ronomical skill; and had he lived longer would, no doubt, have become
very famous. Unfortunately he died about two years after his celebrated
transit, in his _twenty-second_ year only, according to the accounts.
His friend Crabtree, who was then also a young man, is said to have been
killed at the battle of Naseby in 1645.

There is an interesting phenomenon in connection with transits which is
known as the "Black Drop." When an inferior planet has just made its way
on to the face of the sun, it is usually seen to remain for a short time
as if attached to the sun's edge by what looks like a dark ligament (see
Fig. 12, p. 153). This gives to the planet for the time being an
elongated appearance, something like that of a pear; but when the
ligament, which all the while keeps getting thinner and thinner, has at
last broken, the black body of the planet is seen to stand out round
against the solar disc.

[Illustration: FIG. 12.--The "Black Drop."]

This appearance may be roughly compared to the manner in which a drop of
liquid (or, preferably, of some glutinous substance) tends for a while
to adhere to an object from which it is falling.

When the planet is in turn making its way off the face of the sun, the
ligament is again seen to form and to attach it to the sun's edge before
its due time.

The phenomenon of the black drop, or ligament, is entirely an illusion,
and, broadly speaking, of an optical origin. Something very similar will
be noticed if one brings one's thumb and forefinger _slowly_ together
against a very bright background.

This peculiar phenomenon has proved one of the greatest drawbacks to the
proper observation of transits, for it is quite impossible to note the
exact instant of the planet's entrance upon and departure from the solar
disc in conditions such as these.

The black drop seems to bear a family resemblance, so to speak, to the
phenomenon of Baily's beads. In the latter instance the lunar peaks, as
they approach the sun's edge, appear to lengthen out in a similar manner
and bridge the intervening space before their time, thus giving
prominence to an effect which otherwise should scarcely be noticeable.

The last transit of Mercury, which, as has been already stated, took
place on November 14, 1907, was not successfully observed by astronomers
in England, on account of the cloudiness of the weather. In France,
however, Professor Moye, of Montpellier, saw it under good conditions,
and men