404 



NA TURE 



[August 22, 1895 



The >-alue for the twilight band, deduced from these observa- 

 tions, does not measure the full breadth of that band. It gives 

 rather a minimal \'alue for it. For although Mars shows us a 

 disc which is always more than half full, in w hich aspect an 

 illuminated fringe of atmosphere would be more perceptible to an 

 observer placed without than to one placed within it, provideti 

 both were at the same distance off, in the case before us the : 

 outsider is a great deal farther off. In consequence, what would 

 be quite recc^nisable to one standing upon the planet's surface i 

 would be too faint to be seen by him at a distance of forty 

 millions of miles away. The detection, therefore, of any 

 twilight on Mars hints that the extent of that twilight is greater 

 than appears : how much greater, we cannot at present say. A 

 second possible cause affecting the extent of the twilight is the 

 constitution of the Martian atmosphere. That atmosphere is 

 practically cloudless : if, also, it be clearer than our own, the 

 twilight would Ije relatively less for equal amounts of atmosphere, 

 for the amount of twilight is, among other things, a question of 

 the clearness of the air. In a perfectly transparent atmosphere 

 there would be much less twilight than in one charged with 

 solid or liquid ))articles. 



It is to be noted that the evidence of a twilight is independent 

 of any possible change in the value of the corrections. The 

 only corrections that admit of uncertainty are those for the 

 irradiation : and on examining them it will Ix; seen that by no 

 possible alteration can they lie made equal to account for the 

 obscr\ed increase in the equatorial diameter. Whatever altera- 

 tion in them be assumed only affects somewhat the extent of the 

 increase ; it never does away with it. In other words, whatever 

 these corrections, the fact of a twilight remains. 



For the determination of the polar flattening, the measures 

 of October 15 to 23 promise the best result, as the measures of 

 the polar diameter on the 24th were so small, com|)ared with 

 those of the equatorial diameter, as to suggest error. Com]xir- 

 ing, therefore, the polar and equatorial means of October 1 5 to 

 23, we get for the polar flattening 1/196. This, however, is 

 probably too small ; for though the |K)lar cap was nearly non- 

 existent during these obser^•ations, there were, on occasions, 

 .signs of its temporary reappearance, and an almost continuous 

 brightness of the limb where it had previously existed. This 

 by irradiation would incre.i.sc the apparent polar diameter, and 

 so decrease the resulting value for the polar flattening. If we 

 compare each polar determination with Us corresponding et)ua- 

 torial one, deduce the resulting polar flattening, and then take 

 the mean of them all, we have for the polar flattening the 

 value 1/191. 



This is prolably not far from the (ruth, although also pro- 

 liably a little tof) .small, as the polar diameter was unquestion- 

 ably still slightly increase<l Iwyond its real extent, by irradiation 

 from the remains or consequences (vapour in the air, iVc.) of the 

 polar cap. 



This value, 1/191, is also happily accordant with what theory 

 would lead us to expect. Tisserand has found thai with the 

 known rotation of Mars and supjxjsing homogeneity, the planet's 

 flattening .should be 1/175 of the equatori.-il iliameter, while if 

 the strata v,aried in den.sily, after the manner of those of the 

 earth, the [xjlar flattening should \x 1/227 of ■'• Now, assum- 

 ing Mars to have tx;en developed in general accordance with 

 the nebular hypothesis, his strata would Ix: neither homogeneous, 

 on the one hand, nor, on the other, would Ihey vary in density 

 from the .surface to the centre .so markedly as is the case with 

 those of the earth. For Mars being a smaller Imdy, the 

 previurc due to gravity would t>c less, .somewhere between that 

 <if the earth and that of homogeneity, which is nothing, and the 

 pilar flattening shuuhl tic .somewhere lietween 1/227 '*"'' '/'/S 

 <A the e<|uatorial diameter. 1/191 is, therefore, not far from 

 • \\ >a\i\v a priori. It is interesting to have this result 

 • ly with theory, as it furni.shes so much more 

 li' ling in the general evolution of our solar system, 

 much less than I/191 would require that .Mars 

 hail at sfjme time a much swifter axial rotation than 

 he h.is now. which there is not only no ground for thinking, but 

 m'i'-h rptKon for thinking could not have Ix-en the case. For 

 the t'Kils for tidal friction, possessing insufficient 

 ihc one hand and insufficieni oceans on the other, 

 ' - • ' I'l Ix: out of the question. Kven had 



. it is more than doubtful if their 

 I'erially affected his form. For on 

 the earth, which dul possess them, we see that they were practi- 

 cally impotent to alter her shape. Any great change in Mara' 



period of rotation since hecooletl must be looked upon, therefore, 

 as unlikely. 



For the final values of the diameters we have, allowing for a 

 slight irradiation from the remains of the polar cap : — 



True equ.itorial diameter 9''40 "'007 



True polar diameter 9 '35 "'007 



It will be noticed how near these values are to that found by 

 Hartwig from his general discu.ssion several years ago. 



We will now consider the September observations and the 

 first of the Octot)er ones, those taken ujxin the fifth of the 

 month. The first thing we notice about them is the abnormal 

 size of the polar measures, so large as to suggest error. On 

 examination, however, we find that instead of mistake they give 

 us our first recognition of the cause that has so long masked the 

 efl'ect of the twilight fringe. The equatorial measures, it will 

 be seen, come out in fairly good accordance with the October and 

 November determinations, being greater than those taken near 

 opposition, although somewhat smaller than the November 

 ones, the discrepancies falling probably within the errors of 

 observation. The polar measure of CJctober 5 is also much 

 what we should expect, but the ixilar measures of Sep- 

 tember 20 and 23 are ap|xarently unaccountably larger. If we 

 consider, however, the dates at which they were taken, we shall 

 at once perceive a cause ca|iable of pr<xlucing the apparent in- 

 crea.se. For in September and early in (October the polar cap 

 was still in existence. Now the south )x)lar cap is eccentric to 

 the pole, lieing situated .some 5° from it, and from Mr. Douglass's 

 micrometric measures of its position in October, in longitude 59'. 

 As during the observations the south pole was tipped towards 

 the observer, the polar cap was carried, in conse<iuence of the 

 planet's rotation, now in upon the disc, now out Ujxin the limb. 

 Now, if it chanced to be upon the limb at the hour at which the 

 measures were made, its excessive irradiation would protlucc 

 just such apparent increase in the (xilar diameter as was observe<l. 

 <Jn calculating its position for the hours of observation on Sep- ■ 

 tember 20 and 23, it appears that at those times it was in fact 

 upon the sideof the pole toward the limb. Here, then, we have 

 the dius ex maJiina in the matter. To clinch the conclusion, 

 we find on calculating its position for the observation on 

 October 5, when it suddenly measured small again, that at that 

 hour the polar cap was upon the hither side of the pole. Such 

 w.as also the case on October 12. The discreii.incy thus stands 

 accounted for. On October 13, very obligingly, the polar cap 

 practically vanished just in time not to interfere with the most 

 valuable measures at and near opposition. 



That such is the explanation of the change in the [wl.-ir 

 diameter, comes out still more markedly from the July and 

 .-Vugust measures. Turning to those incisures we find that the 

 jxisition of the jK>lar cap is an all-im|X)rtant factor in them. In- 

 deed, it is possible to follow its change of place upon the ilisc 

 from its effect as reflected in the measures. This will appear at 

 a glance from the accompanying diagram of the July and .Xugiist 

 mea.su res of Mr. nouglas,s. .\ similar sequence of position and 

 efl'ect is apparent in Prof. Pickering's measures made at the same 

 time. 



COMI'ARISO.N OF POSITION OK Poi..\R CAP AND MEASI'RI; HI-' 



Poi.AR Diameter. 



The distance of the ]X>int from the medial line shows the 

 angular |X)sition of the jxilar cap from the pole at the times of 

 observation ; 90' ilcnoting its lower, and 90* its upper meridian 

 transit. -At its lower culmination it was at its nearest m the 

 centre of the disc ; at its upper, nearest the limb. The measures 

 show the corresi»nding eflect in irradiation. 



July 6. July 8. July 30. July 33. Aug. 11. Aug. 14. Aug. 31. 



NO. 1347, VOL. 52] 



