than starting the alphabet over again after every score.

[Sidenote: Eros.]

Fortunately, however, on running through it for the fifth time, an object
of particular interest was discovered. Most of these bodies revolve at a
distance from the sun intermediate between that of Mars and that of
Jupiter, but the little planet which took the symbol DQ, and afterwards
the name of Eros, was found to have a mean distance actually less than
that of Mars, and this gave it an extraordinary importance with respect to
the great problem of determining the sun's distance. To explain this
importance we must make a small digression.

[Sidenote: Transit of Venus.]

About the middle of the last century our knowledge of the sun's distance
was very rough, as may be seen from the table on p. 32; but there were in
prospect two transits of Venus, in 1874 and 1882, and it was hoped that
these would give opportunities of a special kind for the measurement of
this important quantity, which lies at the root of all our knowledge of
the exact masses and dimensions of not only the sun, but of the planets as
well.

[Illustration: FIG. 1.]

[Sidenote: The "Black Drop."]

The method may be briefly summarised thus: An observer in one part of the
earth would see Venus cross the disc of the sun along a different path
from that seen by another observer, as will be clear from the diagram. If
the size of the earth, the distance of the sun, and the _relative_
distance of Venus be known, it can be calculated what this difference in
path will be. Now the relative distance of Venus _is_ known with great
accuracy, from observing the time of her revolution round the sun; the
size of the earth we can measure by a survey; there remains, therefore,
only one unknown quantity, the sun's distance. And since from a knowledge
of this we could calculate the difference in path, it is easy to invert
the problem, and calculate the sun's distance from the knowledge of the
observed difference in path. Accordingly, observers were to be scattered,
not merely to two, but to many stations over the face of the earth, to
observe the exact path taken by Venus in transit over the sun's disc as
seen from their station; and especially to observe the exact times of
beginning and ending of the transit; and, by comparison of their results,
it was hoped to determine this very important quantity, the sun's
distance. It was known from previous experience that there were certain
difficult