THE ECLIPSE OF 1871. IO$ 



depth of four or five hundred miles is reduced almost 

 to evanescence. It corresponds, in fact, as nearly 

 as possible, to the two-thousandth part of the sun's 

 apparent diameter, so that it need not be regarded as 

 at all wonderful that hitherto astronomers have failed 

 to detect any signs of so shallow an envelope. Indeed, 

 it is not too much to say that by the aid of the 

 telescope alone it could never have been revealed to 

 them. A brief consideration of its nature will show 

 hovj.Tts discovery has been rendered possible by the aid 

 of the spectroscope. 



In the lower part of our own air there is always 

 present, in greater or less quantities, the vapour of 

 water. This vapour rises from wet earth, from rivers, 

 lakes, and seas, and from the wide expanse of ocean, 

 and occupies a certain portion of the lower atmospheric 

 strata. Tnus these lower strata form, as it were, a more 

 complex atmosphere than those above them. Close by 

 the earth there is air and aqueous vapour, while in the 

 higher regions there is air alone ; l that air being, as 

 we know, composed of a certain admixture of the two 

 gases, oxygen and nitrogen. Now, in the case of the 

 sun a somewhat similar arrangement exists. The 

 lower regions of the solar atmosphere are at all times 

 occupied by certain vapours, which ordinarily do not 

 range to any considerable elevation, simply because 

 they cannot remain in the form of vapour except close 



1 Here I am considering only the main constituents of the atmo- 

 sphere. Relatively minute quantities of other gases are ordinarily 

 present in the upper as well as in the lower regions of the air. 



