2IO LIGHT SCIENCE FOR LEISURE HOURS. 



Nor do we know what the atmospheric pressure is 

 at either level. It would be mere waste of time to 

 discuss a problem all the conditions of which are so 

 vague. 



But it will be worth while to consider the general 

 relations which are involved. 



In the first place, we may leave out of consideration 

 the motion of the hydrogen before it reached the level 

 of 100,000 miles. The retardation we have to inquire 

 into is something taking place within the observed 

 range of the projectile's motion, and we may consider 

 the moving hydrogen precisely as though its motion 

 had been due to some projectile force operating upon 

 it when already at a height of 100,000 miles. Now 

 we have seen that in order to traverse the next 1 00,000 

 miles above that level in ten minutes, it would require 

 an initial rate of motion (at that level) sufficient to 

 carry it to a distance of 350,000 miles from the sun's 

 surface if unretarded. But as the matter (on the 

 hypothesis we are considering) did not reach this 

 distance (250,000 miles from its starting place), but, 

 on the contrary, only traversed a distance of 100,000 

 miles before being reduced to rest, it is obvious that 

 its initial velocity (at level 100,000 miles) must have 

 been greatly in excess of the velocity which, at that 

 level, would correspond to an upward range of 350,000 

 miles in all. In other words, the hydrogen, when at 

 a height of 100,000 miles, was travelling much faster 

 than a projectile would cross that level if projected in 

 vacua at a rate of 255 miles per second. So that 



