The Moon. 195 



letter subjoined to the name of the principal spot ; capital 

 letters are employed for points whose position has been deter- 

 mined by measurement, smaller ones for such as are filled in by 

 the eye ; these letters standing, especially when so required, 

 on the side of the object next to the spot whence it is named ; 

 and the alphabetical succession being determined by the rela- 

 tive conspicuousness of the features when best seen. The 

 system is an ingenious one, but not in all cases easy, or clear, 

 in its application. A more comprehensive and universalry 

 available mode of designating every spot worthy of notice on 

 the lunar disc, is understood to be in the course of preparation 

 by Mr. Birt, and will be a great acquisition to astronomers. 



In order to be able to give some verbal description of the 

 features of the moon, as well as to assist the investigation of 

 supposed changes, it is material to employ a scale of bright- 

 ness, in which the different degrees, though depending of 

 necessity upon mere estimation, are expressed by numbers. 

 Schroter and Lohrmann employed a scale of ten degrees for this 

 purpose, and have been followed by B. and M., who, making 

 the absolute shadows = 0, assign 1° to 3° to the dark grey 

 districts, 4° and 5° to the lighter grey, 6 J and 7° to the white 

 regions, and 8° to 10° to the glittering spots. 1°, 9', and 10° 

 are of infrequent occurrence. 2° and 3° denote the common 

 tone of the " maria/' 4° to 6° that of the brighter landscapes, 

 4° to 7° the rings of most, and the interiors of many craters, 

 6 , 7°, and 8° express the brightness of many peaks and ridges ; 

 but it is remarked by B. and M. that these are never, generally 

 speaking, the most elevated points in the district. 



It is not very probable that many of our readers would wish 

 to undertake the measurement of the heights and depths of the 

 surface. The task would not only be a somewhat troublesome, 

 but to a considerable extent a superfluous one, as it has already 

 been performed with much accuracy by Beer and Mildler, in no 

 less than 1095 cases, comprising most of the principal and con- 

 veniently accessible features of the moon. Many of their 

 results will be found in the following papers ; but they will be 

 given in round numbers, as the extreme preciseness, extending 

 even to single toUes, with which they are specified by those 

 authorities, has of course no other value than that of showing 

 the carefulness of the observation. As all these measures are 

 determined by the lengths of shadows, a trifling difference, as 

 to which we have very little means of judging, in the level of 

 the ground where the shadow terminates, will have so great an 

 influence on the final result, that no such exceeding accuracy is 

 possible. No doubt, much might yet be done, if it were 

 thought desirable to obtain more perfect correctness, by taking 

 the average of many measures, obtained at different times from 



vol. v. — NO. III. p 



