HISTORY OF PHYSICAL ASTRONOMY.

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moon’s mass. Mr. Airy, by a discussion of the totality of the Greenwichobservations, instituted subsequently to their recent reduction upon a uni-form plan, has determined the constant part of the equatorial parallax tobe 57' 4"-94. This value, considering the broad basis upon which it rests,is more entitled to confidence than any other which astronomers havehitherto arrived at.

In consequence of her proximity to the earth, the physical consti-tution of the moon offers to astronomers a more favourable subject of re-search than that of any of the other celestial bodies. Galileo ’s discovery,that her surface is diversified with mountains and valleys, like the sur-face of the earth, was one of the many signal triumphs which thatillustrious philosopher achieved in favour of the Copernican system ofthe world. He concluded, from his observations, that the mountains ofthe moon are much higher than those of the earth, when the relativemagnitudes of the two bodies are taken into account. Many of them,according to his estimation, attain an elevation of four or five milesabove the surrounding plains. Hevelius , Riccioli, and more recentlySchroeter, were conducted by their respective observations to similar con-clusions. Sir William Herschel did not discover any grounds for ascrib-ing such an enormous height to the lunar mountains. He inferred, fromhis observations, that with a few exceptions they did not exceed half amile in height. This conclusion is undoubtedly more consistent withanalogy than that which assigns a higher elevation. It must be admitted,however, that the recent researches of MM. Beer and Miidler, on thissubject, go to support the remarkable results of Schroeter and the moreancient observers. These astronomers have determined the heights of1093 lunar mountains, principally by means of micrometrical measures ofthe lengths of their shadows, and have found several of them to attain anelevation of four miles above the neighbouring plains *.

The numerous spots which appear on the surface of the moon whenviewed with the telescope, besides being calculated to throw light on thephysical constitution of that body, are of some utility in observations ofeclipses, and on this account several astronomers have devoted conside-rable attention to a careful delineation of their relative positions. Chartsof the telescopic appearance of the moon were first executed by Scheiner,but Langrenus, Cosmographer of the King of Spain , was the first personwhose labours on this subject deserve notice. He constructed, and evenactually engraved, thirty maps of different portions of the lunar surface,founded on observations made by himself with a large telescope, at Madridand Brussels . It was he who introduced the practice of naming the dif-ferent spots after mathematicians, philosophers, and other eminent men.Hevelius about the same time devoted four years to assiduous observa-tions of the lunar spots and delineations of their various configurations.He rejected the nomenclature of Langrenus, and named the spots aftercontinents, seas, islands, and other objects on the earth’s surface to whichhe conceived they bore a resemblance. Riccioli was the next astronomerwho constructed charts of the lunar surface. He restored the principle ofnomenclature adopted by Langrenus, but, instead of distinguishing the

* The following are the elevations of a few of the principal lunar mountains, as deter-mined by MM. Beer and Madler:—Dorfel, 23,174 feet; Newton, 22,141 feet; Cas-satus, 21102 feet; Curtius, 20,632 feet; Callippus , 18,946 feet; Tycho, 18,748 feet.A mile contains 5280 feet. The four highest of these mountains, therefore, attain seve-rally an elevation of about four miles. The other two are about half a mile lower.