PLATE XVIII. 



MARE SERENITATIS. PHOTOGRAPHED BY RITCHEY. SCALE, THREE-FOURTHS METER 



TO MOON'S DIAMETER. 



This plate shows the whole of the Mare Serenitatis ; on the upper right-hand corner a part 

 of the M. Vaporum ; on the lower corner of the same side portions of the Mare (or oceanus) Im- 

 brium, known as the Palus Nebularis. The largest vulcanoid near the dark margin is Posidonius. 

 The bright patch showing no distinct structure, which lies on the parallel of Posidonius, about two- 

 thirds across the field, is the problematical Linne 1 . The partly illuminated portion of the mare 

 below Posidonius is the Lacus Somniorum. 



The most noteworthy structures exhibited in this plate are as follows : The great mountainous 

 ridge which traverses the mare in a general north and south direction (this structure more dis- 

 tinctly resembles a terrestrial mountain-chain than any other elevation on the moon); the field 

 abounding in conical elevations in the lower part of the plate ; the crater of Le Monnier just above 

 Posidonius, which has a part of its wall apparently broken down by the mare, and the crater 

 valleys near the upper right-hand corner of the plate. There are a number of clefts, commonly 

 known as rills, which are fairly well shown. A group of these lies just below Plinius, the large 

 crater with a bright central cone emerging from the shadow of the crater wall, situated near the 

 upper margin of the plate. Another notable group is found in the left-hand lower section of the 

 plate. Faint traces of craters may be seen in these clefts. 



It may be noted that a number of the larger vulcanoids here depicted exhibit that tendency 

 to a development in a meridional direction which has been termed in this text " spooning." In 

 Posidonius and the smaller vulcanoid, Jansen,'on the margin above it, the southern (upper) walls 

 are thus indented. 



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