Masch 1, 1899. 



KNOWLEDGE. 



61 



great circle ; whilst the tendency of the Maria to increase 

 in size as we approach the centre seems to suggest that the 

 influence of onr earth was in some way or other the chief 

 determining cause in their formation. 



It is fully recognised now that the " Seas " are amongst 

 the most recent of lunar formations. We cannot otherwise 

 explain the traces of submerged ring-plaias to be seen here 

 and there in the Maria themselves, and the numerous 

 instances of partially demolished rings which border them 

 -the destroyed or injured rampart being always on the 

 .. ' nearer the " Sea.'' Leaving altogether on one side 

 the 4i.estion of any change in the lunar axis as one too 

 curious for our present information, this disposition of the 

 Maria seems to point us backward with no uncertain 

 finger to the time when the moon ceased to rotate as 

 viewed from the earth. Up to that time the earth had of 

 course an equal effect on every meridian ; from that time it 

 has been dominant over the centre of the apparent disc. 



Mr. N. E. Green, truest of astronomical artists, and 

 most experienced of selenographers, has suggested that it 

 was the final set of the great lava tide which the earth 

 produced on the moon which gave rise to the " Seas." 

 The suggestion can hardly be far from the truth, and we 

 are thus able to say that the time when the lunar rotation 

 came into complete accord with the month was also the 

 time when its crust was approaching solidification but had 

 not reached it. It was also the time when our own earth 

 was stUl emitting so much heat as to have a vast influence 

 upon its satellite. For there is a very marked resemblance 

 between the character of the polar regions of the moon and 

 of the limb in general — a circumstance which has long 

 been recognised, but to which M^I. Loewy and Puiseus 

 have given fresh importance by their studies of the photo- 

 graphs they have taken with the equatorial coudr of the 

 Paris Observatory. 



Whatever was the particular mode in which the final 

 formation of the " Seas" was effected, and however it may 

 differ from its greater brethren, the Mare Crisium evidently 

 took its formation about the same time, and in the same 

 way. This is indicated by the meridional ridges which 

 mark its surface. Sinset, in our photograph, has advanced 

 too far for most of these to be seen, but two can be made 

 out pretty clearly. 



Studying the Mare as it appears to-day, we find that its 

 border differs much in its character iu different parts. 

 Our photograph brings out the character of these on the 

 east and south in strong relief. On the east the cliffs are 

 bold and precipitous, broken by only one important pass, 

 which is near the centre, and is fuUy shown on the plate. 

 On the south a number of valleys run up from the Mare 

 into the Highlands, which form on the photograph a 

 particularly bold and interesting region, full of striking 

 detail. These Highlands curve round towards the north, 

 and end in Cape Agarum, a bold promontory which is just 

 receiving the last rays of the sun. Carving to meet this 

 from the north is a similar, but less striking, promontory, 

 which is all but lost in the darkness of approaching night. 

 These two opposite points mark the opening of a deep bay, 

 not seen in the photograph, as it has already passed into 

 the lunar night. But for this bay the Mare Crisium would 

 be roughly circular, its length and breadth being nearly 

 equal. More accurately, it would be liesagonal, the 

 tendency of the outlines of the great lunar formations 

 being to take a polygonal rather than a circular shape. 

 The bay, however, narrow as it looks to the eye 

 through the effect of foreshortening, brings the greatest 

 length east and west of the Mare up to three hundred and 

 fifty-five miles, whilst its length north and south is but 

 two hundred and eighty. The bay is further distinguished 



from the reat of the Mare, in that the greenish tint which 

 can be seen over the greater portion of the floor under high 

 illumination does not extend beyond Cape Agarum. 



The region embraced by the plate is one that, more than 

 most districts of the moon, supplies a considerable variety 

 of colour. As already mentioned, the dark floor of the 

 Mare has a certain greenish tinge, which is absent beyond 

 Cape Agarum, and in the great ring-plain Condorcet, which 

 lies just to the south of that promontory in the darkness. 

 East of the Mare, on the other side of the great pass 

 already referred to, lies the Palus Somnii, its further border 

 strongly marked out by contrast with the dark Mare 

 Tranquillitatis, which comes up so dark in the photograph. 

 The Palus is described as of a peculiar yellow, almost 

 amounting to a golden brown, whUst round the great 



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Sketeli Map. 



ring-plain Taruntius, which stands in the full stmshine 

 due south of the Palus, Elger notes a very decided sepia 

 colour under a low sun. Moving nearly due west from 

 Taruntius, the ring-plain Firminicius lies in the darkness, 

 beyond which is a curious mountain with triple peaks. 

 Round this, on the surrounding plain, appear broad, dark 

 giey streaks, which undergo apparent changes of a 

 periodical character, and which Madler suggested might 

 be due to vegetation. The floor of Mare Crisium, too, has 

 sometimes been noticed to be sprinkled with minute bright 

 points, which Mathieu Williams ascribed to hoar-frost. 



The region therefore is full of interest from the point of 

 view of real or apparent periodic change. It includes also 

 a pair of objects which supply us with perhaps the best 

 attested example of intrinsic change. These are the two 

 crater plains Messier and Messier A, at the extreme south 

 of the plate. It will be seen that the more westerly 

 Messier is distinctly the smaller of the two, and whilst 

 Messier A, the larger and more easterly crater, is nearly 

 circular, its companion is as distinctly elliptical and 



