THE SURFACE OF THE MOON. 85 



and the systems of brilliant stripes which are connected with certain lunar 

 forms. 



" 2. The ' age of the moon,' when a drawing is made, should be stated to 

 the second decimal of the day, because a knowledge of this epoch is essential 

 to a right estimation of the angle of incident light under which the observa- 

 tions are made. Probably the observer will find it convenient to prepare 

 beforehand a table of the moon's age, corresponding to each hour of mean 

 solar time. The mean solar time of the place at the beginning and end of 

 each observation should also be stated. 



"3. Among the chief points to be attended to are — 



a. The steepness of slopes, which may perhaps be best determined by 

 noting the time at which they began or ceased to be illuminated gene- 

 rally. 



h. In ring mountains the difference of level between their exterior 

 and interior bases. 



c. The curvature of their interior, whether greater or less than that 

 of the general surface. Some of them are much raised in the centre, as 

 is evident by the shadows which these parts throw. 



d. Whether the brilliant stripes are elevated above the ground where 

 they pass, and the angle of illumination at which they disappear. 



e. Slopes, height, and breadth of the soft ridges in the Maria. 



f. External fragments round ring mountains. 



g. Relation between mass of wall and area of depression {i. e. would 

 the wall fill up the hollow). 



" 4. In delineating the appearances on the moon's surface, the Committee 

 think the observer must be encouraged to employ various methods. For a 

 general view of the proportional areas, more accurate than any sketch. Pho- 

 tography may be employed. To steady the work, and reduce it to a uniform 

 scale, rnicrometrical measures will be required. In some cases, where these 

 cannot be supplied by the observers separately, they may be obtained at one 

 of the observatories. In drawing by the eye the Camera lucida is available, 

 if the telescope has an equatorial movement by clock — a condition not only 

 desirable, but perhaps indispensable for perfect delineation. 



"5. For convenience of comparison, it appears desirable to recommend 

 that one uniform scale should be employed in the delineations. Though it 

 may seldom be practicable to employ on the moon a power of 1000, the 

 Committee recommend that the drawings should in no case be made on a 

 smaller scale. If the distance of the paper from the eye be assumed at 10 

 inches, a circular space on the moon's surface one mile across will be repre- 

 sented with only a diameter of about one-twentieth of an inch. For objects 

 which require larger representation, the ordinary scale may be doubled or 

 tripled. 



" 6. Both in drawing and describing it appears desirable to employ the 

 method of Beer and Madler, who draw the moon as she appears in the in- 

 verting telescope, but describe the relative situation of her parts by reference 

 to her poles as northern and southern, and sides as east and west, in corre- 

 spondence to the nearest cardinal points of the earth. 



" 7. It is found by trial that Sepia drawings are well-suited for representa- 

 tions of the peculiarities of the moon's surface." 



It is requested that the drawings and descriptions, which may be prepared 

 in conformity with these suggestions, may be forwarded by post to Professor 

 Phillips, Assistant-General Secretary of the British Association, St. Mary's 

 Lodge, York. 



