254 



REPORT — 1872, 



On this day (August 23) Mr. Gledhill described the bounding lines of the 

 light sector, when produced, as cutting two craters outside and above Plato, 

 and the sector itself as "faint, but luminous and well seen." Mr. Pratt's 

 record is as follows : — " The floor was seen as on the 20th inst., similar to 

 Mr. Knott's ; other markings of a more complicated character were very 



Plato, 1869, August 23.— H. Pratt. 



Plato, 1869, August 28.— H. Pratt. 



strongly suspected. The trident-shaped marking a little more slender and 

 elongated N.E. and S.W. than in my sketch of the 20th inst., probably an 

 error in drawing its first appearance. On this drawing I have the following 

 remark : — ' N.W. arm of trident separated from curved streak by a narrow 

 neck of dark surface. The stem as on the 17th.' " 



1869, August 25. — Mr. Gledhill described the floor as " not so dark as the 

 upper part of Grimaldi." 



1869, August 26. — Mr. Pratt writes : " Shading on floor visible, precisely 

 as in sketch of the 23rd of August, 1869 — viz. the long streak from the N.W. 

 round by N. and crossing S.E., with the ray I towards the middle of II E * '^, 

 and the trident- shaped marking on the S.W. part of the floor, with the 

 streak [stem of trident, cZ] extending halfway from spot No. 1 to No. 4." 



1869, August 27. — Mr. Gledhill described the light sector as a very faint 

 object. Spot No. 3 easily seen double, and the floor but little darker than 

 the Mare Imbrium. 



1869, August 28. — Mr. Pratt's record is as follows : — " The shading of 

 the floor was seen as on the 23rd of August, with the addition of the apparent 

 continuation of the streak {d) from spot No. 1 to spot No. 4, and a curved 

 streak commencing abruptly at the shadow of the rock Rupes Smythii (B. 

 & M.'s ^), not sketched, and continued towards spot No. 3, and joining the 

 streak along the north side of the floor." These features are exhibited in 

 fig. 7 ; the opening forming the stem of the trident is seen extending as far 

 as the " Sector," and a new opening, apparently a continuation of the northern 

 opening, extending north of spot No. 7 towards the east border. 



The hypothesis suggested as an explanation of the variations depicted in 

 Mr. Pratt's drawings recognizes the darker portion of the floor as possessing 

 an obscuring character, and subject to changes which do not aff'ect the lighter. 

 Although looking at this hypothesis, as set forth in the above remarks, as 

 explaining the variations observed by Mr. Pratt, yet it is difficult to divine 

 the nature of the darker portion, as it appears to absorb light rather than 

 reflect it. It is noteworthy that it is the darker portion of the floor that 



