56 
POrULAR SCIENCE REVIEW. 
we may suppose that the matter was in reality projected with a 
much greater velocity than 200 miles per second, and was 
brought to rest at a height of 200,000 miles by the retarding 
action of the solar atmosphere cooperating with solar gravity. 
And, of course, we may conceive that these two explanations 
coexist, and that the two causes considered operate with any 
degree of proportional activity, between the relations which 
would 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 pro- 
jectile’s flight, the action of terrestrial gravity being also appre- 
ciably uniform. But in the case of the solar atmosphere 
between the observed levels we have a problem infinitely more 
difficult, because the atmospheric 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. 
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 enquire into is something- 
taking place within the observed range of the projectile’s motion, 
and we m^y 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 100,000 miles 
above that level in ten minutes, it would require an initial rate 
