NUMBER OF THE FIXED STARS. 139 



times of a white, and occasionally even of a bluish green 

 colour. The mariner, driven onwards by the cold south cur- 

 rents of the sea, is unable to recognize the shores, and in the 

 absence of all observations of latitude sails past the harbours 

 which he desired to enter. A dipping needle alone could, as 

 I have elsewhere shown, save him from this error, by the 

 local direction of the magnetic curves. 2 



Bouguer and his coadjutor, Don Jorge Juan, complained, 

 long before me, of the " unastronomical sky of Peru." A 

 graver consideration associates itself with this stratum of 

 vapours in which there is neither thunder nor lightning, in 

 consequence of its incapacity for the transmission of light or 

 electric charges, and above which the Cordilleras, free and 

 cloudless, raise their elevated plateaux and snow-covered 

 summits. According to what modern geology has taught us 

 to conjecture regarding the ancient history of our atmosphere, 

 its primitive condition, in respect to its mixture and density, 

 must have been unfavourable to the transmission of light. 

 When we consider the numerous processes which in the pri- 

 mary world may have led to the separation of the solids, 

 fluids, and gases around the earth's surface, the thought invo- 

 luntarily arises how narrowly the human race escaped being 

 surrounded with an untransparent atmosphere, which though 

 perhaps not greatly prejudicial to some classes of vegetation, 

 would yet have completely veiled the whole of the starry 

 canopy. All knowledge of the structure of the universe would 

 thus have been withheld from the inquiring spirit of man. 

 Excepting our own globe, and perhaps the sun and the moon, 

 nothing would have appeared to us to have been created. An 

 isolated triad of stars the sun, the moon, and the earth would 

 have appeared the sole occupants of space. Deprived of a 

 great, and indeed of the sublimest portion of his ideas of 



2 Cosmos, vol. i. p. 171 and note. 



