1871-] The Eclipse of Last December, 229 



depended upon for determining pressure, the glowing 

 hydrogen even at the base of the sierra* exists at a pressure 

 considerably less than that of our own atmosphere at the 

 sea-level. Wullner assigned the limits of pressure at the 

 sun's surface at two inches and twenty inches of the mer- 

 curial barometer (under the earth's attraction). 



This seemed to dispose conclusively of the theory that 

 the corona is a solar atmosphere properly so-called. Without 

 accepting the somewhat bizarre notion put forward by Mr. 

 Lockyer, that the sierra is itself the solar atmosphere, we 

 seem yet fairly led to believe that that atmosphere into 

 which the glowing hydrogen which forms the prominences 

 has been flung, and in which smaller prominences and the 

 remains of larger ones probably commingle to form the 

 sierra, can extend but to a moderate distance above the 

 summits of the loftiest prominences. For undoubtedly an 

 atmosphere extending even to but 80,000 miles from the sun 

 (and many prominences are far higher) and subjected to his 

 enormous attractive power, could not fail to exert a pressure 

 enormously exceeding that of our own atmosphere at the 

 sea-level, unless this solar atmosphere were of extreme 

 tenuity. I know not, indeed, whether even our so-called 

 vacuum-tubes ought not to be regarded as densely filled 

 with matter, by comparison with the outer regions of a solar 

 atmosphere, if, with a height of some 100,000 miles at the 

 very least, such an atmosphere is to exert at the sun's 

 surface no greater pressure than our own.t 



It was probably such considerations as these which led to 

 the adoption by Mr. Lockyer of Faye's idea that the corona 

 is a phenomenon due merely to the passage of the solar rays 

 through our own atmosphere. It seems at a first view, 

 more especially to those unpractised in considering the 



* By sierra I mean the coloured layer or envelope of prominence matter to 

 which the name of chromosphere has been applied. Sierra harmonises so well 

 with the word prominences, that it is to be hoped this term, employed so many 

 years before by Airy, Leverrier, and others, will before long dispossess so ques- 

 tionable a title as " the chromosphere." 



t There are few problems more difficult than those suggested by the subject 

 here touched" upon. An atmosphere of hydrogen 80,000 miles deep, and at the 

 upper limit existing at the very lowest pressure which our physicists can be 

 assumed to have attained by any process of exhaustion, would yet at a depth 

 of 60,000 miles, under the action of solar gravity, exert a pressure exceeding 

 fully a hundredfold that of our own atmosphere at the sea-level. Nor is the 

 difficulty removed by supposing, as I have done in my Treatise on the Sun, that 

 the real pressure close by the photosphere exceeds manifold Mr. Lockyer's 

 estimate. This supposition, shown by Professor Young's observations last 

 December to be correct, yet leaves the pressure at a depth of 60,000 miles below 

 the summits of the loftiest prominences, or at a height of twenty or thirty 

 thousand miles above the photosphere, altogether less than can be explained 

 by any physical relations we* are familiar with at present. 



