r to these dogmatic
assertions, it may be said that, notwithstanding the multiplication of
monographs and photographs, the knowledge we possess, even of the larger
and more prominent objects, is far too slight to justify us in
maintaining that changes, which on earth we should use a strong adjective
to describe, have not taken place in connection with some of them in
recent years. Would the most assiduous observer assert that his knowledge
of any one of the great formations, in the south-west quadrant, for
example, is so complete that, if a chasm as big as the Val del Bove was
blown out from its flanks, or formed by a landslip, he would detect the
change in the appearance of an area (some three miles by four) thus
brought about, unless he had previously made a very prolonged and
exhaustive study of the object? Or, again, among formations of a
different class, the craters and crater-cones; might not objects as large
as Monte Nuovo or Jorullo come into existence in many regions without any
one being the wiser? It would certainly have needed a persistent lunar
astronomer, and one furnished with a very perfect telescope, to have
noted the changes that have occurred within the old crater-ring of Somma
or among the Santorin group during the past thirty years, or even to have
detected the effects resulting from the great catastrophe in A.D. 79, at
Vesuvius; yet these objects are no larger than many which, if they were
situated on our satellite, would be termed comparatively small, if not
insignificant.

One of the principal aims of lunar research is to learn as much as
possible as to the present condition of the surface. Every one qualified
to give an opinion will admit that this cannot be accomplished by roaming
at large over the whole visible superficies, but only by confining
attention to selected areas of limited extent, and recording and
describing every object visible thereon, under various conditions of
illumination, with the greatest accuracy attainable. This plan was
suggested and inaugurated nearly thirty years ago by Mr. Birt, under the
patronage of the British Association; but as he proposed to deal with the
entire disc in this way, the magnitude and ambitious character of the
scheme soon damped the ardour of those who at first supported it, and it
was ultimately abandoned. It was, however, based on the only feasible
principle which, as it seems to the writer, will not result in doubt and
confusion. Now that photog