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POPULAR SCIENCE REVIEW. 
miles and 200,000 miles ; and let us assume that there was 
no foreshortening. These assumptions both tend, of course, to 
reduce our estimate of the velocity with which matter was 
ejected from the sun. 
Now we need not trouble ourselves by inquiring whether the 
hydrogen wisps which moved upwards before Prof. Young’s eyes 
were themselves ejected, or whether their motion might not 
have been due to the ejection of other matter impinging upon 
these wisps and forcing them upwards. Some matter must 
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 therefore 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 simplified 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 be- 
come 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. 
The calculation is not difficult. The formula for our purpose 
may be thus expressed. Let B be the sun’s radius, or 425,000 
miles ; H the extreme height reached by a projectile from the 
sun ; V the velocity of projection. Then a mile being the unit 
of length and a second the unit of time — 
(379 miles per second is the velocity which would be required 
to carry a projectile away from the sun altogether); and we 
have only to put for H 200,000 (miles) and for B 425,000, to 
deduce the required velocity. 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. 
Now the time in which a projectile with this initial velocity 
