470 



NATURE 



[March i8, 1886 



rapidly in the beginning, and almost always in less than a day 

 they an-ive at their maximum of size. Then they are stationary, 

 I would say in the vigorous epoch of their life, with a well- 

 defined penumbra of regular and rather simple shape. So they 

 sustain themselves for lo, 20, and some even for 50 days." 



I next quote the Rev. S. J. Perry, one of the most constant 

 of modern solar observers : — 



" And. to begin with spot-formation, we find almost invariably 

 that large solar spots start life as little dots, frequently in grou]is, 

 and then grow at once with enormous rapidity. A spot will 

 often attain its full size in 5 or 6 days, although exception.illy 

 large ones occasionally occupy a longer time in their first de- 

 velopment. If no remarkable increase is noticed in a sp>t 

 within 2 or 3 days from its birth, it will in all proba- 

 bility never attain to any considerable size. The solar surface 

 has repeatedly been examined with the greatest care, in regions 

 where considerable spots have broken out on the following 

 day, without detecting any marked disturbance or other sign 

 announcing a probable outburst. No satisfactoiy exceptions to 

 this have as yet been noticed." 



The second state of a spot's history presents us with phenD- 

 mena of reaction, as if the material carried in the first instance 

 below the upper level of the photosphere had produced a dis- 

 turbance in the lower regions, indicated by the increase in the 

 quantity and brilliancy of the faculoiis matter and its separation 

 into small masses, while at times the umbra of the spot becomes 

 distinctly coloured. 



I again quote Dr. Peters : — 



"The notches in the margin, which, with a high magnifying 

 power, always appear somewhat serrate, grow deeper, to such a 

 degree that the penumbra in some parts becomes interrupted by 

 straight and narrow luminous tracts, — already the period of de 

 cadence is approaching. This begins with the foUo«-ing highly 

 interesting phenomenon. Two of the notches from opposite 

 sides step forward into the area, over-roofing even a part of the 

 nucleus ; and suddenly from their prominent points flashes go 

 out, meeting each other on their way, hanging together for a 

 moment, then breaking off and receding to their points of start- 

 ing. Soon this electric play begins anew and continues for a 

 few minutes, ending finally with the connection of the two 

 notches, thus establishing a bridge and dividing the spot into 

 two parts. Only once I had the fortune to w itness the occur- 

 rence between three advanced points. Here fiom the point 

 A a flash proceeded towards n, which sent forth a ray to meet 

 the former when this had arrived very near. Soon this seeoied 

 saturated, and was suddenly repelled ; however, it did not retire, 

 but bent with a sudden swing toward c ; then again, in the 

 same manner, as by repulsion and attraction, it returned to I! ; 

 and, after having thus oscillated for several times, A adhered at 

 last permanently to B. The flashes proceeded with great speed, 

 but not so that the eye might not follow them distinctly. Ijy 

 an estimation of time and known dimension of space traversed, 

 at least an under limit of the velocity may be found ; thus, I 

 compute this velocity to be not less than 200,000,000 metres 

 [or about 120,000 miles] in a second. 



"Tlie process described is accomplished in the higher photo- 

 sphere, and seems not to affect at all the lower or d.irk atmo- 

 sphere. With it a second, or rather a third, period in the spot's 

 life has begim, that of dissolution, which lasts sometimes for 10 

 or 20 days, during which time the components are again 

 subdivided, while the other parts of the lumin jus m u-gin, too, 

 are pressing, diminishing, and finally overcasting the whole, 

 thus ending the ephemeral existence of the spot. 



"Rather a good chance is required for observing the remark- 

 able phenomenon which introduces the covering process, since it 

 is achieved in a few minutes, and it demands, moreover, a per- 

 fectly calm atmosphere, in order not to be cmfounded with a 

 kind of scintillation which is perceived very often in the spots, 

 especially with fatigued eyes. The observer ought to watch for 

 it under otherwise favourable circumstances when a large and ten- 

 or twenty-days'-old spot begins to show strong indentations on 

 the margin." 



The scintillation referred to by Dr. Peters is perhaps 

 associated with a phenomenon which has been described by M. 

 Trouvelot,' who has observed the faculous masses to subdivide 

 into small flakes which vibrate rapidly, producing the efifect of a 

 snow-storm above the umbra, when these dissolve into blue or 

 violet vapours. 



It happens sometimes also near groups of spots which are 

 ^ Bulletin Astronojttipte, vol. ii. 



endowed with great activity that perturbations are observed 

 which are so violent that the adjacent photosphere is shaken to 

 its foundations, cracks, and, on opening, forms sinuous crevasses 

 which extend to considerable distances, sometimes connecting 

 the most distant spots with each other. 



From the instant of their apparition the crevasses — even thoss 

 which are the narrowest — sh 3w a striated and filamentous struc- 

 ture, which presents the greatest analogy with the penumbrae of 

 spots ; only, instead of inchning and forming a sort of sloie like 

 that on the penumbra, the filaments which form entirely the sides 

 of the crevasses are vertical, and are all directed towards the 

 centre of the sun. 



When tliese. crevasses have a certain duration they widen 

 sometimes here and there, especially when they bifurcate and 

 send branches in another direction. In this case it is not rare 

 to see forming a strait and lengthened umbra when, at the same 

 time, the vertical filaments having now more room raise their 

 1 Dwer extremities. 



We have seen that the last act in the history of a spot is its 

 invasion by the faculae. These facula; remain long after the spot 

 has entirely closed up, and in this connection it is important to 

 remark that new spots very often break out in the old place. 

 These of course, unlike the first spot in that position, will appear 

 to be preceded by faculx. 



There is a great deal more that I might say about spots. It 

 is a very tempting subject, but there is so much more to be re- 

 ferred to. The papers which have recently been printed by the 

 Rev. S. J. Perry ^ and M. Trouvel 3t represent some of the most 

 careful modern examination of the solar surface, and there is 

 really a very great deal to be learnt from them ; and fortunate it 

 is that much which this new work has brought out in the 

 plainest way has reference to a region of fact of very great im- 

 portance to any one wlio wants to be able to answer for himself, 

 as well as he can, the question. What is a sun ? For instance, 

 M. Trouvelot discovered about ten years ago that, although, as 

 we shall see presently, ordinary sunspots have a very definite 

 place of their own on the sun, there is a kind of spot 

 which is not so confined. These he calls veiled spots ; and I 

 gather from his description that his opinion is that the photo- 

 sphere is driven down there to a certain extent, but not driven 

 down sufficiently to give us the dark appearance which we get in 

 the other cases. 



Spots caused by Descent of Copied Material 



It has already been suggested in preceding paragraphs 

 that in the spot region we cannot really be dealing with 

 any violent changes of pressure, but we may be dealing with 

 very violent changes of temperature. We can see that it is the 

 most natural thing in the world to suppose that in an atmo- 

 sphere like the sun's, seeing that there i; enormous radiation, 

 and therefore cooling at the exterior, there must be a descent of 

 solid particles into the interior heated region. Now, can we 

 associate this with spot phenomena ? 



Yes, we can, and we are absolutely driven to it. We have 

 already seen that the spot, w'nen it travels over the limb, is a 

 hollow. We also find when wc examine a spot with the 

 spectroscope that certain vapours get very much denser, as if 

 they were being crushed together into a certain limited region 

 either by an upthrust or a dovnfall. Which? Well, the 

 spectroscope answers that question for us quite perfectly, because 

 it shows that the vapours are absorbing, and therefore that they 

 are cooler than the photospheric material immediately underlying 

 them, and that they have not an excess of radiation, as they 

 would have if they came up from below ; the spectroscope then 

 certainly justifies the view that a spot is really the result of a 

 downrush ; the vapours there are cooler, as they should be if they 

 come from a cooler place ; they are denser, as they should be, 

 if they are descending rapidly into a place which is more or less 

 confined ; and, more than all, the change of the refrangibility of 

 certain lines enables us to determine roughly the rate at which 

 tliese descents take place. A very common velocity is 30 miles a 

 second — not 30 miles a minute or 30 miles an hour, but 30 miles 

 a second. 



Our final idea with regard to the spots then is that they are de- 

 pressions, that in fact we may regard them as shallow saucers 

 or cups filled with the cooler vapom's brought from the upper 

 regions of the solar atmo.sphere. This is merely a physical con- 

 ception. What we have next to do, if possible, is to add a little 

 ! Register. 



