476 Royal Society : — 



Atrio del Cavallo, Copernicus, seated in the midst of broad land, on 

 a base of 1 20 geographical miles, rises in many broken stages, brist- 

 ling with a thousand silver-bright crests, — a perfect network of rough 

 and complicated ground, crossed by lights and shades, which have a 

 history of their own, — and toward the inside falls off by many irre- 

 gular terraces, down to an interior plain, as if the whole area had 

 yielded, and the surface had been formed by enormous land-slips. 

 Four sharp notches are traced across the narrow ridge of Gassendi, 

 cutting it deeply, like the hollows left by decomposing lava dykes 

 500 feet broad ; one deeper and broader opening unites the inner 

 plain with the outer Mare Humorum, and one far wider opening 

 leads to an accessory crater, over whose awful depth the cliffs, 

 10,000 to 12,000 ft. high, spread black shadows round some central 

 rocks. In these particulars Copernicus offers a very different aspect. 

 Its high crest, of 10,000 feet, is only cut through by one straight 

 narrow meridional groove, though broken by numerous fissures in 

 other parts, and is in all parts so irregular, partially undulated, and 

 varied with small crateriform points, and enclosed areas, resembling 

 craters, as to offer little analogy to any truncated cone of eruption. 

 The highest summit, on the left-hand (west) side — a huge rock — ^is 

 conspicuous by its broad, deep and extended shade. What suggests 

 a vast lava current, is equally remarkable on the northern slope. 

 Regarding now the central plains of these mountains, we remark in 

 each several low ridges of rather sinuous forms, and several small 

 mounds (half a mile or more across), of which three central digi- 

 tated masses, not 'pierced by craters, are the most elevated, and 

 catch the earliest lights of morning which glance over the rocky 

 borders of the basin. Had the drawings been executed at the 

 instant of sunrise on the central meridian line of the basin, these 

 points would have stood up on 'the soft edge of the light and shade, 

 as bright as the Swiss mountains at sunrise or sunset, but not like 

 them reddened by the optical property of the atmosphere. Gassendi 

 has at least two (I have somewhere a memorandum of more) small 

 craters within the central plain. None such appear in this drawing of 

 Copernicus. In many other lunar mountains the centre is occupied 

 by a crater-formed hill, as Vesuvius stands within Somma ; in 

 others the hill remains a smooth rounded mass, but its crater is 

 lost ; and a further stage of decay seems to be seen in Gassendi and 

 Copernicus, where the central mass is broken into fragments and 

 scidptured by ramified hollows. May we ascribe these effects to 

 the former action of a lunar atmosphere, now absorbed in the oxi- 

 dated crust of the moon ? If so, the lunar mountains have a history 

 of water, as well as records of fire, and we must look on the sinuous 

 ridges of the Mare Humorum with eyes accustomed to the gravel 

 mounds of Norway and Ireland ; study the degraded craters after 

 the models of the Eifel ; and map the *rillen*' with reference to 

 Talleys of erosion as well as of eruption. 



In questions of this kind we shall find such drawings as this of 

 the Roman astronomer of priceless value. Studied, scrutinized, 

 ♦ I have some curious results regarding these beautiful objects. 



