206 LIGHT SCIENCE FOR LEISURE HOURS. 



have travelled at the observed rate or (if the hydrogen 

 was not itself ejected, then) at a greater rate. 



The question which we have to deal with is there- 

 fore this What must be the velocity of ejection in 

 order that matter may pass between the observed 

 heights in the observed time ? 



But it may seem that the problem might be simpli- 

 fied by inquiring what must be the velocity of ejection 

 in order that a height of 200,000 miles should be 

 reached. This, however, introduces the question 

 whether that was really the limit of the hydrogen's 

 upward motion. The wisps seemed to dissolve away at 

 that elevation ; but we cannot assume quite safely that 

 the hydrogen there ceased to move upwards. On the 

 contrary, it seems more likely that it neither diffused 

 itself (so as to become invisible), nor ceased to ascend, 

 at that level ; but simply became invisible through 

 loss of temperature, and therefore of brilliancy. It 

 will be better, therefore, to take simply the flight 

 between the observed levels ; for then we shall be 

 attending solely to observed facts. We may, however, 

 inquire as a preliminary process what would be the 

 velocity of ejection necessary to carry a projectile 

 (moving as if in vacuo} from the sun's surface to a 

 height of 200,000 miles. 



We find thus that a projectile must have an initial 

 velocity of about 213 miles per second to reach the 

 height certainly attained by the hydrogen wisps watched 

 by Professor Young. 1 



1 The calculation is not difficult. The formula for our purpose may 

 be thus expressed. Let R be the sun's radius or 425,000 miles ; H the 



