68 



KNOWLEDGE 



[Apeil 1, 1892. 



moved slower at the sides than at the centre, and faster in 

 the middle, as well as more rapidly at the surface than at 

 the bottom. Consequently he proposed the theory that 

 ice is a plastic substance, capable of yielding to great 

 pressure, and the more so as it approaches the melting 

 point. This theory was not supposed to be irreconcilable 

 with the fact that it will crack under considerable pressure. 

 In small masses this plasticity is not noticeable, but in 

 large masses, and under long-continued pressure, it slowly 

 yields, and will flow like a stiffly viscous fluid. In large 

 masses like a Glacier, this steady, powerful pressure is 

 furnished by the immense weight of superincumbent ice. 

 Many persons consider that both Tyudall and Forbes' 

 theories are true, and so combine the two ; but to the 

 writer it seems that the plasticity of Glacier-ice under great 

 pressure is so well proved as to render the regelation 

 theory almost unnecessary ; and according to it, there 

 ought to be more cracks and crevasses all over Glaciers, 

 and not only in their steeper parts. 



THE GREAT SUNSPOT AND ITS INFLUENCE. 



By E. W. Maunder, F.E.A.S., 



Assistant superinteiulini/ the Solar and Spectroscopic Depart- 

 ments at the RiiyaJ Olisercatory, (Jmemrich. 



IT is a strange circumstance that we seem to have 

 scarcely any observations of Suuspots previous to 

 the invention of the telescope. Only some three or 

 four have come down to us ; yet spots large enough 

 to be seen by the unassisted eye are not at all in- 

 frequent at the times of special solar activity, and from 

 time to time spots occur which are not only visible, but 

 which are even conspicuous."' The writer well remembers 

 when a boy at school, and not in the least likely to have 

 been on the look-out for such an occurrence, having been 

 surprised by the sight of an intensely black speck on the 

 setting sun. On a more recent occasion, when the Queen 

 was holding the review of the troops who had returned 

 from the Egyptian campaign, the morning was a foggy 

 one, and no dark wedges or smoked glasses were needed 

 to tone down the brilliancy of the sun to the eye. It 

 shone feebly through the fog, a dull red ball, but not, as 

 usually, with unvaried surface ; a big black blot was 

 conspicuous upon it, so conspicuous as to attract the atten- 

 tion of the soldiers who were crossing Blackheath on their 

 way from 'Woolwich to Hyde Park to attend the review, 

 and called forth much comment and speculation from them. 

 It may, therefore, be safely assumed that the object on the 

 sun was something not easily overlooked, and it is hard to 

 imderstand, if spots of similar magnitude have occurred 

 upon the sun from time to time, during the ages which 

 preceded the days of Fabritius and Galileo, how they 

 escaped notice. 



But the great Sunspot of the February just past fairly 

 dwarfed that of November, 1882, to which I have referred 

 above, and whenever the sun was sufticiently dull for the 

 eye to be able to look at it without pain, it was not merely 



* The late Mr. John "Williams, formerly Assistant Secretary of 

 the Eoyal Astronomical Society, collected several Chinese accoimts of 

 Sunspots, which had been observed by the naked eye. In the 

 Monihlii JS'otices for April, 1873, he gave a list of such 'observations, 

 extending from A.u. 301 to A.B. 1205. Some of the spots were 

 described as " like a hen's egg," " like a duck," " resembling a large 

 date, " Possibly our foggy atmosphere, ivhich often enables us to 

 look steadily at the setting and rising, and sometimes at the meridian 

 sun, may account for the fact that spots visible to the naked eye 

 have frequently been observed in England, while, as far as I am 

 aware, the Greeks and Romans did not notice them. — A. C. Eaxtaed. 



an easy but a conspicuous object. Nor can we wonder that 

 it was so when we come to consider its real dimensions. 

 For on February ]3th the spot had an extreme length of 

 14° of solar longitude, and a breadth of 8'2° of solar 

 latitude, equivalent to 92,000 and 62,000 English miles 

 respectively ; whilst the entire group of which it formed 

 the principal part was nearly 25° in length, and 10" in 

 breadth, or in miles 162,000 and 7.5,000. The area of the 

 great spot on this day reached the astonishing amount of 

 2940 mil Hans of square miles, or reckoning in the smaller 

 spots which clustered so thickly round the central one, the 

 spotted area of the whole group was 35H0 millions of square 

 miles. Such an area is all but 18 times that of the entire 

 surface of the terrestrial globe ; or to put the matter in 

 another way, some 70 worlds as large as our own could have 

 lain side by side in that immense hollow. 



These figures may seem incredible until we recall how 

 vast is the scale upon which the sun itself is constructed. 

 Thus the photographs of the sun taken at Greenwich, one 

 of which is reproduced in the accompanying plate, are 

 on a scale of 8 inches to the solar diameter, a sufficiently 

 large scale to show much detail. Yet a spot with an area 

 of one million square miles would have a diameter of but 

 one hundredth of an inch on such a photograph ; practically 

 it would be the smallest object which would be measured ; 

 any smaller spot would be qtiite a negligable quantity. A 

 spot of onh/ one million square miles area may be taken 

 as the minutest object that can be satisfactorily dealt with 

 on such a photograph. 



We might look at our giant spot from another point of 

 view. Suppose that an object of the same intrinsic bright- 

 ness as the sun, and of the same apparent size as the great 

 spot, were shining in the midnight sky, how would it appear 

 to us? If we give the sun's "magnitude" as 26i times 

 brighter than an average first magnitude star, then from such 

 a body as I have supjiosed we should receive more light 

 than from 130 million stars as bright as Procyon, more 

 light than from 8680 full moons. The planet Urantis is 

 sufficiently bright to be detected by the unaided eye on a 

 clear and steady night by any keen-sighted person, yet the 

 light by which it shines is practically all derived from the 

 sun, which, as seen from Uranus, is not nearlj- as large as 

 this great spot appeared to us. Even Saturn, bright and 

 readily seen as that planet is, does not receive much more 

 than twice the light from the sun which a region of the 

 sun equal in area to the Sunspot radiates to us. 



If, therefore, the Sunspot were absolutely black, its 

 presence on the sun would imply that the light of the sun 

 was diminished by the amount of 3680 full moons, or 

 130 million stars as bright as Procyon. 



Now it is true that the spot was very far from being 

 absolutely black. The greater part of its surface, the 

 penumbra, was dark only by comparison with the greater 

 brightness of the sun ; and even the darker jjortions, the 

 umbra, still radiated a large amount of light. For it has 

 been found perfectly easy to observe the spectrum of the 

 darkest portion of Sunspots, which aflords clear proof not 

 only that they radiate some light, but tbat they radiate a 

 great deal. Still, if we assume the penumbra to be as 

 much as two-thirds as bright as the general surface, and 

 the umbra as one-quarter as bright, we have a loss of light 

 due to the .spot of some 1456 full moons ; a loss much 

 greater than the total light Neptune receives from the 

 sun, and but little less than Uranus receives. But if we 

 take the figures Sir W. Herschel gives, and regard 

 the brightness of the penumbra as 47 per cent, that of the 

 general disc, and of the nucleus as only 0-7, then our Sun- 

 spot becomes responsible for a loss of light equal to 2200 

 full moons. This is three times as much light as Neptune 



