OUJl SIUE11EAL SYSTEM. 79 



or true movements of the heavenly bodies. If we commence 

 physical cosmography with the most remote nebulae, we may 

 feel inclined to compare this portion of our subject with the 

 heroic, or mythical, periods of history. Both begin in twi- 

 light obscurity the one of antiquity, the other of inacces- 

 sible distance ; and where reality threatens to elude the grasp, 

 imagination becomes doubly incited to draw from its own 

 fulness, and to give outline and permanence to undefined 

 evanescent objects. 



If we compare the regions of space to one of the island- 

 studded seas of our planet, we may imagine we see matter 

 distributed in groups, whether of unresolvable nebulae of 

 different ages condensed around one or more nuclei, or in 

 clusters of stars, or in stars scattered singly. Our cluster 

 of stars, or the island in space to which we belong, forms a 

 lens-shaped, flattened, and every where detached stratum, 

 whose major axis is estimated at seven or eight hundred, and 

 its minor axis at a hundred and fifty times the distance of 

 Sirius from the Earth. If we assume that the parallax of Sirius 

 does not exceed that determined for the brightest of the 

 stars in the Centaur (0"'9]28), it will follow that light 

 traverses one distance of Sirius in three years, while nine 

 years and a quarter are required for the transmission of 

 the light of 61 Cygni, whose considerable proper motion 

 might lead to the inference of great proximity. I take 

 the parallax of this remarkable star (0"'34S3) from 

 Vessel's excellent first memoir ( 34 ). Our cluster of stars 

 is a disk of comparatively small thickness, divided, at 

 about a third of its length, into two branches : we are 

 supposed to be near tin's division, and nearer to the region 



of Sirius than to that of the constellation of the Eagle, 

 VOL.I, it 



