yitly 28, 1881] 



NA TURE 



at a certain height. The line of hydrogen has really divided 

 mtJ two lines of hydrogen, so that there we get, according to the 

 priiicip'e jast laid down, an indication of the fact that the hydro- 

 gen up to a certain height was very nearly at rest, and that beyond 

 part of it was torn away, the line being deflected towards the 

 blue, indicating that it is approaching u<. Now the other 

 Fraunhofer lines in the diagram may be lo iked upon as so many 

 milestones which enable us to measure by the deflection the 



297 



number of miles traversed by the gas in cue second ; for these 

 deflections are nothing more or less than alterations of wave- 

 length, and, thanks to Angstrom's map, we can measure distances 

 along the spectrum in ^^J^^^^ mm., and we know that an 

 alteration of iTrinssTnnfmra. in the wave-length of the F line 

 towards the vi )let means a velocity of thirty-eight miles a second 

 towards the eye ; and that a similar alteration towards the red 

 means a similar velocity from the eye ; so that carrying the part 



iifii nfni ni^H 



Fig. 



-Alteratit 



iofv 



e-Iengtli 



of the line which has the greatest deflection from the normal 

 down to the dots, we find that the velocity of the solar wind 

 under observation at tliat time was something like 1 14 miles per 

 second. (.5 



In the second figure this same prominence is seen a short time 

 afterwards. The tremendous rush of hydrogen has descended 

 somewhat nearer the sun, and bringing that in the same way 

 down to our milestones, we can give that velocity at something 

 like fifty miles per second. The wind velocities measured in 

 this way have amounted to 140 miles a second. The phenomena 

 of convection currents give us velocities which very often amount 

 to forty or si.xty miles a second. 



This method enables us to determine a matter which a few 

 years ago we could not have determined in any other way. I 

 refer to the fact that the motions of the solar winds are to a very 

 large extent cyclonic. These various effects have been produced 

 Ijy varying the position of the slit a very little indeed over a 

 small prominence. 



In the first of the accompanying diagrams it will be seen tliat 

 the hydrogen line indicates by its change of refraugibility that 

 the gas is receding from us, that the waves are being lengthened 

 oat, and that they therefore have approached towards the less 

 refrangible end of the spectrum. In the third diagram we see 

 that in that part of the prominence the ray, were being deflected 

 towardi the violet ; that is to say, they were approaching us. 

 In the middle of the prominence we get indications that they 

 were both receding and approaching, as shown in the sec(jnd 

 diagram. Now if anybody in the moon had as good a method 

 as this of mea-suiing an earthly cyclone, he would see exactly 

 this sort of thin^ — the part of the cyclone receding from him 

 would give a deflection in one direction, the centre of the cyclone 

 would give him both deflections, because he would get currents 

 going in both directions, and on the other side of the cyclone he 

 would get a deflection in the other direction. 



So obvious and so very definite did these observations at last 

 become that a new word had to be coined to separate the forms of 



FiG. 17.— Solar cyclo 



Left-hand diagram, retreating side of cyclone on slit : c 

 on slit. The right-hand side of each diag 



e diagram, both sides on 

 ii the most refrangible. 



right-hand dia.^am, advancing side 



the prominences as seen with a widened slit from the forms which 

 were assumed by the pro.-ninences depending on their rate of 

 motion. * 



Fig. 18 is a diagram of what have for this renson been 

 called " motion forms," because such forms are really not the 

 forms of the prominences at all — have nothi ig whatever to do 

 with the shape of the prominences, but are simply produced by the 

 various changes in the refrangibility of the light brought about 

 by the varying mitions in different parts. It is a very remark- 

 able fact, noticed at the time, that so ne prominences seem to be 

 shot up like so many smoke rings — little cyclones. And many 

 of the strangest motion-forms are due to this cause. The velocities 

 in the same prominence vary very much from the time it leaves 

 the photosphere until it arrives at its greatest elevation in the 

 sun's atmosphere, indeed the variations in anyone prominence are 

 almost as great as the variations observed between any two 

 prominences. 



There is another important fact connected with this : when the 

 phenomena are observed do e to the limb it is very often seen 

 that the dark line on the surface of the sun is broken ; in fact 

 we g it a doubling of the dark "F" line in exicdy the same 

 way as we got this doubling of the line in the prominence itself. 

 That taught us that not only were these motions enormous in the 

 case of vapours ejected from the ^un, but that the subjacent 

 part^ of the sun itself — of the photosphere rather — felt that same 

 influence. 



The next point observed was (and this was an observation very 

 difficult indeed to make near the limb) that whenever we got any 

 v,_-ry considerable velocity we got a new order of phenomena 

 altogether, indicated in these two diagrams (Figs. 19 and 20). 



It was found that the absorption of the hydrogen, or of the 

 magnesium, or of the sodium, as the ca.se might be, was enor- 

 mously reduced ; that for that part of the sun there w as practically 

 no absorption ; but instead of absorption an excessive brilliance 



