A REPORT—1863, 
range to a point not very far west of Plato, the 8.E. aspect of the Alps bordering 
the Mare Imbrium is precipitous—in this respect resembling most of the terrestrial 
chains of mountains bordering large oceans; and the mountains are much closer 
together and more chain-like than in the area towards the Mare Frigoris, where 
they are more or less detached the one from the other. A large extent of surface, 
presenting a great variety of character—mountainous, rugged, pierced with nume- 
rous craters, rising into an elevated crest, sometimes spreading out into considerable 
breadth, at others contracted to a narrow, neck-like kind of isthmus—extends from 
the Caucasus to the promontory La Place, which is the western jutting-point of the 
rugged and mountainous border of the Sinus Tridum. The western part of this 
rugged land is occupied by the Alps. The middle exhibits a decidedly raised 
character, in which Plato appears to have been “sunk.” This walled plain is com- 
paratively shallow. The eastern part of the tract above described is lower, and 
ierced with numerous craters, especially the portion immediately eastward of 
lato; the largest crater (very much smaller indeed than Plato) being a conspicuous 
object under the morning and evening illuminations. The smooth surface of the 
Mare Imbrium comes closely up to the tract above described; and at some little 
distance from the border several isolated mountains, soft ridges, and small craters 
are scattered here and there on the surface of the Mare, which are conspicuous 
objects under every aspect of illumination. The immediate object of this paper was 
to solicit the attention of astronomers to a continuation of the Alps on the northern 
side of the Mare Frigoris. The Alps, as laid down on our lunar maps, do not extend 
beyond the bright ground N. and N.W. of Plato. ‘The boundary common to this 
bright ground and the Mare Frigoris is exceedingly well defined: the contrast 
between the superior brilliancy of the one and the dark grey surface of the other 
is very marked. The bright ground, which is of the rugged character above 
mentioned, gradually rises from the Mare Frigoris to the summit of the ring of 
Plato; and the same is observable of the ascent on the south side from the surface 
of the Mare Imbrium, with the exception of the slight depression of the site of the 
ancient crater Newton (Schriéter). In the Monthly Notices of the Royal Astrono- 
mical Society for May 1863, I described a remarkably dark border common to the 
Mare Imbrium, the Alps, and a bright portion extending from them to the south of 
Plato. Since writing that paper I have ascertained that this dark border is irre- 
spective of any hypsometrical affections, in this respect resembling greatly the 
bright rays extending from the various ray-centres on the moon’s surface, which 
alike cross every variety of depression and elevation, Under an early illumination, a 
soft and slightly elevated ridge, casting a well-defined shadow, is seen extending from 
a crater, Piazzi Smyth (which is some distance 8. W. of Plato), to the small group of 
mountains of which \, at the foot of the Hartwell Mountains, is the principal. The 
slight elevation of this ridge above the general surface of the Mare Imbrium and its 
continuity between Piazzi Smyth and Plato are well seen under both the morning 
and evening illuminations. The visibility of this ridge is fugitive; it disappears 
entirely under even a moderately high sun, and then the dark border, which is only 
manifested under an advanced stage of illumination, crosses it uninterruptedly. 
The dark border greatly resembles, although in an opposite sense, the broad light 
mark crossing Geminus under the mid-day illumination, which is described in my 
paper presented to the British Association in 1859, I have been somewhat par- 
ticular in describing the independence of this dark border and any hypsometrical 
affections, as several patches of a similar kind are observed on the surface of the 
Mare Frigoris, especially near the centre. I have not yet detected much, if any, 
yariation of level on the surface of the Mare Frigoris north of Plato, except a fault 
of an exceedingly well-marked character, not far from the opposite or northern 
border of the Mare Frigoris. This fault clearly indicates a well-marked difference 
of leyel between the southern part of the Mare Frigoris adjoining the bright ground 
north of Plato and the northern portion. It is in this immediate neighbourhood 
that the northern boundary of the Mare Frigoris is exceedingly rugged and rocky, 
running into several promontories which extend towards Plato; and these pro- 
montories, combined with the fault above alluded to, clearly indicate a superior 
level for the land extending between Timzeus and Fontenelle north of the Mare 
Frigoris, Shortly after sunrise, and a little before sunset, this high land is seen to be 
