; 4 S 



NA TURE 



[August 7, 1884 



but when the phenomenon was at is maximum phase it gra- 

 dually crept up from about B till past C, as the sun sank towards 

 the horizon. On the I2th, when the sun was within a few 

 degrees of the horizon, the absorption was well marked up to 

 W.L. 621, i.e. to beyond a, while at the violet end the visible 

 spectrum ended at W.L. 428, or just beyond G. 



The lines A and a were never visible even on the sun, when it 

 was green, and even B could be made out with difficulty from 

 half an hour before sunset onwards, and before it vanished it 

 grew intensely prominent with enormously thick band, on the 

 less refrangible side. The band C, on the more refrangible side 

 of C became very broad and black, while the fine line between 

 this and C remained thin and sharp, and C itself thickened out 

 on the less refrangible side. The rain-band was stronger than I 

 have ever before observed it on the plains, and even with the 

 dispersion produced by a single prism at least eight lines could 

 be measured in it, while many more were visible. The low-sun- 

 band was not very conspicuous, but this was partly due to con- 

 trast with the very strong rain-band. The line W. L. 56S at the 

 more refrangible side of the low-sun-band was very well marked, 

 and the band itself seemed to consist of a series of equidistant 

 lines. 



The apparently much stronger absorption in the red than in 

 the blue end was a very marked feature, which became still 

 more conspicuous when a photograph of the blue end was 

 examined. 



Since the passing away of the abnormal conditions I have 

 made careful observations of the sunset spectrum with the same 

 apparatus, and I find that ordinarily A and a are clearly visible 

 as well as B, though at times they are strongly marked, and a 

 good deal of shading is observable between them ; C, is much 

 thinner, and the rain-band is less prominent than the low-sun- 

 band, which however does not now have the appearance of a 

 number of fine lines. The nearest approach to the green-sun 

 spectrum was observed recently during a severe thunderstorm, 

 which was accompanied by a fall of about ik inches of rain. A 

 very similar though less intense spectrum can be observed almost 

 any evening by taking advantage of the passage of a small thin 

 cloud over the sun's disk. If a lens is used in front of the slit of 

 the spectroscope, the absorption due to the cloud will be seen as 

 a band in the middle of the bright spectrum from the unclouded 

 part of the sun, and owing to the strong contrast, the details of 

 the absorption will be well seen, just as in the case of the 

 spectrum of a sunspot. 



Meteorological Phenomena. — The electrification of the air was 

 carefully studied during the green sun period, and the results are 

 rather curious. From September 3 to 6 the potential of the air 

 was positive in the early morning, diminished to zero between 

 9 and 10 a.m., then became negative, and remained so until the 

 sea-breeze came on in the afternoon, when the charge was 

 positive again, and continued so all night. The amount of 

 electrification varied greatly and rapidly. On the 7th and 9th 

 the potential was positive all day, on the Sth it was negative for 

 a short time. From the loth to the 12th it varied in the same 

 way as from the 3rd to the 6th, and this state of matters was 

 repeated from the 20th to the 27th ; the electrometer readings 

 from the 13th to the 19th having lieen normal. All the negative 



adings were got during a hot land wind from the west. Between 

 the 6th and 9th of September a storm of unusual violence swept 

 o ,-er the Madras Presidency from the south-west to the north- 

 east, 'making itself felt in different ways at different places. 

 The rainfall for September was unusually small all over Madras. 

 The average for fifteen stations was 3-24 inches, not quite half 

 the average for this month during previous years. 



The barometric curves for Colombo, Madras, Belgaum, 

 Allahabad, and Calcutta (Alipore) have been drawn and found 

 to resemble each other closely. All over India there was a 

 minimum between the 6th and the 7th, a maximum about the 

 iSth, another minimum on the 21st, then a rise, and a third 

 minimum on the 27th. 



1 he first essential in any attempt to arrive at an explanation 

 of the cause of the green sun is to ascertain the precise dates at 

 which the phenomenon was first observed in various parts of the 

 world. It is difficult to do this, for people are apt to make more 

 precise statements than their observations warrant. For instance, 

 the sun certainly set with a peculiar silvery gleam, but no green- 

 ness, at Madras on September 9, and yet many persons have 

 assured me that they saw it set green there on that evening. The 

 reason evidently was that after their attention had been arrested 

 by the green sunsets of the loth and nth they remembered 



having noticed something peculiar about the sunset on the 9th, 

 and immediately concluded that the sun had been green on that 

 occasion also. In consequence of this tendency of the mind, the 

 evidence for all the dates given has been carefully tested, and 

 has been found in all cases sufficient to justify the opinion that 

 these dates are correct. 



It appears that in Ceylon, in the south part of the Madras 

 Presidency, and at Ongole in the north, the sun was first observed 

 to be green on the evening of the 9th, and that over the east of 

 the Presidency, when seen at all, it was first seen green on the 

 morning of the loth. The green sun was reported at Belgaum 

 on the Sth, but although the observer was trustworthy, he did 

 not make a note of the fact until afterwards, and 'it is just 

 possible that it may be a mistake. 



The captain of the Cleomene reports a green sun and moon on 

 the 9th, 10th, and nth. when his position was from lat. 8° N. 

 to lat. 16° N., and from long. 83 30' E. to long. 88° 40' E. The 

 chief officer of the s.s. Pelican saw the moon greenish on the 

 night of the 9th, and the sun green on the morning of the 10th. 

 The steamer was more than 1000 miles away from Madras, in 

 lat. 10° 4' N. and long. 64 12' E., wind south-west. 



The green sun was not seen further north than Ongole, except 

 at Vizagapatam, Rajamundry, and Simla, and the dates of obser- 

 vation at these stations are not preserved. It was seen at 

 Bombay, but was so inconspicuous that it escaped notice at the 

 Observatory. 



In Honolulu the sun's disk was seen to be green before it set 

 on September 4 and 5 (Nature, vol. xxix. p. 549). On Sep- 

 tember 4 the master of the Jennie Walker " noticed the strange 

 appearance of the sun, which was greenish," in lat. 8° 20' N. 

 and long. 155° 28' W. A passenger three days out from Hono- 

 lulu for Sydney saw it blue on the 5th and 6th [Ibid., p. 181). 

 On September 2 it was observed bluish green in Venezuela and 

 in Trinidad [Ibid., vol. xxviii. p. 577, vol. xxix. p. 77). It was 

 seen at Panama on the 2nd and 3rd, and at. Cape Coast Castle 

 apparently on the 1st of September. So much for the first 

 appearance. 



The sun was again seen green on September 20, in lat. 

 12° 50' N. and long. 48° 26' E. At almost all stations in 

 Southern India the greenness reappeared on the 22nd, and 

 Hicks Pasha noticed the sun green in the Soudan on the 24th. 



The phenomena of the green sun must be distinguished from 

 those of the remarkable sunsets that occurred all over the world 

 some time later, and the reasons for considering them essentially 

 different are : — 



1. The general appearances of the sunsets were quite dif- 

 ferent. The sunsets accompanying the green sun were lurid, 

 and the horizon so misty that stars were lost in it ; the subse- 

 quent sunsets were remarkable for the play of delicate tints, 

 the rose-coloured after-glow, and the unusual clearness of the 

 horizon. 



2. The spectra were totally different. In the latter case the 

 red end was unusually free from absorption ; A, a, and B stood 

 out clearly, the rain-band was slight, and the low-sun-bands 

 strong. This contrasts in every way with the description of the 

 spectrum of the green sun already given. 



rhree hypotheses have been put forward to account for the 

 phenomena of the green sun : — 



1. That it was due to vapours or dust from the volcanic erup- 

 tion at Krakatao. This was at first proposed by Mr. Pogson. 



2. That the cause was the presence of an abnormal amount 

 of aqueous vapour — an explanation which I offered at the time 

 of the occurrence. 



3. That it was caused by a cloud of meteoric dust. 



The supporters of the Krakatao theory view the phenomena 

 of the green sun and the remarkable sunsets as due to the same 

 cause. The two great difficulties with regard to that hypothesis 

 are (1) that there is no proof of the existence of an air current 

 travelling at the enormous velocity required by the dates of the 

 appearances of the green sun at various places, and (2) granting 

 that there was such a current, how it was that the dust was such 

 a long time in reaching India. Mr. Lockyer gets over these 

 difficulties by assuming the existence of an upper current from 

 east to west along the equator, and an under current from south 

 to north. If, however, Mr. Manley's observations are accurate, 

 as there seems every reason to believe they are, the green sun 

 appeared at Ongole as soon as in Colombo, and at least twelve 

 hours sooner than in Madras ; and, if the Belgaum observations 

 are correct, it appeared there a day before it was noticed at 

 Colombo. Taking, however, only those observations about 



