he
critical moment had arrived. In spite of the long delay, which
exhausted the patience of the assistant, some valuable observations were
obtained, and thus the first passage of a planet across the sun was
observed.

The transits of Mercury are not rare phenomena (there have been thirteen
of them during the nineteenth century), and they are chiefly of
importance on account of the accuracy which their observation infuses
into our calculations of the movements of the planet. It has often been
hoped that the opportunities afforded by a transit would be available
for procuring information as to the physical character of the globe of
Mercury, but these hopes have not been realised.

Spectroscopic observations of Mercury are but scanty. They seem to
indicate that water vapour is a probable constituent in the atmosphere
of Mercury, as it is in our own.

A distinguished Italian astronomer, Professor Schiaparelli, some years
ago announced a remarkable discovery with respect to the rotation of the
planet Mercury. He found that the planet rotates on its axis in the same
period as it revolves around the sun. The practical consequence of the
identity between these two periods is that Mercury always turns the same
face to the sun. If our earth were to rotate in a similar fashion, then
the hemisphere directed to the sun would enjoy eternal day, while the
opposite hemisphere would be relegated to perpetual night. According to
this discovery, Mercury revolves around the sun in the same way as the
moon revolves around the earth. As the velocity with which Mercury
travels round the sun is very variable, owing to the highly elliptic
shape of its orbit, while the rotation about its axis is performed with
uniform speed, it follows that rather more than a hemisphere (about
five-eighths of the surface) enjoys more or less the light of the sun in
the course of a Mercurial year.

This important discovery of Schiaparelli has lately been confirmed by an
American astronomer, Mr. Lowell, of Arizona, U.S.A., who observed the
planet under very favourable conditions with a refractor of twenty-four
inches aperture. He has detected on the globe of Mercury certain narrow,
dark lines, the very slow shifting of which points to a period of
rotation about its axis exactly coincident with the period of
revolution round the sun. The same observer shows that the axis of
rotation of Mercury is perpendicular to the plane of the orbit. Mr.
Lowell has perceive