Feb. I, 1872] 



NATURE 



259 



have now written to you proves for the interior of the earth 

 on the supposition of a thin preternaturally rigid crust. 



I must apologise to you for having troubled you with so 

 long a letter. I did not intend to make it so long when I 

 commenced, but I have been led on by considerations of 

 details, inevitable when such a subject is once entered 

 upon. — I remain, yours very truly, 



William Thomson 



G. Poulett Scrope, Esq., F.R.S. 



THE SOLAR ECLIPSE 



IN the communication to Nature, written from Oota- 

 camund, I promised another when I was in possession 

 of more information as to the work done, not only by the 

 British Association parties, but by those representing the 

 Indian and French Governments. Let me now endeavour 

 to redeem my promise, seeing that since that communi- 

 cation was penned I have had the happiness of hearing 

 from M. Janssen's own lips an account of what he did ; 

 have met Captain Waterhouse, the last representative at 

 Ootacamuiid of Colonel Tennant's party ; have visited 

 Mr. Pogson at Madras, who obligingly gave me an ac- 

 count of the results obtained at Avenashi ; and last, but 

 not least, have learnt since my return home that the Jaffna 

 party were successful, not only with the polariscope, but 

 also with the camera and spectroscope. 



Within a few minutes of the despatch of my last article 

 I found that Captain Waterhouse, who assisted Mr. 

 Hennessy in exposing the photbgraphic plates taken by 

 Colonel Tennant's party, was still at Ootacamund, and 

 this welcome intelligence was soon followed by Captain 

 Waterhouse himself, who was so good as to bring with 

 him a drawing of one of the photographs ; the plates 

 themselves having been taken down the ghaut by Colonel 

 Tennant, with the intention of comparing them at 

 Pothonore with those taken by Mr. Davis. Unfortu- 

 nately, as has been already stated, we missed each other, 

 and so an absolute comparison of photographs did not 

 take place ; but from the drawing it was evident that in 

 the two series the main form of the corona was the same. 

 The photographs I learned were very sharp and good, and 

 one appreciates their value the more when it is known that 

 only a very Uttle time before they were taken, any success, 

 even a partial one, seemed out of the question, so persistently 

 did cloud and mist hang over Dodabet on the eventful 

 morning. I gathered that the spectroscopic observations 

 had also been successful, and that a continuous spectrum 

 with 1474 had been observed. If more lines than this 

 were not seen, it is easily to be accounted for by the rela- 

 tively long focal length of the object-glass employed to 

 throw an image of the eclipsed sun on the slit. 



Not until the morning after my interview with Captain 

 Waterhouse did I learn the whereabouts of Dr. Janssen, 

 who, from a study of the habits of the clouds and their 

 prevailing drift, had concluded that the neighbourhood of 

 Ootacamund was not the best that could be chosen. 

 He had consequently taken his departure, and it seemed 

 at first as if his whereabouts was known to no one. At 

 ast, however, Prof. Respighi and myself came upon his 

 spoor ; he was at Sholoor, on the N.E. flank of the range, 

 at the solitary house of a tea-planter, to which there was 

 no road, but which might be reached on ponies if a guide 



to it could be found. This guide Captain Sargeant, of 

 the Revenue Department, obligingly provided, and in no 

 very long time we reached the beautiful spot which Dr. 

 Janssen had chosen. 



It will be better that I should state his results in his 

 own words. In a letter* to Prof. De La Rive, dated 

 December 26, he thus writes : — 



"J'ai ix.€ favoris^ par un ciel d'unc puretc; presque 

 absolue. Cette circonstance, et surtout les dispositions 

 optiques toutes nouvelles que j'avais prises, m'ont permis 

 de faire sur lacouronne des constp.tations qui ddmontrent 

 son origine solaire (pour la meilleure partie). 



"Dans mon tdlescope,+ le spectre de la couronne s'est 

 montrd non pas continu, mais remarquablement complexe. 

 J'y ai constatd : 



" Les raies brillantes du gaz hydrog&ne qui forme le 

 principal dlemdnt des protuberances et de la chromosphere. 



" La raie brillante verte dcja signalc^e aux (Eclipses de 

 1869 et 1S70, et quelques autres plus faibles. 



" Des raies obscures du spectre solaire ordinaire, 

 notamment D. Ces raies sont beaucoup plus difficiles k 

 apercevoir. 



'• Mes observations prouvent que, inddpendamment des 

 matieres cosmiques qui doivent exister dans le voisinage 

 du Soleil, il existe autour de cet astre une atmosphere 

 trcs dtendue, excessivement rare, a base d'hydrogcne. 



" Cette atmosphere, qui forme sans doute la derniere 

 enveloppe gazeuse du Soleil, s'alimente de la matiere des 

 protuberances, lance'e avec une si grande violence, des 

 entrailles de la photosphere. Mais elle se distingue de 

 la chromosphdre et des protuberances, par une densitd 

 enormcment plus faible, une temperature moins dievee, et 

 peut-ctre par la prdsence de certain gaz dilTcrents. 



" II y a done lieu de distinguer cette nouvelle atmo- 

 sphere solaire. Je propose de la nommer atinospiicrc 

 coronale, ddsignation qui rappelle que c'est die qui pro- 

 duit la meilleure partie des phdnomenes lumineux qui ont 

 dtd ddsignes jusqu'ici sous le nom de couronne solaire. 



" En annongant ce rdsultat, je n'oublic pas, quant a 

 moi, tout ce que nous devons aux travaux qui Font prd- 

 pard, notamment ceux des astronomes amdricains aux 

 dclipses de 1869 et 1S70." 



It will be seen that the importance of the brilhancy of 

 the image, so strongly insisted upon by the Eclipse Com- 

 mittee in their Instructions, had been fully recognised by 

 Dr. Janssen, whose instrument had more light even than 

 those used by the British parties, who used " Browning 

 With " reflectors of 9} inches aperture, and some 6 feet 

 focus. 



Although my account, in this place and at this time 

 can only be of the most general character, the coincidence 

 obtained by Janssen, Respighi, and myself on one point 

 may be briefly referred to, namely, the distinct proof ob- 

 tained by each of us that above the most vivid chromo- 

 spheric layer, and even the prominences, we have hydrogen 

 with its most familiar bright lines, and with much of the 

 " structure " of its spectrum ; these proofs being derived 

 not only from the old method of inquiry, but from the 

 new one employed by Professor Respighi and myself. 



We spent the remainder of the day at Sholoor in 

 mounting the hill at the back of the house to see the 

 observatory, and to admire the wonderful view of the 

 plains of Mysore, which was visible between a break in the 

 hills; while the immediate neighbourhood, with its water- 



* Bibliotheque UniverscIIe, January 15, 1872, p. 103. 



+ Ce telescope a une ouverture de q"" 37, et 1'" 42 seulemeDt de distance 

 focale. Les images y sont de 12 ^ 16 fois plus lutnineuses que datis une 

 lunette astronomique ordinaire. Le spectroscope avait kik construit pour 

 utiliscr toute cette lumi&re. 



