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empire (in Europe and Asia) at 7,900,000 squaremiles, and the British dominions at 6,700,000, thesetwo empires together are almost exactly equal in areato the whole surface of the moon : the part of themoon actually visible to us (taking her librationsinto account) is somewhat more extensive than theRussian empire, while the part totally concealedfrom us is somewhat less extensive than the British empire.
It is important to notice that, under all circum-stances, whether the moon is at her mean distance,or nearer to or farther from the earth (in fact,whatever the size of her disc may be), the earth'sdisc, as supposed to be seen at the moment fromthe moon, is nearly 13| times larger. The actualproportion between the two discs is shown in fig. 7,Plate II.
But the variation of the moon's apparent size,according to her varying distance, must also becarefully taken into account. It is much greater thanis commonly supposed. The observed telescopic meandiameter of the moon is, as already stated, 31'9",while 31' 5"T is taken as the true mean diameter,—that is, the telescopic diameter reduced for the effectsof irradiation. Now, the telescopic semi-diameterwhen the moon is at her nearest to the earth,—that isto say not merely in perigee, but in perigee at a timewhen her orbit has its greatest eccentricity—is foundto be 33' 32"‘l, while, when the moon is farthest fromthe earth, the observed diameter is 29' 22 /,- 9. These