380 The Lunar Apennines. 



its evident height and precipitousness, and Galileo and Hevel 

 naturally thought it the loftiest of the lunar mountains. Its 

 shadow occasionally, about the time of the First Quarter, cuts 

 out a broad and deep notch of the Mare Imbrium reaching to 

 the terminator, and extends for a length of about 83 miles ; 

 or, which comes to the same thing, a spectator at that dis- 

 tance would see across the wide-extended plain its crest rising 

 high enough into the sky to cover the disc of the rising sun ; 

 as on the other hand, at sunset its summit withdraws into 

 darkness when equally removed from the terminator. About 

 the First Quarter, the broad illuminated line of the main ridge 

 runs out so far into the lunar night, that it may even be dis- 

 tinguished by a sharp eye without the telescope ; and though a 

 similar irregularity is obvious when the terminator passes near 

 Theojphilus (85), yet, according to B. and M., it was the projec- 

 tion of the Apennines which led Plutarch, and others of the 

 ancients to infer the rugged character of the lunar globe. 

 Hevel first measured, of course with a very rude kind of micro- 

 meter, the distance of the extreme illuminated point from the 

 terminator at the lunar sunset, and found it T V of the moon's 

 radius,* giving a very fair approximation of 16,800 ft. From 

 its steepness, it is scarcely free from shadow through four days, 

 and the great crest bears traces of it even as late as 24h. 

 before full. During the lunar afternoon and evening, the 

 shadow on the abrupt N.E. descent is replaced by light, and 

 a little before Last Quarter I have seen it, though falling 

 very obliquely to the axis of the chain, reflected from the pre- 

 cipices in a very beautiful and striking manner. The whole 

 mass is comparatively light in colour, unbroken by darkness, 

 excepting where penetrated by deep valleys on the S. ; and there 

 being no luminous streaks here, and the M. Imbrium being of 

 a deep grey, the Apennines can be readily distinguished under 

 the highest illumination. A narrow strip along its loftiest 

 edge reaches 7° — 8° of brightness ; a suggestive fact, in con- 

 junction with many other instances of the like nature, especially 

 in the rings of craters, and one which would come into con- 

 sideration in selenological inquiries ; though its uncertainty 

 and irregularity occasion difficulty in its interpretation. It 

 would seem as though there must be a difference somewhere, 

 either of material or of structure, in the original formation ; or 

 of internal arrangement or reflective power, superinduced at 

 a later period in the process of cooling or consolidation, or 

 from the presence, or relative absence, of some external in- 

 fluence. Among these, singly ,or in combination, would lie 



* Galileo had previously estimated tlie projection of some elevations in various 

 phases at more than so of the diameter (\o of radius). Too large, but not amiss 

 lor him. 



