152 Royal Institution: — 



the green line, which expands in a more decided manner by pressure 

 than does the red. 



I now come to a new field of discovery opened out by these in- 

 vestigations, a branch of the inquiry which I fear you will consider 

 more startling than all the rest — a branch, however, which I have 

 had many opportunities of studying, and which has required me to 

 move with the utmost caution. I allude to the movements of the 

 hydrogen envelope and prominences at which I have before hinted. 



Any one who has observed the sun with a powerful telescope, 

 especially in a London fog (all too # great a rarity unfortunately for 

 such work), will have been struck with the tremendous changes ob- 

 served in spots. Now, change means movement ; and as spot-phe- 

 nomena occur immediately below the level of the chromosphere, we 

 may easily imagine that the chromosphere and its higher waves (the 

 prominences) will also partake of the movements, be they up- or 

 downrushes, cyclones, or merely lateral motions. I have thrown on 

 the screen a photograph of a drawing of a sun-spot observed under 

 the clear sky of Rome by Father Secchi — a drawing I regard as a 

 most faithful counterpart of nature. 



You see how the photosphere is being driven about and contorted — 

 how here it seems to be torn to ribbons by the action of some tre- 

 mendous force, how here it is dragged down and shivered to atoms. 



The spectroscope enables us to determine the velocities of these 

 movements with a considerable approach to accuracy ; and at times 

 they are so great that I am almost afraid to mention them to you. 



Let me first endeavour to give you an idea how this result is 

 arrived at ; and I must here beg your indulgence for a gross illustra- 

 tion of one of the most supremely delicate of nature's operations. 



Imagine a barrack out of which is constantly issuing with mea- 

 sured tread and military precision an infinite number of soldiers in 

 single or Indian file, and suppose yourself in a street seeing these 

 soldiers pass. You stand still and take out your watch and find that 

 so many pass you in a second or minute, and that the number of 

 soldiers as well as the interval between them is always the same. 



You now move slowly towards the barrack, still noting what hap- 

 pens. You find that more soldiers pass you than before in the same 

 time, and, reckoned in time, the interval between each soldier is less. 



You now move still slowly from the barrack, i. e. with the soldiers. 

 You find that fewer soldiers now pass you, and that the interval be- 

 tween each is longer. 



Now suppose yourself at rest, and suppose the barrack to have a 

 motion now towards you, now from you. 



In the first case the men will be paid out, so to speak, more 

 rapidly. The motion of the barrack-gate towards you will plant 

 each soldier nearer the preceding one than he would have been if 

 the barrack had remained at rest. The soldiers will really be nearer 

 together. 



In the second case it is obvious that the interval will be greater, 

 and the soldiers will really be further apart. 



