studied, chiefly by Hevelius, Cassini,
Riccioli, and Tobias Mayer; the idea, however, of investigating the
moon's physical condition, and detecting symptoms of the activity there
of natural forces through minute topographical inquiry, first obtained
effect at Lilienthal. Schroeter's delineations, accordingly, imperfect
though they were, afforded a starting-point for a _comparative_ study of
the superficial features of our satellite.

The first of the curious objects which he named "rills" was noted by him
in 1787. Before 1801 he had found eleven; Lohrmann added 75; Maedler 55;
Schmidt published in 1866 a catalogue of 425, of which 278 had been
detected by himself;[915] and he eventually brought the number up to
nearly 1,000. They are, then, a very persistent lunar feature, though
wholly without terrestrial analogue. There is no difference of opinion
as to their nature. They are quite obviously clefts in a rocky surface,
100 to 500 yards deep, usually a couple of miles across, and pursuing
straight, curved, or branching tracks up to 150 miles in length. As
regards their origin, the most probable view is that they are fissures
produced in cooling; but Neison inclines to consider them rather as
dried watercourses.[916]

On February 24, 1792, Schroeter perceived what he took to be distinct
traces of a lunar twilight, and continued to observe them during nine
consecutive years.[917] They indicated, he thought, the presence of a
shallow atmosphere, about 29 times more tenuous than our own. Bessel, on
the other hand, considered that the only way of "saving" a lunar
atmosphere was to deny it any refractive power, the sharpness and
suddenness of star-occultations negativing the possibility of gaseous
surroundings of greater density (admitting an extreme supposition) than
1/500 that of terrestrial air.[918] Newcomb places the maximum at 1/400.
Sir John Herschel concluded "the non-existence of any atmosphere at the
moon's edge having 1/1980 part of the density of the earth's
atmosphere."[919]

This decision was fully borne out by Sir William Huggins's spectroscopic
observation of the disappearance behind the moon's limb of the small
star Eta Piscium, January 4, 1865.[920] Not the slightest sign of
selective absorption or unequal refraction was discernible. The entire
spectrum went out at once, as if a slide had suddenly dropped over it.
The spectroscope has uniformly told the same tale; for M. Thollon's
observation during the t