154 Royal Institution ;— - 



or nearly so, on both sides of the F line when the pressure of the 

 gas has been increased. 



You see now on the screen a diagram showing the strange con- 

 tortions which the F hydrogen line undergoes at the centre of the 

 sun's disk. Not only have we the line bright, as I have before told 

 you, but the dark one is twisted in places, generally inclining towards 

 the red ; and often when this happens we have a bright line on the 

 violet side. You see it sometimes stopping short of one of the small 

 sun-spots, swelling out prior to disappearance, invisible in a facula 

 between two small spots, changed into a bright line and widened 

 out on both sides two or three times in the very small spots, beco- 

 ming bright near a spot and expanding over it on both sides — very 

 many times- widened out near a spot, sometimes considerably, on the 

 less refrangible side, and, finally, extended as a bright line without 

 any thickening over a small spot. 



Now the other Fraunhofer lines on the diagram may be looked 

 upon as so many milestones telling us with what rapidity the uprush 

 and downrush take place ; for these twistings are nothing more nor 

 less than alterations of wave-length, and, thanks to Angstrom's map, 

 we can map out distances along the spectrum from F in jo" WuUXTo^ 8 

 of a millimetre from the centre of that line ; and we know that an 

 alteration of that line yjjxnr'nwD' urillim. towards the violet means 

 a velocity of 38 miles a second towards the eye (i. e. an uprush), and 

 that a similar alteration towards the red means a similar velocity 

 from the eye (i. e. a downrush) . The fact that the black line inclines 

 to the red shows that the less bright hydrogen descends ; the fact 

 that the bright line (where both are visible side by side) inclines to 

 the violet shows that the more vivid hydrogen ascends ; and the alte- 

 ration of wave-length is such that 20 miles a second is very common. 



Now, observations of the lateral motions at the limb are of course 

 made by the chromospheric bright lines seen beyond the limb. Here 

 the velocities are very much more startling — not velocities of uprush 

 and downrush, as you now know, but swinging and cyclonic motions 

 of the hydrogen. 



I will first show you a cyclone observed on the 14th of March ; 

 but before I do so let me make one remark. Although the slit used 

 is as narrow as I can make it, let us say -g-^y (I have not measured 

 it) of an inch, a strip of this breadth, of the sun's image, is some- 

 thing considerable, as the glorious sun himself is painted by my 

 object-glass only about *94 inch in diameter, so that after all the slit 

 lets in to be analyzed a strip some 1800 miles wide. 



Now, suppose we have a cyclone of incandescent hydrogen some 

 1500 miles wide tearing along with a very rapid rotatory motion, it 

 is clear that all this cyclone could fall within the slit, and that, if the 

 rotatory motion were sufficiently rapid, the spectroscope should sepa- 

 rate the waves which are carried towards us from those which are rece- 

 ding. It does this : as you see, we have an alteration of wave-length 

 both towards the red and violet, amounting to something like 40 miles 

 a second. Now it should be clear to you that, by moving the slit 

 first one way and then the other, we may be able to bring it in turn 



