ON MAPPING THE SURFACE OF THE MOON. 249 



while on the N.N.E. it is depressed. The north boundary consists of cliffs 

 which do not appear to rise much, if at all, above the surface of Hipparchus, 



HipiDarclius. 



N.N.E. 



their faces looldug towards the depressed land IV A* " . It is this feature 

 which occasions the evanescent character of Hipparchus, so that in about 

 forty-eight hours after the terminator has passed it, this boundary is no 



longer discernible. It is in the neighbourhood of IV A" ' that the cliffs be- 

 gin to rise to any extent above the surface of Hijiparchus, and it is here that 

 the east boundary commences. This boundaiy passes through IVA*'^, 

 IVA^30^ the mountains IVA^37 and IV A^"', the west slope of IV A^^'^ to 

 IV A^^"', where the S.W. boundary commences. 



The south angle of Hipparchus is filled with the formation IV A^ -^, which 

 is shghtly depressed below its level, the land filling the angle between Alba- 

 tegnius and Ptolemtseus being higher. 



*4. A crater just S.W. of a line joining the south point of Rhaeticus 



with the S.W. point of Eeaumur. H. of B. & M. 6"-18, mag. 0-37. 



Shown by Lohrmann. 



This crater, although neither large nor bright, is yet sufficiently conspicuous 



to form a point of the " First Order." Its position, second order, x 4, as given 



by B. & ]yi., is •0145 west of its position as measured on the photographs. 



This crater is very interestingly situated on the depressed land IV A* " , 

 extending between Ilhaticus, Horrox, and Ecaumur, and is surroimded by 

 mountains and cliffs disposed in a nearly circular form at an average dis- 

 tance of 24"-73. These mountains, however, must not be regarded as 

 b)' any means intimately connected with the crater ; for the ranges of 

 which they form parts have over a very extensive area a general direction 

 S.S.W.-Jf.jSr.E. ; but the interesting feature is, that with the exception 

 of those to the N.E., the summits of the mountains in the neighbour- 

 hood of IV A* ^ attain their greatest altitudes at or about the distance 

 above named. We are not without numerous instances of craters being 

 accompanied by strong evidences of their ha\'ing been centres of considerable 

 disturbance ; and although but few radiating marks are foimd characterizing 

 IV A" *, yet they are not entirely wanting, as doubtless large apertures wiU 

 show ; so that it is not unlikelj' that while the general mountain formations 

 originated by the operation of a force of a very extensive character, the out- 

 burst which produced IV A* '^ might within its range of action have modified 

 the surrounding surface. It is noteworthy that IV A* * and the group IV A* "^ 

 are opened upon a " Eay from Tycho," which is coincident with the high 

 ranges forming the west borders of Ptolcmocus and Alphousus, and also with 

 the crater and ridge on Arzachel. It is probable some of the highest peaks'in 

 the central portion of the southern hemisphere may be found on this ray. Its 

 direction is S.S.E.-N'.N.W., and, as mentioned under IV A ^, is to be referred 

 to a more recent convulsion than that which produced the lines of disturbance 

 having a general direction S.S.W.-N.N.E. 



5. A mountain-peak on the S.W. border of Reaumur, A of B. & M. 



