6 I The Surface of the Moon. 



which they overspread and frequently obliterate. Some general 

 rules may be traced, but exceptions are numerous and unac- 

 countable ; and while on the earth a similarity of structure and 

 composition in neighbouring objects would be in all probability 

 attended with similarity of colour, this is so far from being 

 universally true in the full moon, that it is no uncommon thing to 

 see one of two adjacent and resembling craters or ridges con- 

 spicuous at that time for its brightness, while the other shows 

 scarcely a trace of its existence. This interesting subject will 

 come before us again in detail ; it is only adverted to in this 

 place as occasioning some preliminary obstacles to the student 

 in regard to the identification of many of the lunar spots, which 

 from this cause becomes perplexing and uncertain, in propor- 

 tion as the relief of the surface gradually disappears with the 

 increasing angle of illumination. 



Difficulties, if such there be, arising from the supposed 

 varying conditions of the lunar atmosphere, will present them- 

 selves as matters of local inquiry. 



In addition to the statement respecting the apparent 

 dichotomy of Venus in p. 455 of the last number of the Intellec- 

 tual Observer, it may be remarked that the same phenomenon 

 of disagreement with calculation was observed by De-Vico and 

 his associates at Eome in 1839, the terminator becoming 

 sensibly straight three days after the time specified in the 

 tables. 



