December, 1902.] 



KNOWLEDGE 



277 



VISIBILITY OF THE CRESCENT OP VENUS. 



TO THE EDITORS OF KNOWLEDGE. 



SiKs, — III July last a letter froui Dr. Rvle appearLnl iu 

 KsowLEUiiE, from which, aud Mr. Maumler's note, it 

 wouM appear that the seein-; the creseeut with the uakeJ 

 eye is aluiDst. if uot altoi^ether iui]iossible. I'enuit me 

 therefore to relate the following: — 111 either Oetolx-r or 

 November, l!<Ol, I was viewing Veuus, ami looking for 

 Mercury one evening. Finding the latter I called two of 

 my men, who were working near by, to come aud look at 

 what they had probably never seen before. When I 

 pointed the direction of Mercury, one of them at ouce 

 made it out with the naked eye. I then showed it to him 

 with the telescope — a o-inch refractor. Veuus at the time 

 was very bright, and to test his sight I asked him " What 

 shape she was" "r His rei>ly at ouce was " Just like the 

 new moon," and adiled, to my astonishment, " Is that the 

 rest of the star, the dark round we see " ^ I then let him 

 look through the glass, and he said "Yes, just the same, 

 only much clearer." I cannot say as to astigmatism of 

 his eye, as I do not know anything about it ; but I feel 

 sure that there was no imagination at work. He did not 

 even know the [danet's name, and all he did, when I 

 ix)inted Venus out, was to shade his eyes with his hand, 

 aud answer at ouce. 



A. W. Mansergh, Lt.-Col. 



Martness House, Portadown, 

 1st November, I'.nyi. 



SYNCHRONISM OF SOL.\R STORMS AND 

 TERRESTRIAL MAGNETIC DISTURBANCES. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — From the report of the July meeting of the 

 B. A. A., I gather that the above subject has been ouce 

 again under discussion. The Rev. Walter Sidgreaves' 

 essay on the " Connexion between Solar Spots and Earth- 

 Magnetic Storms" (R. A. S. Mem., Vol. LIV.) is about 

 the only purely scientific pronunciamento which I have 

 had the pleasure to read on this controversy. Both sides, 

 however, to my thinking, oommlt the error of allowing 

 their respective views to be too much induenced by basing 

 them exclusively on tlie speculative relations between 

 earth-magnetic stonns and " sun-spots," as if these latter 

 were the one and only solar storm phenomenon subject to 

 periodic cycles. I liken a sun-spot to the black storm- 

 cloud we see here on the earth, the cloud being, how- 

 ever, merely the scenery of the storm proper, in which 

 lightning and thunder alternate, from which rain or hail 

 may fall, aud underneath which a tornado may do havoc. 

 But if these real storm-ingredients are absent, the mere 

 passage of a cloud across the same landscape would lie of 

 little consequence. Similarly, then, a solar spot taken 

 as a dark mirk on the sun's surface denotes simply an 

 interruption in the luminous photosplieric clouds. Such a 

 spot is not necessarily the one absolute requirement to 

 produce a terrestrial magnetic storm, and in this sense it 

 would, indeed, seem advisable to droji the idea of an 

 obsolitte coniie.rion between the two phenomena. 



There are quite a number of solar periodic phenomena, 

 partly well known and partly justifiably surmised, which 

 accompany the spot cycle, and these may be much more 

 directly concerned in any action at a distance. To begin 

 with, as far ;is solar spots go, every observer can see 

 directly whether he has to do with an active or quiet s|iot. 

 Again, he can see whether, on rounding the edge of the 

 disc, the neighbourhood of a spot is studded with eruptive 

 prominences or surrounded by brilliant faculse, but both i 



f. prominences or surrounded by bri] 



p fftjUA ^'^JU j9-i^^<^. 



these he may see without a spot being visible near. But 

 if no spot is visible, aud the.se eruptive prominences and 

 brilliant facula; (indicative of a solar storm much more 

 than the dark s{)ots /Jtr xc) are well within the sun's disc, 

 their discovery is not so easy, aud I feel sure that many 

 tremendous outbursts are thus entirely lost to direct 

 telescopic or spectroscopic observation. Even then we 

 have no absolute evidence that distortion and reversals of 

 lines, etc., constitute, with disturlied spots, the whole 

 range of solar storm phenomena, and we are in entire 

 ignorance as to what extent electric potentials of un- 

 suspected magnitude may enter the arena of the upheaval. 

 These electric contributions are so far entirely invisible to . 

 us, but the belief in their actual existence is none the less 

 justified. 



Therefore I do uot see what purpose is served by 

 debating as to whether one or another particular solar 

 longitude of a sun-spot is the more productive of terrestrial 

 magnetic storms. At the same time one might argue, 

 theoretically at least, that the line of least resistance 

 would be also the shorteit connecting line betv,'een sun and 

 earth, and that, therefore, near or on the central meridian 

 a solar disturbance (not necessarily a spot) may have an 

 enhanced effect. . 



In these debates on the relationship of solar phenomena V 

 aud terrestrial magnetism, I notice that any attempt to ' 

 account for the following circumstances is invariably left j 

 out of consideration : — » 



(1) How is the incontrovertible fact explained that T 

 terrestrial magnetic st«iims do not only synchronise with L 

 the spot cycle in tok^ but accumulate in two annual ' 

 maxima about the eijuinoxes ? 



(2) Why should a solar south latitude disturbance I 

 more frequent!}' prove effective on the earth's magnetic I 

 condition iu March (spring equinox) than a north latitude 

 disturbance, aud the reverse take place at the autumn 

 equinox ? 



{o) How is it that terrestrial magnetic storms recur in j 

 subordinate periods, synchronous with the sun's sidereal j 

 rotation period, almost corresponding to the res|)ective I 

 latitude of the solar disturbance ; aud why should this I 

 same suliordinate synchronism be more strikingly apparent 

 during the spring and autumn equinoxes Y 



Possibly a correct answer to question 1 would settle the 

 second portion of question o, but unless these three main 

 questions can be satisfied, I feel lj^)und to say that what- 

 ever be the immediate cause of the terrestrial magnetic 

 disturbance, this cause must be located on the sun or in 

 extremely close jiroximity to its j>hotospheric surface ; must 

 partake of its rotation, differentiated according to latitude ; 

 must be more effective in spring and autumn through 

 more favourable relative position of sun and earth ; and, 

 finally, must be synchronous with the spot period as a 

 whole. Here then I part company with Mr. Sidgreaves, 

 inasmuch as he says on page 91, loc. cit. : — 



" It seems therefore true that recorded observations, 

 while clearly asserting a real connexion between sun-spots 

 and magnetic storms, are (ujainst anij theory tvhich jdaces 

 the cause of matjnetic disturbance within or near the sun." 



Helmholz's theory placed the cause of the sun's 

 sustained heat radiation on that body itself. From the 

 sun we also receive nearly all our light, aud I do not see 

 why we should show such hesitation to credit it also with 

 sufficient electromotive force to influence the earth's 

 magnetic condition. Could we but have an analyzing 

 magnetoscope, after the analogy of the analyzing spectro- 

 scope, which would enable us to watch maijneticaJiy any 

 selected portion of the sun's surface, we might come 

 immeasurably nearer to a solution of this vexed question. 

 Albert 4jufR£d Buss. 



O^ 



<W^ 



^ 



<ihi 



