rotation--that is to say, once every twenty-four hours. Schiaparelli's
discovery, if it were received as correct, would put Mercury, as a
planet, in a class by itself, and would distinguish it by a peculiarity
which had always been recognized as a special feature of the moon, viz.,
that of rotating on its axis in the same period of time required to
perform a revolution in its orbit, and, while this seemed natural enough
for a satellite, almost nobody was prepared for the ascription of such
eccentric conduct to a planet.

The Italian astronomer based his discovery upon the observation that
certain markings visible on the disk of Mercury remained in such a
position with reference to the direction of the sun as to prove that the
planet's rotation was extremely slow, and he finally satisfied himself
that there was but one rotation in the course of a revolution about the
sun. That, of course, means that one side of Mercury always faces toward
the sun while the opposite side always faces away from it, and neither
side experiences the alternation of day and night, one having perpetual
day and the other perpetual night. The older observations, from which
had been deduced the long accepted opinion that Mercury rotated, like
the earth, once in about twenty-four hours, had also been made upon the
markings on the planet's disk, but these are not easily seen, and their
appearances had evidently been misinterpreted.

The very fact of the difficulty of seeing any details on Mercury tended
to prevent or delay corroboration of Schiaparelli's discovery. But there
were two circumstances that contributed to the final acceptance of his
results. One of these was his well-known experience as an observer and
the high reputation that he enjoyed among astronomers, and the other was
the development by Prof. George Darwin of the theory of tidal friction
in its application to planetary evolution, for this furnished a