1886.] Sun-spot Spectra Observations at Kensington. 355 



With regard to the extensions of volume to which I have referred, 

 I find that if we assume that metallic iron can exist in any part of 

 the sun's atmosphere, and that it falls to the photosphere to produce a 

 spot, the vapour produced by the fall of one million tons will give us 

 the following volumes : — 



Volume in 



Temperature. Pressure. cubic miles. 



2,000° C 380 mm "8 



10,000 760 „ 1-8 



20,000 5 atmos 0*7 



50,000 760 mm 8*8 



50,000 190 „ 35-2 



If we assume the molecule of iron to be dissociated ten times by 

 successive halving, then the volume occupied will be 1024 times 

 greater, and we shall have — 



Volume in 



Temperature. Pressure. cubic miles. 



50,000° C 760 mm. ...... 9,011 



50,000 ........ 190 „ 36,044 



In these higher figures we certainly do seem nearer the scale on 

 which we know solar phenomena to take place; the tremendous 

 rending of the photosphere, upward velocities of 250 miles a second, 

 and even higher horizontal velocities according to Peters, are much 

 more in harmony with the figures in the second table than the first. 



I may mention in connexion with this part of the subject, that 

 the view of the great mobility of the photosphere which this hypo- 

 thesis demands, so soon as we regard metallic prominences as direct 

 effects of the fall of spot material, is further justified by the fact that 

 if we assume the solar atmosphere, that is the part of the sun outside 

 the photosphere, to be about half a million miles high, which I 

 regard as a moderate estimate, the real average density of the sun is 

 very nearly equal to one-tenth that of water, instead of being slightly 

 greater than that of water, as stated in the text-books.* 



We can then only regard the photosphere as a cloudy stratum exist- 

 ing in a region of not very high pressure. It is spherical because it 

 depends upon equal temperatures. 



The second direction in which I have attempted to develop the 

 hypothesis has relation to the circulation in the sun's atmosphere. I 

 have taken the facts of the solar atmosphere as a whole, as they are 

 recorded for us in the various photographs taken during eclipses since 



* The density referred to water = 1*444 and to the earth 0-255, according to 

 Newcomb. 



