xxviu.] FORMS OF PROMINENCES. 415 



have all seen drawings of them, and the drawings of water- 

 spouts that I have seen represent the reality very well. If 

 we imagine a bank of fog about fifty or sixty feet high filled 

 with little fog-spouts, we get exactly what I then saw, and 

 what one often sees in quiet prominences on the sun. It may 

 be that what I and others have likened to the trunks of trees 

 may be really, somewhat akin to these fog-spouts, with the 

 enormous difference, however, that we are dealing with water 

 and aqueous vapour in one case, and with the photosphere of 

 the sun and incandescent hydrogen gas in the other. 



FIG. 122. Tree-like prominences. 



These quiet prominences are built up entirely of hydrogen. 1 

 When I say " quiet " it must be understood that the word is a 

 relative one. I have seen a quiet prominence as big as a dozen 

 earths born and die in an hour. That is not at all an un- 

 common thing. And there are several facts which indicate that 

 when such a prominence disappears, it does not mean that the 

 stuff disappears ; it means that it changes its state, that is to 

 say, it chiefly changes its temperature. 2 



1 That is a substance which gives us some of the lines observed when hydrogen 

 is examined. I do not mean that the hydrogen is such as we know of here. 



2 M. Trouvelot states that he observed at Meudon at 9h. 25m. on Aug. 16, 1885, 

 a brilliant prominence 4', or 108,000 miles, in height, which, two hours later, 

 had grown to 9' 27". That is to say, it then reached to at least (for it was 

 probably to some extent foreshortened) a quarter of a million miles from the 



