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38 r jsMOa.temperature they now possess, and which we designate as76° of a mercury thermometer, had a temperature of ahout— 1400° or even many thousand times lower IIt still remains for us to consider two hypotheses in relationto the existence of a fluid filling the regions of space,cf which one— the less firmly-based hypothesis— -refers to thelimited traiisijarency of the celestial regions and the other,;founded on direct observation and pelding numerical results,is deduced from the regularly shortened periodsof revolutionof Encke's comet. Olbers in Bremen, and, as Struve has observed,Leys de Cheseaux at Geneva, eighty years earlier"^drew attention to the dilemma, that since we could not conceiveany point in the infinite regions of space unoccupied bya fixed star, i. e.,du sun, the entire vault of heaven must appearas luminous as our sun if light w^ere transmitted to usin perfect intensity; or, if such be not the case, we must assumethat light experiences a diminution of intensity in itspassage through space, this diminution being more excessivethan in the inverse ratio of the square of the distance. Aswe do not observe the whole heavens to be almost uniformlyillumined by such a radiance of light (a subject consideredby 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. Theresults obtained by iSir William Herschel from gauging theBet) to the heating influence of the earth's radiation, and the coolingpower of its own into space, woukl — indicate a medium temperature betweenthat of the celestial spaces 132^( Fahr.) and that of the earth'ssurface 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 atmospheretends to prevent the thermometer so exposed from attaining these extremelow temperatures: first, by imparting heat by conduction ;secjndly,by impeding radiation outward." Sir John Herschel, in the—Edinburgh Review, vol. 87, 1848, p. 222. "Si la chaleur des espacesplanetaires n'existait point, notre atmosphere eprouverait un refroidissement,dont on ne peut fix'jr la lim'.te. Probablement la vie des planteaet des animaux serait impassible a la surface du globe, on relcguce dansline etroite zone de cette surface." (Saigey, Physique du Globe, p. 77 .^* Traits de la Cometc de 1743. avec nne Addition sur la force de lahumiere et sa Propagation dans Vither, ct sur la distance des ^toiles fixes;par Loys de Cheseaux (1744). On the transparency of the regions ofspace, 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 alscSir 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 PhiwsTransact., vol. xxxi., for t lo year 1720, p. 22-2(3.