212 LIGHT SCIENCE FOR LEISURE HOURS. 



hypothesis we should lean, or rather which cause we 

 should consider as chiefly operative. 



In the first place, it is obvious that we cannot dis- 

 miss the hypothesis of retardation entirely, for glowing 

 hydrogen travelling through an atmosphere even of 

 extreme tenuity at an average rate of 167 miles per 

 second must needs be enormously retarded. But I 

 think that, apart from this, we cannot for a moment 

 accept the belief that the hydrogen wisps which 

 Professor Young watched as they slowly vanished at a 

 height of 200,000 miles were then travelling upwards 

 at the rate of about 150 miles per second. So acute 

 an observer could not but have recognised the fact 

 that the hydrogen was still in rapid upward motion at 

 that time. We are compelled then, as I judge, to 

 regard retardation as operative to at least some con- 

 siderable degree in that upper half of the hydrogen's 

 course. 



This being so, I do not know that a single word of 

 what I have said on the hypothesis of retardation being 

 solely operative need be altered. The italicised words 

 at the close of the remarks made on that view must 

 still be used in stating the conclusion to which careful 

 reasoning would lead us. 



And here I approach the point to which these 

 remarks have been tending. If we regard the hydrogen 

 erupted or in motion in these jet prominences as not 

 less dense than other matter partaking in the motion 

 of primary ejection, the above conclusion, interesting 

 as it is in itself, yet has no bearing on the subject of 



