April 1, 1892.] 



KNOWLEDGE 



69 



receives, considerably more than Uranus receives, and is 

 much more than one-fourth of that which falls upon 

 Saturn. 



These figures, of coiu'se, represent not the average size 

 of the group, but its greatest size. It would appear to have 

 been quite as large, perhaps even larger, on February 11th 

 and 12th as on February 13th, the date for which the 

 preceding figures have been computed. But it had passed 

 through many changes before it attained so great a de- 

 velopment. We cannot trace it back to its formation, for 

 that took place upon the further side of the sun, and it 

 yet remains to be seen whether it will end its course in the 

 visible or the invisible hemisphere. The first time it came 

 imder observation was November 1.3th, 1801, when it was 

 seen at the eastern edge of the sun. It was already a 

 fair-sized spot, with an area of about 470 miUions of 

 square miles when first seen, and it increased in size from 

 day to day. At this time, November 19th and 20th, it 

 resembled the general type of the more important spot 

 groups in its appearance, that is to say, it was divided into 

 three principal regions. The largest spot of the group 

 led, then followed a few small scattered spots, and the 

 procession was closed by a large spot of irregular outline. 

 Usually the leading spot in such a group tends to become 

 very well defined in outline and circular in shape, and it 

 increases in size, whilst the rest of the group tend to 

 die out. It was not so ^ith the present group. After 

 passing the central meridian of the sun great changes were 

 seen, in which the following spot assumed the circular 

 form of the first, and the leader broke up into a chain of 

 small spots. This chain, however, conformed to the 

 typical spot history, and before the group had been carried 

 out of sight round the western edge of the sun, it had 

 become reduced to a single circular spot, making with the 

 following spot a pau- very similar in appearance and size. 



The gi-oup was next seen at the eastern edge on 

 December 12th, and remained visible until December 24th, 

 returning into view on -lanuary 9th, and disappearing at 

 the western edge for the third time on -January 21&t. No 

 very noteworthy changes occurred during these two 

 appearances. But when the group reached the east limb 

 for the fourth time, on February 5th, it was altogether 

 changed in appearance and size. It was now five times as 

 large as when it had been last seen on -January 21st, and 

 it was to increase stiU further in size. The distressmgly 

 bad weather we had in England during the next week 

 prevented further o"bservations at Greenwich until Feb- 

 ruary 13th, the date upon which the photogi'aph was taken 

 which is reproduced in the accompanying plate. From 

 this date it diminished in area somewhat until Feb- 

 ruary iKth, when it reached the western edge. It returned 

 to our view on March 4th, shrunk in size beyond all recog- 

 nition. The entire area of the group was now only about 

 250 millions of square miles, and of the largest spot only 

 120 millions. It revived a good deal during the following 

 days, but though it attained very important dimensions, 

 and showed several interesting changes, it at no time 

 approached the magnificent proportions it had exhibited 

 during February. It reached the western edge on 

 March 17th, and if it still continues in existence may be 

 expected to reappear again on the eastern border on 

 March 31st or April 1st. 



It was, therefore, only during a small portion of its life 

 that the group attained the enormous development sho\Mi 

 on our photogi-aph. It happened with this spot, as it 

 often happens with other large groups, that its greatest 

 development took place very nearly at the time when the 

 rotation of the sun brought it nearest to the centre of the 

 disc. The days, therefore, when the actual area was 



greatest were those when the effect of foreshortening in 

 diminishing the apparent size of the group was least ; so 

 that it was seen most nearly in its true proportions just at 

 the time when these were at their maximum. 



It was, therefore, only for three or four days that the 

 spot was effective in diminishing the sun's light to the 

 extent we have mentioned. StiU that diminution would 

 appear to be so serious in actual amount that even if we 

 regard it as lasting but three days it would seem reasonable 

 to expect that it would make itself felt here. And certain 

 effects were undoubtedly experienced, though perhaps not 

 those which might have been naturally looked for. and 

 though it is not by any means certain that they had any 

 direct connection with a diminished radiation from the sim. 

 it might, perhaps, have been supposed that a loss of 

 Ught equivalent to that afforded by over 2200 fuU moons 

 would have seriously affected terrestrial weather. It is 

 possible that it may hereafter be proved that it has 

 such an effect, but up to the present nothing of the kind 

 has been satisfactorily established. And even supposing, 

 what is qiute possible, that the diminution of radiation 

 over the spot itself is not compensated by an increased 

 radiation from other parts of the surface, it must be fully 

 borne in mind that the greatest loss due to the spot must 

 be but a very small fraction of the total solar radiation. 

 The difference due to the greater distance of the earth from 

 the sun at one part of its orbit far outweighs it. For the 

 extreme difference due to that cause is one part Lq fifteen, 

 but the greatest difference which, on the assumptions we 

 have made, we can ascribe to even such a spot as that of 

 February 18th would be no more than one part in 320, or 

 only three times the effect produced by Venus in transit. 

 Any dinct effect upon the weather, therefore, must, in any 

 case, be of the slightest, the more so that Sunspots are 

 short-lived, and the time during which they present their 

 full face to the earth is necessarily limited to three or four 

 days. 



But if the weather shows little or no sign of Sunspot 

 influence, that influence does really exist, and makes itself 

 tmmistakably evident ; and our February Sunspot did not 

 fail to make its own indindual record. Down in the cellars 

 of the Royal Observatory, Greenwich, and the Kew 

 Observatory, and at many other similar institutions all over 

 the world, delicate magnetic needles are kept carefully 

 suspended so as to be free to move at the shghtest impulse. 

 The position which such a needle takes up in England is 

 neither a horizontal nor vertical one, nor does it point truly 

 to the geographical north. This deriation from the true 

 north is what is known as the magnetic decUnation, or 

 the ordinary " variation of the compass." But such a 

 needle do3s not remain stationary in the position it takes 

 up. Day by day it appears to make an effort to follow the 

 sun as it crosses the sky. The greatest deviation from 

 the mean position towards the west takes place about 

 two o'clock. The needle then begins to move back again, 

 and by about ten o'clock at night has returned to its 

 average position. There is usually but little movement 

 during the night. 



This curious daily motion varies in character and ex- 

 tent at different times. Notably it varies with the season 

 of the year, but for our present purpose its variation from 

 year to year is more important. For it has been clearly 

 "established that, »■/(/•« "v tuk-r aivrwie results for .swre-ssiif 

 i/i-firs, we shall find that this motion is greater in amplitude 

 and force in strict proportion to the number and size of 

 the spots upon the sun. 



That there is this riineral connection between the mag- 

 netic variations and Sunspot frequency is admitted. But 

 when we come to the question as to whether individual 



