ON MAPPING THE SURFACE 01' THE MOOX. 217 



corresponding prominent featiu-es in the two photographs. As before re- 

 marked, the photograph of October 4, 1865, is that of full moon ; the one 

 employed for the smaller and more striking detail is that taken by AVarren 

 De La Eue, Esq., on February 22, 1858, enlarged on glass to 8-75 inches in 

 diameter. On neither of the six ai'cas above mentioned does a point of tho 

 first order occur, and the nearest to areas IV A*, lY A^ is the central moun- 

 tain of Albatcgnius in IV A-^. From the veiy nature of the apparent changes 

 effected in the visible disk by libration, it is clear that such measures as those 

 just alluded to cannot possess much claim to accuracy when referred to the 

 mean projection, except ivhen made in the immediate neighhourlwod of a point 

 of the first order, or near a point that has been well identified ; all other 

 positions can only be considered as approximate ; indeed, when the lines 

 measured approach the tenth part of the moon's scmidiameter in length they 

 are quite useless; still, with a proper amoimt of care the approximation 

 capable of being attained may be sufficiently close for all the purposes of a 

 map, especially if all well-detcrmiucd positions be distinctly indicated. The 

 reader may easily convince himself of the difiiculty of combining portions of 

 photographs taken at differing intervals from the epoch of mean libration by 

 simply making enlarged tracings of such portions and supervising the one on 

 the other ; he will soon see they will not fit ; added to this is the effect 

 produced by variation of distance ; two photographs taken at mean libration 

 will not quite agree, the features of that taken at perigee will manifestly be 

 larger than if it were taken at apogee. 



Notwithstanding these difliculties, numerous objects have been inserted on 

 Areas IV A«, IV A.^, IV A ^, and IV An from the photograph of February 22, 

 1858, and other sources, and a drawing made of the area IVxi^. This has 

 been enlarged to a scale of 400 inches = moon's diameter*. Each object, as 

 it is inserted in the map and drawing, is entered in Form No. 3. (See Eeport, 

 1865, p. 296.) Appendix III. contains a catalogue of these objects. The num- 

 bers in each of the above-named areas are as follows: — IV A", 88, IV A/^, 21, 

 IVA^, IMjIVAi, 25. 



There are a few points of interest which attach to the features thus in- 

 serted. It is well known that Tycho is the centre of the most magnificent 

 system of rays or lucid bands on the moon's surface, and that this system is 

 seen to the greatest advantage at the time of full moon ; accordingly, the 

 photograph of October 4, 1865, furnishes the best means for depicting under 

 that aspect, the rays emanating from Tycho. Three of these rays cross the 

 areas above-mentioned; the two eastern rays cross the areas IV A«, IV A?, 

 and the west ray crosses the areas IV A/^, IV A''. These rays Avill be referred 

 to in Appendix III., which contains an abbreviated catalogue of the objects 

 alread}- mapped and inserted in the drawings ; nevertheless it may be proper 

 to mention here that all three are coincident with ranges of high land, as seen 

 in the photograph of Febniary 22, 1858, which in some places are much 

 broken, and in others rise into rocky eminences. The middle of these rays 

 passes along the east border of Albatcgnius, and the western along the west 

 border. The west border of Ptolempeus forms part of the eastern ray. 



Another feature bearing remotely on the above-nam.ed areas is the exist- 

 ence of two " raj-- centres" in the neighbourhood of Fvirncrius. These ray- 

 centres are depicted bj^ Hevelius in his Sclenographia, figs. and P, similar 

 as Beer and Mitdler remark to two pairs of crab-claws, the rays going north- 



* It was exhibited at the Meeting. 



