PLATE XIII. 



PHOTOGRAPHED BY RITCHEY WITH 40-INCH TELESCOPE, USING YELLOW COLOR 

 SCREEN AND ISOCHROMATIC PLATE. 



This plate shows more than half of the fourth quadrant or the southwest quarter of the moon's 

 visible surface, taken at about three-fourths full. The area extends from the equator on the lunar 

 margin to about 55 south latitude, and from near the polar axis westwardly two-thirds the distance 

 to the margin of the visible field — a district rich in instructive objects. 



On the lower part of the plate is a portion of the Mare Tranquilitatis ; on the middle of the 

 left-hand side a portion of the Mare Nectaris. The observer should note the features of contact of 

 these maria with the higher ground against which they lie, especially that there are some indica- 

 tions of a gradual passage from the rough surface of the upland to the relatively smooth floors of 

 the maria, and also that several of the rings (at least five) facing the M. Tranquilitatis have the side 

 towards that area destroyed. The wrinkles on the floor of this sea are fair but not good examples 

 of the mountain-like ridges that are found on those areas. That on the margin of the M. Nectaris, 

 extending northward from a crater half in the shadow, is noteworthy. 



About a score of the vulcanoids in this field show the tendency to " spooning " or elongation 

 of the crater in a general north and south direction, in some instances rather northeast and south- 

 west. In the northeast part of the field some of them pass into crater valleys with a distinct 

 northeast and southwest axis. In a few instances the axes of these deformed craters are inclined 

 to the southeast and northwest. So that there appear to have been three different lines of strain 

 developed on this part of the lunar crust. 



The large, deep vulcanoid with the steep, ragged peaks rising from its floor, near the dark 

 margin on the left, and about one-third the distance from the bottom of the plate, is Theophilus, 

 one of the noblest structures on the moon. The width of the crater is about sixty-four miles ; the 

 greatest height from the floor to tlie crest of the wall eighteen thousand feet. The central mass, 

 composed of several sharp peaks, rises about six thousand feet above the lava plain. In the 

 center of these masses there appears to be an obscure crater about half a mile in diameter. The 

 terraces in the inner wall of the cone are indistinctly shown. 



It is to be noted that Theophilus in its development has partly invaded Cyrillus, the next 

 large vulcanoid on the southeast, and also that the older structure seems more ancient with less 

 steep slopes and exhibits a generally ruined appearance. Cyrillus is also more " spooned " or 

 drawn out in a north and west direction than Theophilus. South of Cyrillus, at a distance of half 

 its width, is Catherina. This crater is met by another of half its diameter which has developed 

 on one side of its floor. From near the southeastern margin of Catherina a beautiful row of small 

 craters extends eastwardly for a distance of over two hundred miles to the large vulcanoid Abul- 

 feda. This is perhaps the most noteworthy crater row on the moon. 



The long curved wall extending from Piccolomini, near the upper left hand corner (the large 

 crater with its floor in shadow), to the east side of Catherina is the Altai Mountains. It should be 

 noted that this step-like structure obscurely extends northwards to the M. Tranquilitatis, where it 

 forms an irregular ridge-like promontory. 



It should be observed that about a dozen of the larger vulcanoids have either a crater or a 

 cone in the central part of their flat bottoms. In some instances on the brightly illuminated parts 

 those structures exist, but are not revealed by the illumination. 



The larger details of the general surface of the moon on the area to the left of the Altai escarp- 

 ment are perhaps better shown here than in any other plate. They are rarely so well revealed in 

 even the best telescopes. In the best seeing the trained eye has a chance to observe perhaps one-half 

 more than is here shown. Note near the margin southwest of Catherina the existence of obscure 

 ancient craters, their walls broken and shoved about, as well as the mingling of small cones and 

 craters, suggesting that craters began with dome-like cones (see p. 30). 



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