Tlie Moon. 199 



paratively small a globe, and the sharpness of outline and 

 detail consequent upon the absence of a vaporous atmosphere, 

 must give them an effect which would seem very astonishing 

 to a terrestrial spectator. Further to the S. this E. coast 

 declines to a kind of pass, not well shown by B. and M., who 

 admit that this part of their map is deficient in boldness, but 

 much better drawn by Schroter : on the other side of this in- 

 terruption the mountains rise again, and on the S. the great 

 summits Picarcl a and /3 spring up to 14,200 and 15,600 feet, 

 rivalling our loftiest Alpine peaks ; beyond these the great 

 headland, named Promontorium Agarum by Hevel, runs out into 

 the plain with a rounded summit nearly 11,000 feet in height, 

 supported by cliffs 8° bright a few days after the full, when they 

 are most directly enlightened. Birt has detected a crater upon 

 it. In this direction broad bays and c ' fiords " penetrate the 

 mountain border towards the S., and in the young crescent are 

 filled with shade. The W. edge of the " Mare " is less boldly 

 defined between the craters Gondorcct and Eimmart, and is made 

 up of hillocks and ridges, intermixed with isolated mountains, 

 like lofty islands in the sea. 



lb was among these that Schroter found a distinct and always 

 recognizable crater, abouttwenty-threemiles in diameter, remark- 

 able for its dark grey colour under every angle of illumination, 

 to which he gave the name Alhazen, and which, from its proxi- 

 mity to the W. limb, he continually used for the purpose of 

 measuring the libration. Having thus had it constantly before 

 his eyes, he was the more surprised at its variable aspect : at 

 first it was a depressed grey surface within a ring; then 

 frequently, and even in the 27-foot reflector, like a longish flat 

 ridge, and these appearances were interchangeable ; sometimes, 

 too, while the neighbouring objects were as sharp as usual, it 

 would be so indistinct that " he did not know what to make of 

 it:" and on one occasion (1797, March 1), after ten years of 

 observation, when libration was most unfavourable, as having 

 carried its W. edge to within 28" of the limb, and the ter- 

 minator had crossed over to the other side of the " Mare," 

 and it consequently ought to have been very ill seen, he found 

 it extraordinarily distinct : its form, however, was one pre- 

 viously entirely unknown, that of a very deep bright irregular 

 crater, whose ring was barely complete towards the S., and 

 open, with a prolongation of its E. edge, at the opposite end. 

 The cause of this difference, he thought, must lie in some modi- 

 fication of the lunar atmosphere, such as he believed that he 

 could trace in many instances, which would be capable of 

 masking the depths of a crater, and giving it a grey and flat 

 aspect, or changing altogether its appearance. In Bode's 

 Jahrbuch for 1825, Kunowsky, an accurate observer, asserted 



