38 r jsMOa. 



temperature they now possess, and which we designate as 

 — 76° of a mercury thermometer, had a temperature of ahout 

 — 1400° or even many thousand times lower I 



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

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

 cf which one — the less firmly-based hypothesis— -refers to the 

 limited traiisijarency of the celestial regions ; and the other, 

 founded on direct observation and pelding numerical results, 

 is deduced from the regularly shortened periods of revolution 

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

 served, Leys 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.,du sun, the entire vault of heaven must ap- 

 pear as luminous as our sun if light w^ere 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 Halleyt 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 iSir William Herschel from gauging the 



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

 power of its own into space, woukl 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, 3p Fahr., in the Polar Sea. 

 Under th.e 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- 

 jndly, 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 eprouverait un refroidis- 

 sement, dont on ne peut fix'jr la lim'.te. Probablement la vie des plantea 

 et des animaux serait impassible a la surface du globe, on relcguce dans 

 line etroite zone de cette surface." (Saigey, Physique du Globe, p. 77 .^ 



* Traits de la Cometc de 1743. avec nne Addition sur la force de la 

 humiere et sa Propagation dans Vither, ct sur la distance des ^toiles fixes; 

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

 space, see Olbers, in Bode's Jnhrbuchfur 182G, s. 110-121 ; and Struve, 

 Etudes d'Asir. Stellairc, 1847, p. 83-93, and note 9.5. Compare alsc 

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

 151, 152. 



t Halley, On the InfinVij of the Sphere of Fixed Stars, in ihe Phiws 

 Transact., vol. xxxi., for t lo year 1720, p. 22-2(3. 



