tions that the black drop remained very conspicuous for fully a
minute. The transit was also observed in the United States, the reports
from which speak of the black drop as very "troublesome."

Before leaving the subject of transits it should be mentioned that it
was in the capacity of commander of an expedition to Otaheite, in the
Pacific, to observe the transit of Venus of June 3, 1769, that Captain
Cook embarked upon the first of his celebrated voyages.

In studying the surfaces of Venus and Mercury with the telescope,
observers are, needless to say, very much hindered by the proximity of
the sun. Venus, when at the greatest elongations, certainly draws some
distance out of the glare; but her surface is, even then, so dazzlingly
bright, that the markings upon it are difficult to see. Mercury, on the
other hand, is much duller in contrast, but the disc it shows in the
telescope is exceedingly small; and, in addition, when that planet is
left above the horizon for a short time after sunset, as necessarily
happens after certain intervals, the mists near the earth's surface
render observation of it very difficult.

Until about twenty-five years ago, it was generally believed that both
these planets rotated on their axes in about twenty-four hours, a
notion, no doubt, originally founded upon an unconscious desire to bring
them into some conformity with our earth. But Schiaparelli, observing in
Italy, and Percival Lowell, in the clear skies of Arizona and Mexico,
have lately come to the conclusion that both planets rotate upon their
axes in the same time as they revolve in their orbits,[12] the result
being that they turn one face ever towards the sun in the same manner
that the moon turns one face ever towards the earth--a curious state of
things, which will be dealt with more fully when we come to treat of our
satellite.

The marked difference in the brightness between the two planets has
already been alluded to. The surface of Venus is, indeed, about five
times as bright as that of Mercury. The actual brightness of Mercury is
about equivalent to that of our moon, and astronomers are, therefore,
inclined to think that it may resemble her in having a very rugged
surface and practically no atmosphere. This probable lack of atmosphere
is further corroborated by two circumstances. One of these is that when
Mercury is just about to transit the face of the sun, no ring of
diffused light is seen to encircle its disc as woul