after two years, by
a still more weighty contribution to lunar science in Mr. Neison's
well-known book, accompanied by a map, based on the survey of Beer and
Maedler, but adding some 500 measures of positions, besides the
representation of several thousand new objects. With Schmidt's _Charte
der Gebirge der Mondes_, Germany once more took the lead. This splendid
delineation, built upon Lohrmann's foundation, embraced the detail
contained in upwards of 3,000 original drawings, representing the labour
of thirty-four years. No less than 32,856 craters are represented in it,
on a scale of seventy-five inches to a diameter. An additional help to
lunar inquiries was provided at the same time in this country by the
establishment, through the initiative of the late Mr. W. R. Birt, of the
Selenographical Society.

But the strongest incentive to diligence in studying the rugged features
of our celestial helpmate has been the idea of probable or actual
variation in them. A change always seems to the inquisitive intellect of
man like a breach in the defences of Nature's secrets, through which it
may hope to make its way to the citadel. What is desirable easily
becomes credible; and thus statements and rumours of lunar convulsions
have successively, during the last hundred years, obtained credence, and
successively, on closer investigation, been rejected. The subject is one
as to which illusion is peculiarly easy. Our view of the moon's surface
is a bird's-eye view. Its conformation reveals itself indirectly through
irregularities in the distribution of light and darkness. The forms of
its elevations and depressions can be inferred only from the shapes of
the black, unmitigated shadows cast by them. But these shapes are in a
state of perpetual and bewildering fluctuation, partly through changes
in the angle of illumination, partly through changes in our point of
view, caused by what are called the moon's "librations."[928] The result
is, that no single observation can be _exactly_ repeated by the same
observer, since identical conditions recur only after the lapse of a
great number of years.

Local peculiarities of surface, besides, are liable to produce
perplexing effects. The reflection of earth-light at a particular angle
from certain bright summits completely, though temporarily, deceived
Herschel into the belief that he had witnessed, in 1783 and 1787,
volcanic outbursts on the dark side of the moon. The persistent
recurrenc