330 ANNUAL OF SCIENTIFIC DISCOVERT. 



tains ; from -which fact, and from the great distances they extend, it 

 would seem that they are not such lava-streams as have flowed, for ex- 

 ample, from Etna. It has been supposed that a force acting, as it were, 

 centrifugally or explosively, and therefore differently from the force to 

 which we attribute the upheaval of mountain-chains upon the earth, 

 has formed the lunar craters, and overspread the adjacent surface with 

 the ridges or rays in question. 



In Professor Phillips' recent contributions to a Report of the Physical 

 Aspect of the Moon, he notices another class of phenomena, certain 

 remarkable rills in the mountains mapped as Aristarchus, Archimedes, 

 and Plato. The last exhibits a large crater ; and a bold rock which 

 juts into the interior has been seen during the morning illumination 

 to glow in the sunshine like molten silver, casting a well-defined shadow 

 eastward. The object known as the Stag's-hom Rill, east of the 

 mountain Thebit, appears to be what geologists call a fault or dyke, 

 one side being elevated above the other. Professor Phillips mentions 

 a group of parallel rills about Campanus and Hippalus, and he traces 

 a rill across and through the old crater of the latter mountain. All 

 the rills appear to be rifts or deep fissures resembling crevasses of a 

 glacier ; they cast strong shadows from oblique light, and even acquire 

 brightness on one edge of ibhe cavity. Their breadth appears to be 

 only a few hundred feet or yards. The mountain Gassendi is remark- 

 able for rough terraces and ridges within the rings which form the 

 crater. In the interior area there are central elevations of rocky char- 

 acter, which are brought into view by the gradual change in the di- 

 rection of the incident solar rays as the lunar day advances. In Lord 

 Rosse's magnificent reflecting telescope, the flat bottom of the crater, 

 called Albategnius, is seen to be strewed with blocks not visible in in- 

 ferior telescopes ; while the exterior of another volcanic mountain 

 (Aristillus^) is scored all over with deep gullies radiating towards its 

 centre. 



The reader need not be reminded that our knowledge is limited to 

 one hemisphere or face of the moon, in consequence of the period of 

 its rotation upon its axis corresponding with the period of its revolu- 

 tion round the earth. 



Depressions in the Moon's Disk. These have long been recognized. 

 Mr. Key, in a paper recently presented to the Royal Astronomical 

 Society (G. B.), records his observations of certain large depressions 

 in the western limit of our satellite, not of comparatively small gullies 

 lyinnr between elevated ranges, but of vast tracts, the general level of 

 which lies very considerably beneath the mean level of the moon's sur- 

 face. On the 20th of September last, while observing with a twelve- 

 inch glass speculum the moon, then a few hours past her first quarter, 

 he was astounded at observing that the limb of the moon was entirely 

 out of shape ; that it was in fact irregularly polygonal, as if several 

 large segments had been cut off the spherical limb. Since then other 

 astronomers had verified this observation. Rev. Mr. Webb, of the 

 Hardwick Observatory, had also measured these depressions with a 

 micrometer, and proved that they were actually flat. 



Lunar Mountains. At the last meeting of the British Association, 

 Professor Phillips gave a detailed account of his examination of the 

 moon's surface through an equatorial with an object-glass of six inches. 



