134 THE CHEMISTRY OF THE SUN. [CHAP. 



by blowing through the open end of the tube, and while still 

 blowing whirls it round rapidly in a vertical plane in which an 

 observer is standing, that observer will note that when the 

 whistle is approaching him in one part of the curve, and the 

 waves are therefore being crushed together, the note will appear 

 higher than when it is receding from him in the opposite part of 

 the curve, where the waves are being, as it were, pulled asunder. 



Now to apply this to the light from the hydrogen promi- 

 nences in which the effects are most pronounced. The long 

 notes of light are red, and the short notes are blue, and if we 

 sharpen or shorten any light note in any part of the spectrum 

 we shall give that light a tendency to go towards the blue, and 

 if we lengthen or flatten it we shall give it a tendency to go 

 towards the red ; so that, for instance, if a mass of hydrogen gas 

 giving the line or note in the green indicated by F, is approach- 

 ing us with a velocity comparable to the velocity of light, the 

 line will change its position in the spectrum towards the blue ; 

 and if we are careful to note the exact amount of change of 

 refrangibility, as it is called, we shall have then an absolute 

 method of determining the rate of relative motion of that mass 

 of gas. This will help us in more ways than one. Suppose we 

 observe the gas at the limb of the sun, we shall then, if we get 

 any change of refrangibility, be justified in calling it a solar 

 wind, because the motion thus indicated would be very nearly 

 parallel to the surface of the sun ; but if on the disk of the sun 

 itself take a spot, for instance, in the very middle of the disk 

 we get any change of wave-length such as I have referred to, 

 it is perfectly clear that we shall no longer be dealing with what 

 we can justly call a wind, it will really be an upward or down- 

 ward current. So that this principle enables us at the limb of 

 the sun to determine the velocity of solar winds, and at the 

 centre of the sun to determine the velocity of uprushes or 

 downrushes. 



If the hydrogen lines were invariably observed to broaden out 



