hanges dependent upon phase vary with the nature of the
reflecting surface, following a totally different law on a smooth
homogeneous globe and on a rugged and mountainous one. Now the phases of
Mercury--so far as could be determined from only two sets of
observations--correspond with the latter kind of structure. Strictly
analogous to those of the moon, they seem to indicate an analogous mode
of surface-formation. This conclusion was fully borne out by Mueller's
more extended observations at Potsdam during the years 1885-1893.[811]
Practical assurance was gained from them that the innermost planet has a
rough rind of dusky rock, absorbing all but 17 per cent. of the light
poured upon it by the fierce adjacent sun. Its "albedo," in other words,
is 0.17,[812] which is precisely that ascribed to the moon. The absence
of any appreciable Mercurian atmosphere followed almost necessarily from
these results.

On March 26, 1800, Schroeter, observing with his 13-foot reflector in a
peculiarly clear sky, perceived the southern horn of Mercury's crescent
to be quite distinctly blunted.[813] Interception of sunlight by a
Mercurian mountain rather more than eleven English miles high explained
the effect to his satisfaction. By carefully timing its recurrence, he
concluded rotation on an axis in a period of 24 hours 4 minutes. The
first determination of the kind rewarded twenty years of unceasing
vigilance. It received ostensible confirmation from the successive
appearances of a dusky streak and blotch in May and June, 1801.[814]
These, however, were inferred to be no permanent markings on the body of
the planet, but atmospheric formations, the streak at times drifting
forwards (it was thought) under the fluctuating influence of Mercurian
breezes. From a rediscussion of these somewhat doubtful observations
Bessel inferred that Mercury rotates on an axis inclined 70 deg. to the
plane of its orbit in 24 hours 53 seconds.

The rounded appearance of the southern horn seen by Schroeter was more or
less doubtfully caught by Noble (1864), Burton, and Franks (1877);[815]
but was obvious to Mr. W. F. Denning at Bristol on the morning of
November 5, 1882.[816] That the southern polar regions are usually less
bright than the northern is well ascertained; but the cause of the
deficiency remains dubious. If inequalities of surface are in question,
they must be on a considerable scale; and a similar explanation might be
given of the deformations