Oct. 20, 1 881] 



NATURE 



597 



kind which it gives out coming from the body behind than it 

 gives out to rci'lace it by virtue of its o\\n emiN>ion, and ace 'rd- 

 ingly ve should .'^ee tlie place of tho.^e bright Imes, or what 

 would be brjglit lines if the gas were the. e alone, dark on a bright 

 ground. By following out tliat theory he was enabled to i lentify 

 a great number of the dark lines in the s ilar spectrum "i'h the 

 bright lines given out by elements which \se kn iw at the surface 

 of the earth, such as iron, magne ium, an'l so forth. Now this 

 throws a most important light on the constituti m of thi sun. It 

 indicate^ that even in the outer, and, comparatively, therefore 

 cooler portion oT the sun, there mu-t still be a temperature so 

 enorm lus as to be above the boiling-point of iron, and above 

 the boiling-point of some of the raott rcfrac-ory metals. And 

 now I will refer to a later application of the spectro cope which 

 was made by a gentleman whom I see before me. First I should say 

 that in the year 1S42, in o'lservinj a total solar eclip-e, a new 

 pheno-uenon was witnessed, or at least a phenomenon which, if 

 not new, had n^t previously attracted general attention. The 

 dark body of the moon was seen to bj suroundrd by ro-e- 

 eoloured prominences having the appearance of mountains. 

 What were these ? What could possibly be tbeirnalure? \\ e had 

 but a small time to observe them ; the greatet duration of a total 

 eclipse of the sun i^a little over four minute;, and these eclipses 

 occur only once perhaps in two years or so, and uhen they occur 

 the t'ttality extends over a strip along the earth's surface of only 

 inconsinerable breadth, with probably a givat portion of it falling 

 on the ocean, so tuat if we were there present in a ship we could 

 hardly make any ob-ervations but what could be taken by the 

 naked eye. Tiie study of the^e prominen-.es and the nature uf 

 them must have been tiierefore a slow matter to get on u ith, so 

 long as we were limited to the observation of them during 

 the period of a total eclipse. The change of height of those 

 prominences shows that they belong not to the moon, but to the 

 sun. Of course, as the moon moved over ihe b^dy of the sun 

 toey would, if they belonged to the sun, tend to get shorter and 

 shorter as they were covered in, and would reveal themselves 

 gradually in the same way behind the opposite side, which is 

 just what happened. In 1S60 special provision was made for 

 the O'sen'aiioir of these prominences, and Mr De La Uue un- 

 dertook to mase a series of photographs, which led to so ..e most 

 important results. They showed, am ng other thnigs, tliat in 

 some ca>es the prominences, whatever they were, were not at all 

 attached to the body of the sun, but were su-pended as clouds 

 around it. They could n:>t, therefore, be mountiins clearly. 

 Mr. Lockyer, for some considerable tiaie prior to 1868, had been 

 devising in his uii-id a possible mode of rendering those promi- 

 nences viible, and studving their nature without waiting for or 

 being de I enden' upon the rare phenomenon of a total f clipse. 

 If the light which th ise prominences gave out consisted of bright 

 lines, then, by applying a spectroscope of high | ower to the 

 study of tho^e bodies, we might so far reduce the intensity of 

 the intervening portions of the spectnim where there is the 

 diffused light coming froui the immediate neighbourhood of 

 the sun's disk as to render them visible. At last he 

 was rewarded by success, and the announce-uent of this 

 discoveiy was made to the Royal .Society. Meanwhile 

 M. Janssen had gone out to India to observe a total 

 eclipse, and the special subject which he took up was to observe 

 the spectrum of those pr.iiinneMces, which he did with ^uccess. 

 The idea struck hi u, " Why should not this be done any day?" 

 He tried, and the next day he succeeded. In point of ab olute 

 time this was before the ob ervation of Lockyer, although at the 

 time nt account of it had reached this country, so th3t the two 

 observations \\ ere perfectly indepeedent of each other. Well, 

 subsequent improvements in the method of observing those pro- 

 minences have enabled us to see them at will, so that they may be 

 observed from day to day, when we choose, from hour to hour, 

 from minute to minute. T'e forms of them can be seen, and it is 

 found that they move with a^t(mndillg velocity. They are pro- 

 jected upwar 's from the sun with a velocity soineti jies of ICO or 

 even of 140 miles per .second. Their forms were such as we 

 might natuially attribute to the ejection of gas from ihe body of 

 the sun. It h 'd been conjectured that they might be of the 

 nature of auroral discharges. Their feature; however indicate 

 that they are projections of actual matter from the sun, and 

 moreover the n iture of their motion indicates the same. This 

 gives us, then, a new idea of the vastness of the chmges which 

 are crmtinuHlly going on at the surface of the sun. 



Now what IS the origin of these changes ? It seems to me 

 that the most reasjnable idea that we can form respecting them 



is something derived from what takes place io our own eai-th, 

 and what we can observe here. Suppose the .'un to be shinin^, 

 we will ^ay on a summer's day. If we 1 )ok horizontally with a 

 telescope eveiything is seen to be in a state of tremor ; the air is 

 far from homogeneous What is the reason of th u ? The 

 greater ])art cf the sun's heat passes throu,.;h the upper strata of 

 the atuiosphere and reaches us, the air being tran-parent with 

 regard to a liri;e po'tion o' the sun's heat. It warms the sur- 

 face of the earth. That in turn warms the air in cont.ict with it, 

 and farther radiites forth heat of a kind for which the air is 

 opique. The Consequence is that the lower ponions of the air 

 in contact with the earth get warmer, and that unequally accord- 

 ing to the nature of the ground — more on stones and gravel, 

 for instance, and less on grass and so forth. Being warmer 

 they get lighter, and therefore there is a constant ascent, a 

 constmt mixing of the hot and cold portions by curtents of 

 convection. As this goes on continually a stratum of air of 

 considerable height becnnes warmed in this manier, and some- 

 times an exchange by convection or something of the nature of 

 convection takes place on a very grand scale. Let us take the 

 case of summer weather. Suopose we have a succe sion of hot 

 days accompanied by a good deal of evaporation, gradually these 

 several currents of convection ca use a warming of a stratum 01 

 air below of considerable height, which is also well supplied with 

 moisture fr.nn the evaporaii m. At last, taking the stratum as a 

 whole, the equilibrium becomes unstabU, and there is an up- 

 rush ; hence there is a kind of chimney formed, through which 

 the air flo.vs upwards, and then spreads out laterally overhead. 

 This appears to be what takes ]ilace in our summer thunder- 

 storms. The heated and moist air forms for itself a chimney, 

 and in aseending there is a rapid depontion of what was 

 previ usiy vapour of water in the no v condensed slate of water 

 itself, and a rapid fall of rain occurs after a time Thi- appears 

 from so ore caase or other t > be the occasion of the devel-ifiment 

 ot a great deal of electricity, which is manifested in the forai of 

 lightning. While this action goes on you have the in draught 

 towards what I will call the chimney from all sides ; the vapour 

 .ooner or later gets condensed, and there is a fall of rain accom- 

 joanied by lightning. .Sometimes there is hail even in sum iier j 

 fir when the air charged with vapour gets to a particular height 

 the vapour becomes condensed and forms rain ; but it may be 

 thit the stratum of the upper air that i; pierced throa-jh is below 

 the freenng-poiut, and, the rain falling through this, it gets 

 frozen. I will just call attentio-.i to one fact ; according to this 

 view , you see you may have a g-neral current of wind over the 

 country — say, for the sake of illu tration, from west to east. 

 .Suopose there is a region to the west where an ascei'.ding current 

 has beet forme 1 ; then there is an in-draught from all -ides 

 to thit place, and when the thunderstorm has not yet come on 

 you are in a comparative calm, because the general direction of 

 the wind being from the west, and the in-draught carrying the 

 air from the east, the two together tend to neutralise one 

 another ; or you may have actually a wind blowing towards the 

 region of the thunderstorm. Acordinglv we know people often 

 say that thunderclouds move against the wind. I shall have 

 occasion to refer in my next lecture to the development of atmo- 

 spheric electricity in reference to some speculations in regard to 

 phenomena accompanying changes in the condition of the sun ; 

 but at present I merely refer to this process as illustrative of what 

 seems to be the most natural supposition to make regarding the 

 oriiin of these disturbances which are found to be continually 

 taking place at the surface of the sun. The outer portions of 

 the sun are the source of a gigantic amount of radiati in of heat 

 and light which passes out in all directions. By this radiation 

 those riuter portions mu;t tend to a certain extent to cool down, 

 and consequently, as the same ph\'sical conditions hoi 1 good, 

 if the same physical laws hold good, at the surface of the sun 

 that we have on our own earth, you may easily suppose that, 

 havin; become cooler than they were, the substances become 

 specific illy heavier, and accordingly give rise to cu -rents of con- 

 vection similar to tho-e that we h.ave in our own atmo cohere 

 fiom a -imilar cau;e, but operating in one respect in a different 

 way, because in the solar atmosphere there is a co:ling from 

 above, but in that of the earth a heating from below. Those 

 minor currents of convection ascending and descendin.; naturally 

 enougli give rise to that mottled appearance which is always 

 seen on the sun's surface, because if the interior of the sun be 

 hotter than the portions which have cooled by radiation, then 

 the ascending portions would naturally, being at a higher tem- 

 perature, be brighter, the descending portions darker, and small 



