our space to enter into the circumstances which
affect the length of these intervals. The question, in fact, is not a
very simple one. All the necessary information is given in the almanac.
We merely notice that the planet is most favourably seen as an evening
star in spring, and as a morning star in autumn.[11]

The observer with an equatorial has of course no difficulty in finding
Mercury, since he can at once direct his telescope to the proper point
of the heavens. But the observer with an alt-azimuth might fail for
years together in obtaining a sight of this interesting planet, if he
trusted to unaided naked-eye observations in looking for him. Copernicus
never saw Mercury, though he often looked for him; and Mr. Hind tells me
he has seen the planet but once with the naked eye--though this perhaps
is not a very remarkable circumstance, since the systematic worker in an
observatory seldom has occasion to observe objects with the unaided eye.

By the following method the observer can easily pick up the planet.

Across two uprights (Fig. 10) nail a straight rod, so that when looked
at from some fixed point of view the rod may correspond to the sun's
path near the time of observation. The rod should be at right-angles to
the line of sight to its centre. Fasten another rod at right angles to
the first. From the point at which the rods cross measure off and mark
on both rods spaces each subtending a degree as seen from the point of
view. Thus, if the point of view is 9-1/2 feet off, these spaces must
each be 2 inches long, and they must be proportionately less or greater
as the eye is nearer or farther.

[Illustration: _Fig. 10._]

Now suppose the observer wishes to view Mercury on some day, whereon
Mercury is an evening star. Take, for instance, June 9th, 1868. We find
from 'Dietrichsen' that on this day (at noon) Mercury's R.A. is 6h. 53m.
23s.: and the sun's 5h. 11m. 31s. We need not trouble ourselves about
the odd hours after noon, and thus we have Mercury's R.A. greater than
the sun's by 1h. 41m. 52s. Now we will suppose that the observer has so
fixed his uprights and the two rods, that the sun, seen from the fixed
point of view, appears to pass the point of crossing of the rods at
half-past seven, then Mercury will pass the cross-rod at 11m. 52s. past
nine. But where? To learn this we must take out Mercury's declination,
which is 24 deg. 43' 18" N., and the sun's, which is 22 deg. 59' 10" N. The
difference, 1 d