A GREAT SOLAR OUTBURST. 209 



make one or other the sole cause of the observed excess 

 of velocity. 



Now, to determine the actual height which must 

 be reached by a projectile from the sun (in vacuo) so 

 that it may pass from a height of 100,000 to a height 

 of 200,000 miles in ten minutes, I have gone through 

 a series of calculations which need not be discussed 

 here, leading to the result (which may be accepted as 

 trustworthy) that 350,000 miles is the required height, 

 and therefore 255 miles per second the requisite initial 

 velocity. In this case the hydrogen wisps watched by 

 Professor Young were in reality travelling at a rate of 

 about 150 miles per second when they reached the 

 highest visible part of their course and vanished from 

 view as if by a process of dissolution. 



On the other hand, it is not possible to determine 

 the nature of the motion of hydrogen wisps, retarded by 

 the resistance of the solar atmosphere, so as to travel 

 from a height of 100,000 miles to an extreme height 

 of 200,000 miles in ten minutes. We are very far 

 from knowing how to deal satisfactorily with the motion 

 of a solid projectile through our own atmosphere, which 

 may be regarded as appreciably uniform during the 

 projectile's flight, the action of terrestrial gravity being 

 also appreciably uniform. But in the case of the sular 

 atmosphere between the observed levels we have a 

 problem infinitely more difficult, because the atmos- 

 pheric pressure must be greatly less at a height of 

 200,000 miles than at a height of 100,000 miles, the 

 solar gravity at these heights being also very different. 



in. P 



