comete de 1743, avec une Addition sur la force de la 

 Lumiere et sa Propagation dans l'6ther, ct sur la distance des etoiles fixes; 

 par Loys de Cheseaux (1744). On the transparency of the regions of 

 space, see Olbers, in Bode's Jahrbuckfur 1826, s. 110-121 ; and Struve, 

 Etudes d'Astr. Slellaire, 1847, p. 83-93, and note 95. Compare also 

 Sir John Herschel, Outlines of Astronomy, $ 798, and Cosmos, vol. i., p. 

 151, 152. 



t Halley, On the Infinity of the Sphere of Fixed Stars, in the Philos. 

 Transact., vol. xxxi., for tfie year 1720, p. 22-26. 



