July 1, 1889.] 



♦ KNOWLEDGE ♦ 



191 



as to diCFer not mei-ely in degree but in kind (just as suns 

 differ from giant planets, thess from our earth and her 

 fellow-planets of the — so-called — terrestrial order, these from 

 asteroids, and asteroids from meteors). Of six hundred stars 

 examined by Secchi, one-half showed the spectrum cliarac- 

 teristic of the giant suns, the .spectrum belonging to Sirius 

 (known to be a giant sun) and to all the bluish-white .stars — • 

 a spectrum characterised by the great strength and breadth 

 of the hydrogen lines. As Sirius gives out 200 times as 

 much light as our sun, he probably has a surface 200 times 

 as large, a diameter l-t times as large, and a volume .3,000 

 times as large. Assigning to him a volume only 1,000 times 

 as large, we must regard him as belonging to a higher order 

 among suns : and probably all the orbs which have a similar 

 spectrum belong to the same order. 



Now, if our sun can eject bodies with a velocity of 500 

 miles per second, as he has been observed to do, which would 

 imply the power of ejecting bodies with such velocity that 

 not only would they pass for ever away from him, but they 

 would never have their velocities i-educed by his retarding 

 action below 22 miles per second, we may fairly expect that 

 among meteors coming to the solar sy.stem from our sun's 

 fellow-stars, would be some travelling with velocities of 20 

 or 30 miles per second, in addition to the velocities acquired 

 under the sun's attractive influence while they were draw- 

 ing near to him. (These added velocities, I may remark in 

 passing, would be considerably less than those which the 

 sun can impart to bodies approaching him under his oun 

 attractive influence only.) But among bodies ejected from 

 the giant suns would certainly be many travelling with 

 much greater velocities than 20 or 30 miles per second — 

 probably many with velocities of more than 100 miles per 

 second, even after they have passed into interstellar space. 

 Such bodies, travelling in flights, would account perfectly 

 for the meteor systems with persistent radiant points 

 detected by Mr. Denning. 



In other words, the conclusions which, on an}- other 

 theory of the origin of meteors and comets than that which 

 I have advanced, appeared so improbable that even the 

 strongest evidence seemed for awhile insuflicient to establish 

 them, are found to follow so naturally from the processes 

 which my theory indicated, that we might almost have 

 expected them to be established bv just such evidence as 

 Mr. Denning has obtained. Only, of cour.se, his description 

 of these radiants as stationary, as well as his opinion as to 

 the close approach to the determination of the radiant 

 which he supposes himself able to make, must both be 

 rejected. 



It is interesting to note, in conclusion, that, with velocities 

 averaging lOO miles per second, meteors would not require 

 many thousands of years to traverse such distances as sepa- 

 rate star from star (that is, .sun from sun), instead of the 

 period of several millions of years required for these journeys 

 as heretofore viewed. But thousands of years are as mere 

 seconds in cosmical astronomical time. So that star-born 

 meteors of the orders recently recognised might fairly be 

 described asJ^Uting from sun to sun. 



Jlcilcts. 



The Rditor dce3 not hold himself responsible for the opinions or 

 statements of correspondents.] 



\ I^ETl'KR FROM rUOFKSSOR 

 II. I'ICKERING. 



KXTR\CT FROM 

 W 



.... I was much interested in your article on the 

 Orion nebula, as I am preparing an article on the subject 

 myself. I think no one win examine the photograph care- 



fully without being struck by the similarity to some of the 

 coronal forms. I was rather disappointed in Mr. Roberts' 

 photographs. In detail they may be slightly better than 

 the Harvard pictures, as his focal length is more than twice 

 ours. But in extent they certainly do not excel, and I 

 scarcely think equal, one of ours taken a year ago last 

 February, with an exposure of only 90 minutes." You 

 will find a lantern slide copy in the rooms of the Royal 

 Astronomical Society. I notice in your article you speak 

 of the nebula to the north of the great nebula. Our nega- 

 tives show them to be connected by a nebulous mass°of 

 considerable complexity. In fact the whole sword handle 

 is simply one great nebulous mass. I hope to have my 

 article ready for distribution in a few weeks. . 



May 27, 1889. W. H. Pickering. 



EXTR.VCT FROII A LETTER FROM PROFESSOR 

 E. S. HOLDEX. 



.... I am much interested in the resemblances which 

 you point out in Mr. Roberts' wonderful photographs of 

 the Orion nebula and the coronal forms. Mr. Roberts was 

 kind enough to send us prints of his Andromeda nebula, 

 but I have not seen any copy till now of the Orion nebula. 

 It is most interesting. With the original negatives before 

 you, you are in a position to judge of the analogies to 

 groups of synclinal structure in the corona and in the 

 nebula. I have looked over my own notes of the nebula 

 (central parts only), and find nothing of importance which 

 can strengthen your argument except perhaps the existence 

 of cometic tails to the stars Bond 6S5, 703, 741, the three 

 s.p. the trapezium. These look like the polar rays of the 

 corona. . . . We expect to send a (photographic) expe- 

 dition from here to South America to observe the December 

 eclipse. I shall be very glad of any information which you 

 can give us as to the best stations (healthiest, most con- 

 venient of access, most likely to be clear), and for any 

 notice of the plans of other parties, so that we may 

 co-operate with them. ... E. S. Holden. 



Lick Observatory: May 28, 1889. 



To the Editor of Knowledge. 



Sir, — Your article on lightning photographs has interested 

 me greatly. Among the many ingenious suggestions which 

 it contains, what appears to me to be a very obvious one as 

 to the cause of the dark flash has been omitted. We know 

 from common observation that there is blue lightning and 

 pink lightning, and we know that the two colours act very 

 differently on the photographic plate. I have before me a 

 photograph of a young lady in a blue dress, wearing a red 

 rose. The blue dress has come out absolutely white, and 

 the ])inkish rose .seems nearly black upon it. May there not 

 be the same eflfect with lightning ?— I am, Sir, yours 

 obediently, G. H. Clarke. 



[The so-called black flash is absolutely darker than the 

 background of sky on which it appears. The red rose or 

 pink lightning would have a slight eflfect in darkening the 

 plate, though a slower one than the blue dress or the blue 

 lightning. We have to account for something which cuts 

 down the light of the background on which it is seen, and 

 not merely for a difference of rate of light action. — Editor.] 



THE DARK FL.V.^ill. 



2'o the Editor o/" Knowledge. 



Sir, — With the June part of Knowledge there appeared 



facsimiles of a considerable number of lightuing-flivshes, 



many of wiiich were very curious, and nearlv all strikin"lv 



beautiful. " " ^ 



