346 REPORT— 1866. 



give the resiilts of the measures as they stand; especially as the object is to 

 indicate to sclenographcrs the work to be done, rather than to claim for the 

 present work anything like perfection ; stOI it is hoped that no little progress 

 is now being made in selenography. 



Wherever the word " crater " is used in the catalogue, it invariably signi- 

 fies a pit, cavity, or more or less round depression, in which at sunrise or 

 sunset a well-marked interior shadow is seen on the side next the sun, the 

 opposite side reflecting more or less strongly the sun's rays. It is also 

 retained with the same signification for all those objects designated craters 

 by previous selenographers. 



It has been suggested to employ the term " cratcrlet" for the round white 

 glistening objects which are so numerous, particularly in the mountainous 

 regions of the moon, associating with it the idea of a more recent formation 

 than the larger objects. These small objects frequently occur in rows, as 

 mentioned by Schmidt (see Appendix to last Report, 1865, j)p. 305-307), and 

 called by him crater-rills. It is in rills and crater-rills that Herr Schmidt 

 is disposed to seek for new formations (see Eeport, 1865, p. 299), and recom- 

 mends the particular study of them with this view. As it is important to 

 have some distinguishing feature between ordinary craters (including even 

 large formations) and craterlets, I would suggest for this purpose size or 

 magnitude, all pits having a smaller diameter than mag. 0-3 or 5" being re- 

 garded as " craterlets ;" bright craters exceeding 5" will form a class inter- 

 mediate between " craterlets" and " light-centres." 



In a few instances objects are mentioned as bright or lucid spots, the exact 

 nature of which it is difficult to determine. The recent obscuration or pro- 

 bable filling up of the crater " Linnc " on the Mare Serenitatis, and a lucid 

 spot having been seen in its place, suggests that such objects should be cai-e- 

 fully observed, especially when the Terminator is near them. IV A^^'', 

 ly A^^^, and IV A^ '"^ may be specified as examples. 



Aeka IV A*. 



Introduction, 



The positions in this area are more easterly than given by Beer and Miidler, 

 or by Lohrmann. They are all taken in the fu-st instance from De La Rue's 

 photograph of the 4th of October, 1865, G. M. T. 9'' 0" 4^ Libration in 

 longitude at 11*^ 40'"-6 =— 0° 39'-7, and latitude =-0° 20'-9 ; so that the 

 middle of the apparent disk was not very far distant from the point which is 

 central in the orthographic projection. The libration in longitude was, how- 

 ever, of an extent sufficient to remove the point 0° of longitude so far to the 

 west as to require a correction of -020 to all measures on the photograph be- 

 tween 0'^ and 15° of latitude. On making this correction for the ordinates Y, 

 taken from the photograph, of Messier, TheophUus, and Dolloud, the south 

 half of the central meridian exactly coincided. The central mountain of 

 Albategnius diff'ered from the three ordinates above-mentioned -OlO. There 

 does not appear to be any error in the mean of Lohrmann's seven measures, 

 or in the computation as given by B. & M. of the ordinate Y of the central 

 mountain ; still in this respect the photograph diff'ers from the computed 

 quantity. The central mountain in Thcophilus lies in nearly the same 

 parallel as that of Albategnius, the computed difi'erence of Y being '36659, 

 the measured -37400, which also gives the position of the central mountain 

 in Albategnius more castwardly than the computed. As the central moiui- 



