Aug. 3, 1883.] 



KNOAVLEDGE ♦ 



75 



It is remarkable that ZoUner's observations should agree 

 so well -with calculations based on a formula -which is, on 

 the face of it, erroneous.* 



ZoUner's -svork on the brightness of Mars when 

 gibbous led to the strange result that the defalca- 

 tion of the brightness of Mars when gibbous is greater 

 than in the case of the moon. Instead of an inclina- 

 tion of 52° for the Martian mountains, Zollner inferred 

 an inclination of more than 75°. It must be admitted, I 

 think, that this would be absurd, even though wo had 

 reason to believe that the whole surface of ]\Iars is solid, 

 which is certainly not the case. An equally complete, and 

 much more natural explanation, is obtained, if we suppose 

 the Martian air to be ordinarily somewhat cloud-laden, the 

 clouds resembling in shape our own summer clouds, and 

 lying far enough apart to show the Martian surface. This ex- 

 planation is given and illustrated at p. 65 of m}' " Essays on 

 Astronomy," where it is shown that as a consequence of 

 the supposed state of things the edge of Mars's disc should 

 appear brighter than the central part, which is actually the 

 case. But I must here note that there is a mistake in what 

 I there say, to the effect that ZoUner's observations would be 

 equally well e.xplaine d by supposing that " when the sun 

 is near the horizon of Mars, heavy mists hang in the air." 

 The aspect of Mars when gibbous at once negatives this 

 subsidiary explanation (as I ought to have noticed when I 

 penned the above words, and as I should have noticed if I 

 had followed my usual rule of reasoning out the matter 



* Thia table is taken from Klein's " Das Soiinonsystem." In 

 the original paper by Zollner the logarithms of the quantities 

 above tabulated are given, and Klein has been at great pains to 

 substitute the natural numbers. But he fails to notice two mis- 

 takes in the original tables, where, opposite 139° and 219°, Zollner 

 sots as the logarithm of the brightness calculated by Lambert's 

 method, 1-9985 and 1-9774 respectively, instead of 1-89S5 and 

 1-9079 respectively. Thus in Klein's table there is given opposite 

 219° the value 94-93, and opposite 139° the value 99-60 which ai-o 

 obviously erroneous. Hero is another instance of the absolute 

 neceiixitij of examinitiij everythiiuj resemhling a table which one 

 may desire to make use of. No one would believe how readily mis- 

 takes will escape the notice of oven the most careful tabulist. I 

 am satisfied that the very care taken by Klein to make his table 

 instructive caused him to overlook points aii'ecting its general 

 accuracy. It w-ould be unfair not to point this out. There are 

 tWQ ways in which tables may become erroneous : — Fii-st, when 

 tables are simply pitchforked into a book by a compiler, one may 

 be tolerably certain that they are erroneous; but secondly, when a 

 careful worker like Klein really gives labour to a table, it will 

 happen that while engrossed on such special work, he overlooks the 

 nocuesity far general supervision. 



at the time of writing). For, near the terminator of 

 gibbous Mars the light darkens perceptibly, precisely as 

 it should do if my first explanation be correct (of which 

 I now entertain no doubt) ; Vjut if morning and evening 

 mists prevailed in Mars, the part near the terminator would 

 be rather brighter than the part near the true limb. Com- 

 paring this with my reasoning as to the first explanation, 

 and noting how absolutely untenable is ZoUner's theory of 

 sugar-loaf hills all over Mars, it appears to be as nearly 

 demonstrated as the nature of things renders possible that 

 the Martial clouds are for the most part cumulus clouds. 



(Tu he continued.) 



THE DISCOVERY OF THE CHIEF 

 DIVISION m SATURN'S RING. 



MISLED by a categorical statement on page 217 of 

 Breen's " Planetary Worlds," I last autumn wrote 

 to Knowledge (Vol. II. p. 294) to inquire whether a 

 letter written by Wallis to Huyghens, on the subject of 

 the supposed duplicity of Saturn's Ring, was still e.^ctant, 

 and, if so, where it was to be found. It was subsequently 

 pointed out by Mr. Herbert Rix (p. 471) that it was not 

 Wallis at all who wrote to invite the attention of Huyghens 

 to what had been seen (or imagined) by William Ball, but 

 the first President of the Royal Society, Sir Robert Moray. 

 I am happy to learn, through Mr. C. Leeson Prince, 

 F.R.A.S., that Moray's letter to Huyghens has been found 

 in the library at Leyden by Dr. Bakhuyzen. The wording 

 of this letter seems to me to be conclusive as to the idea 

 which its writer entertained of the nature of the duplicity 

 of Saturn's Ring, assuming it not to be a single appendage. 

 " Quant au plus eloigne," says Moray, (" la planete Saturne) 

 il y aurait plaisir a decouvrir que son corps tourna autour de 

 son axe pendant que ses anses demeurent ferme, et si c'est un 

 cercle comme vous avez prouve qu'il poitvra bien estre sans 

 difficulte il ne pouvra avoir aucun mouvement autour 

 du corps du planete si ce n'est qu'il garde tousiours une 

 meme plaine. Mais il est temps que vous considerez avec 

 grande attention la figure de ces anses a present pour ^•oir 

 si vous n'y voyez rien qui vous fasse avouer que ce n'est 

 pas un corps de figure circulaire qui embrasse la disque 

 du planete, mais deux ; ce qu'il y aura quelque 

 (word undecipherable) de croire :i ce qu'il me semble 

 selon les observations qu'en ont fait le Dr. Ball et son frere 

 tous deux de nostre societe, avec un fort bon telescope 

 de 36 pieds." Hence it would seem that Moray was not 

 quite convinced by Huyghens' reasoning that the ring of 

 Saturn was a circle ; but rather imagined that each of the 

 ans» was a separate curve, as shown in the strange draw- 

 ing — or paper-cutting — by Ball, reproduced in Vol. I. of 

 the "Philosophical Transactions" for 1665 and 1666, and 

 engraved in facsimile on p. 307 of your second volume. 

 At any rate, it is ob\-ious tliat neither Ball nor he had the 

 most distant idea that Saturn was surroimded by two concen- 

 tric rings. In the communication from which I have made 

 the above extract. Dr. Bakhuyzen goes on to intimate that 

 Joseph Campani may possibly have seen the outer and 

 inner rings of Saturn, and even " traces of the inner dark 

 ring," and he refers to a figure of the planet by Campani, 

 in justification of this' surmise. Mr. Prince, however, has 

 this engraving in one of the numerous early astronomical 

 volumes in his library, and he informs me that no indica- 

 tion whatever of a di\-ision in the ring exists in it. So 

 far, then, I would venture to suggest, every attempt to 

 deprive Oassini of the merit {quantum valeat) of having 



