ON MAPPING THE SURFACE OF THE MOON. 27 
8.W. edge is nearly but not quite straight. It is slightly curved, with the 
convexity towards the centre of the crater. This edge measures 5-78. The 
curvatu~e of the remainder of the rim is much greater, and the convexity 
outwards. 
20(x). Part of the “ fault” from IV A’? to Rheeticus. This fault is fully 
described under IV A*”? in letterpress to areas [VY A*, IV AS, pp. 19, 20, and 
Report Brit. Assoc. 1866, pp. 255, 256. 
21(x). A craterlet opened on the S. slope of the mountain IV A** and 
adjoining IV Af 19 onthe N.E. It is shown by B. & M. but not by Lohrmann, 
and is the eighth in order upon IV A*. Longest diameter (which aligns with 
N.W. angle of IV Af 19, and §. angle of [TV Af 16)—5’"-31, Diameter at aright 
angle to the normal line, viz. a line joining N. edge of Diony.ius and the 8. edge 
of Agrippa, =3'"-54, mag. 0-26. With a morning terminator advanced beyond 
Copernicus, the interior shadow of the W. rim is crescentiform, the chord 
being at aright angle to the direction of the longest diameter ; breadth of 
shadow 2”-70, of illuminated interior 3-17. Proportion of shadow to illu- 
minated interior as 1:1:174. It is deeper than the somewhat similarly 
situated crater IV A® 15, 
This craterlet is well situated for observing the effect of libration on the 
precipitous W.N.W. wall of Hipparchus. This wall, which has a direction 
8.8.W.-N.N.E., is at times brought by libration into such a position that 
the eye looks along its E.S.E. slope; but when the roon is in the opposite part 
of her orbit, it is so situated with regard to the eye that the slope is more 
readily seen, and probably a portion of it lower than the craterlet may be 
detected. In the latter case it would rea!ly be further from the eye; but the 
slope would be seen under a larger angle, while the craterlet would by fore- 
shortening be seen under a smaller angle ; the difference, however, would be 
but slight, and scarcely appreciable by the eye. 
22(x). A small eminence nearly central in IV A#16(Horroa), diameter 3-17, 
23. A small hillock in IV Af 16 N.N.E. of IV Af 22, 
24, A small hillock in IV Af 16 §.8.E. of IV A %, 
25(x). The lower part of the E. interior slope of TV A*16, The upper 
part is IV A* 5, Bibi 
26. A mountain N. of Horrow;. length of crest 4-94, breadth of base 35. 
This mounta‘n is elevated on the N. slope of IV # 16, 
27. Amountain W.N.W. of IV A826; length of crest 4-01, breadth of base 
4"-01. The crest of this mountain is a continuation of that of TV Af 7, witha 
slight break between them. The W. end is separated from the E. exterior 
slope of IV A®1*by a narrow valley which lies in the line of fault IV A* ”%, 
and opens into the broader valley IV A* #1! (see TV Af #1). 
28. A crateriform depression on the N.W. slope of Horrow ; diameter 5'"13, 
mag. 0°30. 
_ **29(x). A mountain W. of Rheticus. Well shown by Lohrmann, 8. of 
his 62, Sec. I., but indifferently by B. & M. Length of crest 6°71, breadth 
of base 8"-02. This mountain, with IV A*® (Lohrmann’s 62, Sec. I.), forms 
the W. part of the secondary rampart around Rheticus (see letterpress to 
areas IV A*, IV AS, p. 12, and Report Brit. Assoc. 1866, p. 248). 
30(x). A depression N. of IV Af “5; length E.—W. 10:25, breadth N.-S. 
6-25. This depression, which is very shallow, lies in the valley IV A* 31, 
and also in the line of cliffs IV A* W.N.W.-ES.E., No. 1 (see letterpress, 
areas IV A*, IV AS, p. 34, and Report Brit. Assoc. 1866, p. 270). The 
fault LV A”! TY Af *° TY A*” appears to have dislocated this depression, 
as there is the appearance of the W. part on the higher level west of 
D2 
