276 



KNOWLEDGE 



[December 1, 1894. 



NOTICE. 



•-♦-* 



With very great regret the announcement has to be made 

 tliat j\Ir. Ranyard. owing to serious illness, has been unable 

 to edit this issue. Mr. Ranyard has been brought to a very 

 low and weak condition, and as he recovers a sea voyage 

 will be necessary, but he hopes to resume his labours before 

 very long. In the interim, Mr. E. W. ]\Iaunder has kindly 

 offered to supervise the astronomical pages of Knowledge. 



THE CENTRAL EQUATORIAL REGION OF 

 THE MOON. 



By T. G^\TN Elger, F.R.A.S. 



THE contrast between the northern and southern 

 hemisphere of the moon, as regards the number 

 and size of large ramparted enclosures, is evident 

 on glancing at any map or photograph of its visible 

 surface. Though the former includes many notable 

 objects of this description, only a few of them can compare 

 in dimensions or complexity of structure with those on the 

 opposite side of the lunar equator. A distinction almost 

 as striking exists between the two quadrants of this 

 southern hemisphere, the south-western containing an 

 almost endless variety and bewildering number of for- 

 mations of a type which is much more scantily represented 

 in the other quarter. The inequality in the distribution 

 of the maria, or so-called "seas," is also very remarkable. 

 If a great circle is drawn from about N. lat. 20° on the 

 western limb, through the intersection of the first, or 

 central, meridian with the equator, to the corresponding 

 S. lat. on the eastern limb, the greater portion of the 

 superficies north of this line consists of low-lying 

 dusky plains, whose monotony is often unrelieved by any 

 conspicuous object, while the region south of it, very 

 distinctly brighter, is the area on which the results of those 

 volcanic forces which have given to the moon its extra- 

 ordinary aspect are developed to the greatest extent. 



The beautiful plate which accompanies this paper, 

 though only including a portion of the surface extending 

 from S. lat. 30° to abo\it 20° on the north side of the 

 equator, admirably illustrates the distinction which has 

 been referred to. Ptolomaus, the great central ring- 

 mountain so prominently represented thereon, with the 

 chain of irregularly-shaped formations running north from 

 it, and the apparently inextricable confusion of hkarn' 

 groups flanking it on the -west, may thus be compared 

 with the smaller and generally different features portrayed 

 on the northern half of the picture. 



Ptolemaus is undoubtedly one of the most perfect and 

 typical examples of a certain class of formations, termed 

 "walled-plains," which can be found on the moon, and 

 from its position, near the centre of the disc, can be more 

 conveniently scrutinized and studied than almost any 

 other. These formations differ from many which often 

 bear a superficial resemblance to them, not only in extent, 

 but in the hypsometrical relation which exists between 

 their interior, or, as selenographers term it, " the floor," 

 and the region beyond the limits of the circumvallation. 

 In most other enclosures this Hoor is depressed, sometimes 

 to the extent of many thousand feet, below the outside 

 region, but in the walled-plain there is seldom any 

 appreciable difference in level between the interior and 

 exterior. Less marked, but still characteristic of these 

 objects, is the nature of the rampart and its slopes, within 

 and without. In many of the so-called ring-moantains, 



the latter are only slightly inclined outside, rising to the 

 summit of the wall with an extremely gentle gradient, 

 while they descend to the floor with a very steep declivity. 

 The border of the walled-plain is also, as a rule, less 

 continuous, being more frequently interrupted by gaps, 

 crossed by transverse valleys and passes, andbroken by more 

 recent craters and depressions. 



To return, however, to the notable object which figures 

 so prominently on the plate. In order to appreciate its 

 actual size and the scale of its surroundings, it must be 

 remembered that a distance of one hundred and fifteen 

 miles intervenes between the sides, or rather more than 

 that from London to Birmingham ; so that the whole of 

 the principality of Wales, with one or two of the bordering 

 counties in addition, could easily be accommodated within 

 its limits. In fact, its dimensions are such that, though 

 there are peaks on the rampart rising to four thousand, 

 five thousand, and 

 even six thousand 

 feet; if it were pos- 

 sible for an obser- 

 ver to be stationed 

 near the centre of 

 this great amphi- 

 theatre, he might 

 well imagine that 

 hewas standing on 

 a boundless plain, 

 for, except at one 

 or two points on 

 the west, there 

 would be nothing 

 to break the 

 monotony of the 

 prospect. 



At the particular 

 phase represented 

 in the plate, the 

 sun has not long 

 risen on Ptole- 

 maus and the 

 chain of great 

 rings south of it ; 

 hence its rays fall 

 very obliquely on 

 the enclosed plain , 

 and reveal details 

 which, a few hours 

 later, would be 

 wholly invisible. 

 Among the most 

 noteworthy of 

 these, is a number 

 of shallow, circu- 

 lar, saucer-shaped 

 depressions, about 

 four or five miles 

 in diameter, con- 



Index to Map. 



Aliacensis. 



Regioiiiontaniis. 



Worner. 



Purbac'li. 



Arzaeliel. 



Alpetragius. 



7. Alplionsus. 



8. Albateguius. 



9. Ptolemaus. 



10. Ilipparclms. 



11. Herschel. 



G, 



12, 

 13. 

 It, 

 15, 

 16. 

 17, 

 18, 



Godin. 



Agrippa. 



Triesneckei". 



Hjginii.s. 



Ai'iadanis. 



Bode. 



Manilius. 



fined for the most 

 part to the eastern 

 half of it. Owing 

 to their insignificant depth, they are so evanescent that, 

 unless they are looked for at a very early stage of lunar 

 sunrise, it is impossible to see them, though, minute 

 craters found within them can be traced long after they 

 have disappeared. Objects of this kind, though not un- 

 common on the " seas," are nowhere so closely aggregated 

 as here. They impress one with the idea that they 

 represent old crater-rings, which are covered and partially 



