ON MAPPING THE StTHFACE OF THE MOON. 241 



localities of IV A« and IV A? may be easily ascertained by reference to exist- 

 ing maps, the large formations, Hipparchus, Albategnius, and Ptolemsus, 

 being sufficiently indicated. 



The plate contains a portion from Beer and Madler's map corresponding to 

 areas IV A« and IV A^. The smaU circle above on the same scale represents 

 one degree at the centre of the disk in mean libration. The large circle be- 

 low represents a similar area on the scale of 200 inches to the diameter, that 

 of the British Association Outline Map. It contains 279-27 square miles 

 (English), and is seen under an angle of 16"-277 + . 



It jf ojily at the centre of the disk that one degree is seen under an ano-lc 

 of 16"-277; in other parts of the disk the reduction is in the proportion°of 

 the angle subtended at the centre x cosine of the angular distance from the 

 centre ; thus at a distance of 12° from the centre, 16"-277 x cos 12°=15"-922 

 and at a distance of 60^, 16"-277 X cos 60°=S"-139. " ' 



The arrangement of subzones to each observer in zone II. area IV A* is as 

 follows :—J}-o. 1, S. lat. 0° to 1°; No. 2, 0° to 2°; No. 3, 1° to 3°; No. 4, 

 2="^ to 4°; No. 5, 3° to 5°; and No. 6, 4° to 5° S. lat. : No. 6 of area IV A» 

 will be allotted to No. 1 of zone IV area IV A?, the numbers of which are 

 as foUows :— No. 1, S. lat. 5° to 6°; No. 2, 5° to 7°; No. 3, 6° to 8° ; No- 4 

 7° to 9° ; No. 5, 8° to 10° ; and No. 6, 9° to 10° S. lat. 



The very high probability, if not certainty, that the crater " Linne " has 

 undergone a physical change since it was first figured by Riccioli in 1653, 

 induces the belief that if lunar objects were observed upon a regular system 

 from time to time, other instances of {)iferre<l physical change may be detected, 

 especially among the smaller features. With this view, the objects in each 

 area of two subzones, arranged in the order of their visibility, as far as ascer- 

 tained, are specified, that observers may have as little trouble as possible in 

 selecting objects for observation. 



The folloflning numbers refer to the accompanying catalogue. Those 

 marked vrith two asterisks (**) are conspicuous, those with one (*) are easy, 

 those with a dagger are difficult (f), and those with a double dagger (J) are 

 very difficult, and but rarely seen. Nearly the whole of the objects recorded 

 in area IV A^ have been observed with an aperture of 4} inches, object- 

 glass by Cooke, power 230. The exceptions are mostly noticed. It is re- 

 commended that conspicuous objects should be examined first. 



Area IV A*. 

 ^ No. 1. Lat. 0° to 1° S.— 1**, 43, 46, 47, 49, 58, 59, 60, 65, 66, 68, 70, 

 /2, /4, 85, 87. 



No. 2. Lat. 0° to 2° S.— 1**, 11, 15, 19, 38, 40, 43, 44, 45, 46, 47, 48, 

 49, 50, 58, 59, 60, 61, 62, 63, 64, 65, 66, 68, 69, 70, 72, 74, 77, 78, 84, 85, 

 87. 



No. 3. Lat. 1° to 3= S.— 2**, 10**, 4*, 5, 11, 15, 16, 19, 20, 31, 36, 37, 

 38, 39, 40, 41, 42, 43, 44, 45, 48, 49, 50, 58, 61, 62, 63, 64, 65, 66, 69, 73, 

 77,78,83,84,86. 



No. 4. Lat. 2° to 4° S.— 2**, 10**, 4*, 5, 8, 11, 12, 13, 14, 10, 20, 21, 

 24, 25, 26, 27, 28, 29, 30, 31, 32, 34, 35, 36, 37, 38, 39, 41, 42, 49, 54, 56, 

 57,67,73,77,81,82,83,86. >>>,,>,, 



No. 5. Lat. 3° to 5° S.— 2**, 10**, 51**, 6*, 7*, 9*, 18*, 3, 5, 8, 12, 13, 

 14, 17, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 42, 49, 

 52, 53, 54, 55, 56, 57, 67, 71, 73, 79, 80, 81, 82, 88, 75t, 76J. 



No. 6. Lat. 4° to 5° S.— 51**, 6*. 7*, 9*, 18*, 3, 17, 22, 23, 24, 28, 33, 

 34, 35, 49, 52, 53, 54, 55, 71, 79, 80, 81, 88, 75$, 76+. 



1866. g 



