' '--I 



ASTRONOMY. 



[st'UFACB OF TUK 



placa with the full moon at the time of the vernal equi- 

 nox, and the greatest possible difference in tho latitude 

 Ixuvlon U lii. 17m. 



PHYSICAL CONSTITUTION AND TKLBSCOPIC APPEARANCE 



or TUB MOON. No celestial object U butter known to 

 m, by means of the telescope, than the moon ; and in- 

 strument* hare been constructed which shorten iU dis- 

 tance by a thousand times, and thus brings it optically 

 within fen than two hundred and fifty miles of the earth, 

 wing it, the observer is immediately struck with 

 the roughness of its surface, and the numerous circular 

 formations, which appear as if surrounded by a steep and 

 high wall : whilst those are by far tho most numerous, 

 there are also caves and hilly prominences. There exist 

 also immense mountain chains, which for oiteut and 

 height appear to surpass even those of the earth. Im- 

 mense ridges, rising up from the open plains, are re- 

 markable for their height and brightness, as well as for 

 the numerous peaks and eminences into which the sur- 

 face is separated. One of those is very visible in the 

 interior of the crescent of the moon, when about half- 

 full, and can even be seen as a notch in that portion, 

 with the naked eye, from the shadow which it casts upon 

 the open plain. This part is seen in the preceding 

 representation of the moon when half-full (Fig. 49), and 

 the mountain chain to which it belongs is a part of the 

 Lunar Apennines, some of the peaks of which are calcu- 

 lated to be upwards of five thousand feet in height. 

 Although, however, the general surface of the moon is 

 rough and uneven, and the large circular mountains are 

 extraordinarily plentiful, especially in the southern he- 

 misphere, yet some portions are nearly level and of great 

 extent. When the moon is full, these portions appear as 

 dark grayish spots, and give it that appearance winch has 

 been likened to the human face ; and as the moon cannot 

 cast any shadows at this time, it follows that this must 

 be their natural colour. These are known by the name 

 of teat, and still retain that appellation ; although when 

 examined by means of the telescope, they have not any 

 appearance of being sheets of water, but seem to be large, 

 level, alluvial tracts of land, on which craters, peaks, 

 cavities, and long banks are plentifully scattered. These 

 seas take up a large portion of the lunar surface, although 

 it is only the most extensive which are seen with the 

 naked eye. The smaller ones are known by the names of 

 bays, bikes, and marshes. They vary considerably in 

 intensity of colour, and some of them have been even 

 noticed of a grayish tinge. The greater number of 

 these seas were named by Riccioli, who, according to 

 the spirit of the age, called them, in the astrological 

 manner. Mare luibrium, llumorum, Serembutes, 

 &c. If we view the other portion of the moon 

 when at the full, they might be mistaken for a 

 crude, chaotic mass of crystallised or frozen frag- 

 ments, without sliape or termination. It is only 

 when examined in its various crescent aspects, with 

 good telescopes, that the various formations are 

 teen, as it were, raised above its surface, which to all 

 appearance they are when viewed in this bold relief. 



The most remarkable of tho lunar mountains are those 

 of the circular shape, which are not, however, equally 

 regularly detiued all over its surface, some of them Doing 

 quite perfect, and others misshapen and broken. The 

 larger of tho annular mountains, taken in general, do 

 not possess tho same regularity of structure as the 

 smaller ones. For the greater part they have a level 

 interior, which, in many instances, U very irregular ; and 

 the wall U but seldom equally high or broad at all parts, 

 which will not, however, be perceived at first ; and it is 

 only when the sun has just commenced to illuminate the 

 tops, that the shadows are noticed to be so rough and 

 irregular. The exterior is frequently surrounded by 

 high rocks, which are often more elevated than tho wall. 

 In some few instances, tho interior surface is perfectly 

 level, as in Plato, Archimedes, and the eastern portion of 

 Stoflus ; but, for tho most part, they are covered with 

 mountains, mountain chains, and craters of various 

 depths and sizes. In several instances, the chain of 

 mountains in the interior is quite straight and regular, 



and divides this surface into two parts inoro or less 

 equal. These mountains are not in general so elevated 

 as those which surround them. It is but i.uvly tint 

 those which are level in the interior, and muy bo 1 

 initiated the walled plains, are quite circular in form. 

 The wall which surrounds them is generally uneven, and 

 in many parts is completely broken away: sometimes these 

 appear as gaps, and in other cases the broken portion is 

 completely levelled with the soil In some cases the wall 

 is wanting for a sixth part of the circumference ; in 

 others for one-fourth, or even one-third ; bu in these in- 

 stances there is mostly some trace of a continuation of 

 the real wall by a differonco of brightness, or a succession 

 of small hills. The walled plains may thus be considered 

 to bear some affinity to the bays and some neighbour- 

 ing portions of tho great seas, as in the splendid 

 Sinvi Iridum, which, for a considerable portion of its 

 boundary, bears a resemblance to some of the larger 

 walled plains. 



By far the greater number of the walled plains are 

 seen in the southern hemisphere, whore they are inter, 

 linked among each other in great plenty, and where it is 

 a difficult matter to distinguish them properly, or to map 

 them with the requisite distinctness. In the northern 

 hemisphere they are less common, but appear to greater 

 advantage from being more isolated ; and it is here we 

 see the more regular specimens of this class, as Plato, 

 and Archimedes. It has been remarked that, in numerous 

 instances, the walled plains appear ranged in a row in a 

 north and south direction ; and if the southern part be 

 viewed at the first and last quarter, in that portion which 

 separates the Mare Xubiitm from the hilly parts of the 

 south-west quadrant, a row of these formations becomes 

 plainly visible, some of which are of considerable magni- 

 tude and curiously entwined with one another. Similar 

 to the circular-walled plains in shape, but mostly of 

 smaller dimensions, are the circular-walled concavities, 

 whose diameter varies considerably ; and which, like tho 

 latter, are best seen in the northern part of tho moon, 

 though they are plentifully distributed in other parts, in 

 the mountain chains, in the level seas, and even in the 

 interior of the walled plains. Tho interior is mostly re- 

 gularly concave ; and in general they are furnished with 

 a central peak, which occurs likewise, though not so fre- 

 quently, in several of the walled plains, as may be seen 



Tig. so. 



in tho accompanying representation (Fig. 60). Tho 

 interior portion of the wall is much steeper and consider- 

 ably smoother than the outer. The central peak has no 

 connection with the surrounding wall, and never attains 

 to the same height, and frequently not even the same 

 elevation, as the surrounding country ; while in many 

 oases they are so small as to be seen with tho utmost 

 difficulty. The smaller walled concavities are almost 

 always without them. The central peak is in general 

 very steep ; but it sometimes happens that it takes tho 

 form of a mountain of gradual ascent, and, in some 

 instances, a mass of mountains, as in Gassendi. In the 

 great annular mountain Tycho, which presents such a 

 fine appearance at the time of full moon, the central 

 peak is very steep and high. In some of tho atiniil.tr 

 mountains, instead of tho central part being occupied by 

 a peak, it is frequently the locality of a deep crater, 

 although this is not so exactly situated in the centre, as 

 the peak generally is. It has been remarked that tho 

 very dark walled plains and concavities are mostly with- 

 out any central mountain or peak, as ia the case in 



