THE MOON. 151 



ually become possible to construct a topographical chart of 

 the Moon, based upon actual observations ; and since, in the 

 opposition, the entire half-side of the Earth's satellite presents 

 itself at the same moment to our investigation, we know more 

 of the general and merely formal connection of the mountain 

 groups in the Moon, than of the orography of a whole terres- 

 trial hemisphere containing the interiors of Africa and Asia. 

 Generally the darker parts of the disk are the lower and more 

 level ; the brighter parts, reflecting much sunlight, are the 

 more elevated and mountainous. Kepler's old description of 

 the two as sea and land has long been given up ; and the 

 accuracy of the explanation, and the opposition, was already 

 doubted by Hevel, notwithstanding the similar nomenclature 

 introduced by him. The circumstance principally brought 

 forward as disproving the presence of surfaces of water on 

 the Moon was, that in the so-called seas of the Moon, the 

 smallest parts showed themselves, upon closer examination 

 and very different illumination, to be completely uneven, pol- 

 yhedric, and consequently giving much polarized light. Ar- 

 ago has pointed out, in opposition to the arguments which 

 have been derived from the irregularities, that some of these 

 surfaces may, notwithstanding the irregularities, be covered 

 with water, and belong to the bottoms of seas of no great 

 depth, since the uneven, craggy bottom of the ocean of our 

 planet is distinctly seen when viewed from a great height, 

 on account of the preponderance of the light issuing from be- 

 low its surface over the intensity of that which is reflected 

 from it. (Ammaire du Bureau des Longitudes for 1836, 

 p. 339-343.) In the work of my friend, which will shortly 

 appear, on astronomy and photometry, the probable absence 

 of water upon our satellite will be deduced from other optical 

 grounds, which can not be developed in this place. Among 

 the low plains, the largest surfaces are situated in the north- 

 ern and eastern parts. The indistinctly bounded Oceanus 

 Procellarum has the greatest extension of all these, being 

 360,000 geographical miles. Connected with the Mare Im- 

 brium (64,000 square miles), the Mare Nubium, and, to 

 some extent, with the Mare Humorum, and surrounding in- 

 sular mountain districts (the Riphcei, Kepler, Copernicus, 

 arid the Carpathians), this eastern part of the Moon's disk 

 presents the most decided contrast to the luminous south- 

 western district, in which mountain is crowded upon mount- 

 ain.* In the northwest region, two basins present them- 

 * Beer and Madler, p. 273. 



