40 COSMOS. 



is manifested during the twenty-five days immediately pre- 

 ceding and succeeding the comet's periheHon passage. The 

 value of the constant is therefore somewhat different, because 

 in the neighborhood of tho sun the highly attenuated but 

 still gravitating strata of the resisting fluid are denser. Gi- 

 bers mauitained*' that this fluid could not be at rest, but 

 must rotate directly round the sun, and therefore the resist- 

 ance oftered to retrograde comets, like H alley's, must difler 

 wholly from that opposed to those comets having a direct 

 course, like Encke's. The perturbations of comets having 

 long periods of revolution, and the difierence of their magni- 

 tudes and sizes, complicate the results, and render it diffi- 

 cult to determine what is ascribable to individual forces. 



The gaseous matter constituting the belt of the zodiacal 

 light may, as Sir John Herschelf expresses it, be merely the 

 denser portion of this comet-resisting medium. Although it 

 may be shown that all nebulce are crowded stellar masses, 

 indistinctly visible, it is certain that innumerable comets fill 

 the regions of space with matter through the evaporation of 

 their tails, some of which have a length of 56,000,000 of 

 miles. Arago has ingeniously shown, on optical grounds,! 

 that the variable stars Avhich always exhibit white light 

 without any change of color in their periodical phases, might 

 afford a means of determining the superior limit of the dens- 

 ity to be assumed for cosmical ether, if we suppose it to be 

 equal to gaseous terrestrial fluids in its power of refraction. 



The question of the existence of an ethereal fluid filling 

 the regions of space is closely connected with one w^armly 

 agitated by Wollaston,s^ in reference to the definite limit of 

 the atmosphere — a limit which must necessarily exist at the 

 elevation where the specific elasticity of the air is equipoised 

 by the force of gravity. Faraday's ingenious experiments on 



* Olbers, in Schum., Ash: Nachr., No. 268, s. 58. 



t Outlines of Astronomy, $ 5-56, 597. 



X " En assimilant la matiere tres rare qui rempUt les espaces cilestes 

 quant a ses proprietes refringcntes aux gas terrestres., la densiti de cette 

 matiere nc saurait depasscr une certaine limite dont les observations des 

 itoiles changeantes, p. e. celles d' Algol ou de (3 de Persie, peuvent assigner 

 la valeury — Arago, in the Armuaire pour 1842, p. 33G-345. " On com 

 paring the extremely rare matter occupying the regions of space with 

 terrestrial gases, in respect to its refractive properties, we shall find thai 

 the density of this matter can not exceed a definite limit, whose value 

 may be obtained from observations of variable stars, as, for instance, 

 Algol or /3 Persei." 



$ See WoUaston, Philos. Transact, for 1822, p. 89; 8ir John Herschel, 

 op. cit., § 34, 36. 



