October 1, 1801.] 



KNOWLEDGE. 



191 



without warrant from experience, since spurious instances 

 of this kind of change might be counted by the dozen. 

 What I would urge, accordingly, is not that any single 

 alleged fact bearing on the point should be taken as 

 established, but that a more determinate plan of investiga- 

 tion should be adopted than has hitherto been in use. 

 Few will deny that a strong case for further enquiry has 

 been made out. The evidence at hand is at any rate of 

 sufficient weight for the purpose of directing special 

 attention to certain objects, the rumoured colour- variations 

 of which can be tested only by patient watching. Now, 

 under this system, sundry processes of variation undoubtedly 

 tend to become arrested, which seemed to show decidedly 

 enough in the twilight of casual notice. Whether or not 

 the changes imputed to red stars will fall into this category, 

 remains to be seen. My personal conviction is that some 

 of them will prove genuine, but it is founded, I admit, on 

 a scanty basis of experience. Nevertheless, /• Velorum made 

 so striking an exception to the otherwise unfailing and 

 emphatic redness of Gould's "red stars," so far as I was 

 able to review them at the Cape, that I find it difficult to 

 conceive of the change as apparent only. Properly speak- 

 ing, however, all observations of this nature yet recorded 

 ought to be regarded as merely indicative. A fresh start 

 should be made with a view- to ascertain — first, whether 

 colour-variations really occur ; secondly, what is their 

 cause, if they do occur. Not that these two enquiries 

 need be prosecuted separately or successively ; the better 

 plan would be to carry them on at the same time, and by 

 the same means. In neither should the telescope bo 

 implicitly relied upon. The data aflbrdod by it should 

 have their meaning probed and deepened by the concurrent 

 aid of the spectroscope. A useful adjunct, moreover, 

 might be found in the photographic determination of 

 relative magnitudes ; for changes of tint would presumably 

 be accompanied, and might even prove to be strictly 

 measurable, through changes in the chemical intensity of 

 the emitted light. 



A. M. Clerke. 



Ballysodare, Co. Sligo, Ireland, 



\%th Seiiteiiihti; 18!)1. 

 Dear Sir, — Looking over your interesting paper on the 

 Pleiades in the May number of Knowledge, I was sur- 

 prised to see that in your list of first magnitude stars (p. 

 !)1) you give Canoj^us only sixth in order of brilliancy ! 

 From my own observations in India I have no doubt tliat, 

 with the exception of Sirius, Canopus is certainly the 

 brightest star in the heavens. Indeed, on one or two occa- 

 sions I found it very little, if at all, inferior to Sirius. 

 Gould gives (" U.A.," p. 342) the order of brightness : — 



Sirius 0-1 I Arcturus fS 



Canopus 0-4 aLyne l-O 



a Centauri ... 0-7 | Rigel 1-0 



and, setting aside a Centauri (with which I am not familiar), 

 this is the order in which I should place them. 



I am aware that there is a difference of opinion with 

 reference to the relative brightness of Arcturus, Capella, 

 and Vega, but to anyone who has seen Canopus there can, 

 I think, be no doubt that it is brighter than any star in 

 the northern hemisphere, I am aware that it has been 

 suspected of variation — some observer in Chili, in 1801, 

 thinking it brighter than Sirius ! — but in its normal state 

 it should, I think, certainly stand second on the list. 

 Yours very truly, 



J. E. GoiiE. 

 I see in the " Cape Observations " Sir John Herschcl 

 gives the following sequence, observed Mai-ch 2Hth, 1838 : — 



[I have given the readers of Knowleixje the benefit of 

 Mr. Gore's letter, though I imagine that it was not written 

 for publication. 



Accurate photometric measures of the brighter stars in 

 the southern heavens are much needed. The magnitudes 

 given in the article referred to are from Prof. Pickering's 

 " Photometric Catalogue for the Northern Sky " and from 

 the " Uranoraetria Argentina '' for the rest of the heavens. 

 There must surely be some mistake about the estimates 

 of Gould and Sir .John Herschel of the brightness of Siriu.s. 

 But if we adopt their order of magnitude, it makes no 

 difference in the fact to which I wish to draw attention — 

 that out of the twelve brightest stars in the heavens seven 

 lie in the brilliant girdle of stars referred to, and three 

 are intimately associated with it, being situated only just 

 on the opposite border of the Milky Way. — A. C. R.^xy.vrd.] 



ARK THE LUXAE R.V.YS DYKES, OR DUE TO FISSURES? 

 To till' Editor of Kno\\xedge. 



Sir, --The paper on " Lunar and Terrestrial Vol- 

 canoes," in Knowledge, August 1st, pages 145-7, and 

 your remarks thereon, are most interesting to students, 

 and I trust you may allow me a small space for a few 

 words about the rays. 



With Mr. Hutchinson's conclusion, that the long white 

 streamers radiating from Tycho and other craters cannot 

 be due to " faults," most selenographers, who understand 

 a little geology, will at once agree. 



A fault is not only a fissure, but also a more or less 

 vertical " slip," or dislocation, of strata, which leaves one 

 side much higher than the other ; hence, if the rays were 

 due to faulting, we should (frequently) be able to detect 

 this inequality by the shadows at sunrise and sunset. 



As a matter of fact this feature is not seen in the rays, 

 and, when we realize their width, this explanation of them 

 may be safely set aside as untenable. 



Mr. Hutchinson inclines to look on the rays as trap 

 dykes, or vast cracks into which some white molten rock 

 has been extruded: but, as you have clearly pointed out, 

 tiiey are far too wide and indefinite along the edges to 

 permit of this as a solution. 



As Prebendary Webb so clearly points out, page 74 of 

 his " Celestial Objects," '■ the chances against so general 

 and exact a restoration of level, all along such multiplied 

 and most irregular lines of exposure, would be incalculable.' 



If the rays were due to vast fissures, filled in with 

 whiter material, the distortion (by displacement) of the 

 detail would be quite extraordinary. Where a ray-dyke, 

 ten miles wide, cut a cleft or ridge at an acute angle, the 

 severed extremities could not possibly remain in line. 



The entire region around Tycho, especially to the west, 

 would in fact have been so revolutionized by the vast 

 fissures and dykes, that the normal detail would have been 

 iittiitij (ililitcnitiil if the rays were due to this cause. 



So that this also, as a solution for these singular white 

 streamers, may safely be put aside as untenable. 



With regard to the view that they may be due to snow- 

 deposit, along the margins of minute iixsiirt'<. we are at 

 once met by the difficulty of understanding how they come 

 to be so long, so radially grouped, and so minute as to be 

 nowhere visible. The difficulty, again, of conceiving such 



