276 



KNOWLEDGE. 



[Deoehbeb, 1902. 



iliffcront state of tbinj^s which we observe, viz., that the 

 retrions uear the liiul>, as we sec them, have a much higher 

 ivlleetive cai)aeity tliaii those near the centre. Nor is it 

 (lidicult to see that this is actually the case. In the centre 

 of the disk we see si)re;ul out before us the fjreat grey 

 |ihuiiti, the floors of the craters and ring-j)laiu.s, tiie 

 liottoms of the valleys. Aud we note that these low-lying 

 regions are far darker than the mountain tops ; in other 

 words, tlie lowest regions are the darkest ; the most 

 elevated are the brightest. But as we apiiroach the limb 

 till- effect of the foreshortened ]>resentati<>n is to hide tlie 

 valleys and crater-tloors mi^ire aud more from view, whilst 

 the mountain sides are more fully jtresented to us. Tlie 

 brightness of the lunar limb is therefore due to the fact 

 that it is here that the highlands are best seen, aud the 

 lowlands least. On the planet Mars, which also shows a 

 liright limb, the similar effect may be due to the presence 

 of clouds, too thin to be seen when looked down upon from 

 above — and probably also much rarer under the midday 

 sun than at morning or evening — but becoming quite 

 evident when crowded together near the limb by the effect 

 of foreshortening. Just as to a spectator on the earth, 

 t he horizon seems much more frequently aud more densely 

 tlouded than the zenith. 



The interest attaching to the brightness of the lunar 

 liiglilands dejieiids upon the cause to which it is assigned. 

 It is probable that the actual material of the grey plains 

 and crater-floors is in itself darker than that of the 

 moimtain crests and ridges. But there must be an action 

 continually going on — slight audSipw, but cumulative — 

 tending to clean the upper slopes^^K to transfer material 

 down to the lower districts ; a kinorpf weathering. The 

 meteor rain, which the moon experiences in its proportion 

 equally with the earth, must tend to rake the surfaces of 

 the steep ridges, and to drive the matter it disturbs down- 

 ^vards. So, too, the alternate expansion and contraction 

 of free particles lying on the slopes, as the lunar day and 

 night succeed each other, will mean the gradual " creeping" 

 downwards. Thus, however " dead " the moon may be, 

 there must be even now a steady kind of denudation at 

 work, a continual deposit of material in the lowlands from 

 the surface- matter of the mountain slopes. 



The interesting point is that this deposited material 

 seems darker than that uncovered by its removal, as if on 

 tlie moon, as on the earth, some process of tarnishing or 

 oxidation were at work ; as if the lunar atmosphere, rare 

 as it must lie beyond all terrestrial experience, is not 

 wholly inefficient in this direction. 



%ttttX^. 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



A NEBULOUS STAR IN SCUTUM SOBIESKI. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — Prof. Barnard informs me that the star in 

 Scutum Sobieski, S.D. — 10°, 4713,* referred to in my 

 article on " Nebulous Stars and their Spectra," iu your 

 Uctot)er number, is undoubtedly and conspicuously 

 nebulous. He has not only photograjjhed the dense glow 

 encircling it more than a dozen times with varied expo- 



* Through an oversight, the erroneous designation B.D. 10° 

 I:il3 got printed in my article. The star's place for 1900 is R.A. 

 18h. i!G-9m., Dec. —10° 52'. 



sures, but observed it visually as well. Moreover, Prof. 

 Frost has kindly examined a spectrograph of the star 

 taken by Mr. EUerman with the Yerkes refractor in the 

 autumn of IHO'.I, and finds it to be of the most authentic 

 helium type. Besides hydrogen and helium absorption, 

 Sclieiner's " high-temperature " magnesiuin-liue at A. 4ri->ii 

 was recorded between H/3 and Hy. No bright lines could 

 be detected. The object, accordingly, conforms to prece- 

 dent, and ratifies the association of a helium spectrum 

 with an inchoate condition. The experiment would be 

 worth trying of taking a spectrograph on a plate suffi- 

 ciently sensitive iu the green to test the emission of the 

 chief nebular ray by the halo attached to this interesting 



Agnes M. Clebke. 

 London, October -JSrd, 1902. 



THE CANALS OF MARS. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — On seeing that Mr. Maunder had reached 

 slightly different results with regard to the exact nature 

 of the delusion to which these appearances were due, I 

 was at first rather at a loss to account for the fact, since 

 the subjects of my experiments had all seen the canals on 

 pei'fectly unstippled sui'faces. Lately, however, the idea 

 occurred to me that perha]is Mr. Maunder had made his 

 drawings more hard and definite in outline than I had, 

 aud I therefore wrote to Mr. Maunder suggesting this 

 explanation, aud shortly after received a letter from him 

 stating that in his drawings he had shaded in the " seas " 

 with Indian ink, and had used smooth white Bristol board 

 on which to draw them, whereas I had drawn mine in 

 black lead, on rough, slightly yellowish drawing paper. 

 The difference in character between these two kinds of 

 drawing seems to me to explain adequately the slight 

 disparity in our conclusions. 



It will be understood, of course, that if a drawing is 

 stippled over with minute dots, which are individually 

 indistinguishable, the net effect is not that of a speckled 

 surface, but merely that of a general and even darkening 

 of the space so stippled. This darkening has the same 

 effect as the lightening of the dark areas would have, i.e., 

 it makes the drawing more difficult to see, and the eye 

 having thus less to definitely lay hold of, has more free 

 play, and is more easily deceived into representing 

 shadowy lines. 



The probability of the truth of this explanation becomes 

 greater, of course, when it is considered that even my 

 comparatively feeble dra^viugs had to be placed iu the 

 shadow before canals could be easily seen. 



Thus the inability of the subjects of Mr. Maunder's 

 exjieriments to see canals on unstippled surfaces does not 

 point, as might at first sight appear, to the hypothesis 

 that the red areas of Mars are speckled over with oases, 

 but merely to the fact that the light areas are not so much 

 brighter thau the dark areas as they were represented on 

 Mr. Maunder's drawings, and to a smaller extent on my 

 own. 



As everyone knows, the markings on the planet itself are 

 anythiug but strongly marked. Of course this darkening 

 of the red areas may be caused by oases, but of this we 

 have no proof, and it seems simplest to suppose, for the 

 time being, that excej>t for the large spots whose existence 

 no one can doubt, there are on the Martian continents no 

 markings visible to our jiresent telescopes, and certainly 

 no canals ; at least in our present astronomical conception 

 of the meaning of the term. 



B. W. Lane. 



