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The W. side is much darker than the rest. It is, in fact, as dark, if not darker, than any part of the floor of Grimaldi. Riccioli extends 106 miles from N. to S., and is nearly as broad. It includes an area of 9000 square miles. ROCCA. An irregular formation, 60 miles in length, near the limb S.E. of Grimaldi, consisting of a depression partially enclosed by mountain arms.
The elevation of mountains, till lately, was in no way attempted to be ascertained but by the use of the quadrant, and their height was so generally exaggerated, that Riccioli, one of the most eminent astronomers of the seventeenth century, gives it as his opinion that mountains, like the Caucasus, may have a perpendicular elevation of fifty Italian miles . Later observers have undertaken to correct the inaccuracy of these results through the application of the barometer, and thus, by informing themselves of the weight of the air at a certain elevation, proceeding to infer the height of the situation.
The more striking of these systems were recognised and drawn at a very early stage of telescopic observation, as may be seen if we consult the quaint old charts of Hevel, Riccioli, Fontana, and other observers of the seventeenth century, where they are always prominently, though very inaccurately, portrayed.
The "seas," or, more correctly, plains, excited our travellers' curiosity to a very high degree, and they set themselves at once to examine their nature. The astronomer who first gave names to those "seas" in all probability was a Frenchman. Hevelius, however, respected them, even Riccioli did not disturb them, and so they have come down to us.
But how does the case really stand? Copernicus and Tycho Brahe held the distance to be twelve hundred semi-diameters; Kepler, who is received to have been perhaps the greatest astronomer that any age has produced, puts it down as three thousand five hundred semi-diameters; since his time, Riccioli as seven thousand; Hevelius as five thousand two hundred and fifty ; some later astronomers, mentioned by Halley, as fourteen thousand; and Halley himself as sixteen thousand five hundred .
Hevelius singularly reduced these figures, which Riccioli, on the contrary, doubled. All these measures were exaggerated. Herschel, with his more perfect instruments, approached nearer the hypsometric truth. But it must be finally sought in the accounts of modern observers. Messrs. Boeer and Moedler, the most perfect selenographers in the whole world, have measured 1,095 lunar mountains.
A conspicuous little crater stands at the S. end of it, and two others some distance to the W. The smaller component of Damoiseau contains a low central ridge. RICCIOLI. An immense enclosure, near the limb, N.E. of Grimaldi, bounded by a rampart which is very irregular both in form and height, though nowhere of great altitude, and much broken by narrow gaps.
Galileo explained the phenomena of the lunar light produced during certain of her phases by the existence of mountains, to which he assigned a mean altitude of 27,000 feet. After him Hevelius, an astronomer of Dantzic, reduced the highest elevations to 15,000 feet; but the calculations of Riccioli brought them up again to 21,000 feet.
He then went on: "Galileo was wonderfully successful considering that the telescope which he employed was a poor instrument of his own construction, magnifying only thirty times. He gave the lunar mountains a height of about 26,000 feet an altitude cut down by Hevelius, but almost doubled by Riccioli.
Not but that the lunar surface had already been diligently studied, chiefly by Hevelius, Cassini, Riccioli, and Tobias Mayer; the idea, however, of investigating the moon's physical condition, and detecting symptoms of the activity there of natural forces through minute topographical inquiry, first obtained effect at Lilienthal.
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