ON MAPPING THE SURFACE OF THE MOON. 247 



taiii of Albategnius is the only ijoint of the first order south of the equator 

 between 0° and 15° of south latitude and 0° and 10° west longitude, all posi- 

 tions in this and the neighbouring areas are measured from it either directly 

 or indirectly on the photograph (glass) of 1858, February 22 (De La Rue), this 

 giving the features near the middle of the disk more distinctly than that of the 

 full moon. It is to be regretted that points of the first order are so few ; the 

 method of determining points of the second order involving a certain amount 

 of inaccuracy, very few are found to coincide with the positions as given by 

 measurements on the photographs. B. & M.'s points of the second order 

 are indicated thus, x 7, &c. The crater IV A* * and the mountain-peak 

 IV A^ 2^ on the east of Halley, may form suitable points of the first order for 



the areas IV A" and IV A^. There should at least be one point of the first 

 order in each area. 



In the greater portion of this area three distinct levels may be recognized 

 under suitable illumination ; that on the south-west consists of the northern 

 part of Hipparchus. The central level is that surrounding the crater IV A* ^j 

 it is lower than that of Hipparchus ; the surface of Eeaumur constitutes the 

 third level ; it is on the east of the area. 



Two great lines of disturbance from Tycho can be traced on this area, of 

 which the western is the most prominent, and appears as a lucid ray in full 

 moon. It presents the character of a gently sweUing raised land across the 

 smoother fioor of Hipparchus in the southern part of the area, and rises into 

 somewhat lofty cliff's on the western borders of IV A" "^, and Rhaeticus in the 

 northern. The eastern line of disturbance can be well traced to Tycho 

 along the western borders of Ptolemaeus and Alphonsus through Arzachel, 

 having in its interior produced a ridge on which a crater is opened, it is 

 thence continued along the west border of Thebit and east border of Purbach, 

 and crosses IV A" ^ and IV A* ^. In areas IV A", IV A^ a portion only of 

 this line of disturbance apjiears lucid at the time of full moon. 



Points of the First Order. None. 



Points of the Second Order indicated thus x . 



X Y S. lat. W.long. 



IV A* * -04013 -05665 2 18 3 15 



IVA«5.. -04769 -02440 2 44 124 



IVA«s -07672 -00145 4 24 5 



IVA«7 -07527 -04726 4 19 2 43 



**1. EniETicus. — Lohrmann's map, 234, Sect. I., K. The part south of 

 equator. 



Ehseticus is a well-marked walled depression, apparently of a nearly tri- 

 angular form, situated on the equator. It is one of the few formations 

 that can have both the sun and the earth in its zenith. Its walls are per- 

 fect with the exception of two passes, one at its south angle, the other at its 

 north-west. The west wall, which is the highest, is nearly straight, and 

 forms part of the high land which appears in the full moon as the " Ray 

 from Tycho" passing through the east border of Albategnius. This wall is 

 dislocated by the fault IV A"? l^, IV A^ 20^ ly A« 72, ^nd it is near the point 

 of dislocation that the north-west pass is found. 



The north and east walls form a somewhat semicircular mountain-range, 

 but the north being the flattest, tends to impart to the formation a triangular 



