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° 43' 18" N., and the sun's, which is 22° 59' 10" N. The
difference, 1° 44' 8" N. gives us Mercury's place, which it appears is
rather less than 1-3/4 degree north of the sun. Thus, about 1h. 42m.
after the sun has passed the cross-rod, Mercury will pass it between the
first and second divisions above the point of fastening. The sun will
have set about an hour, and Mercury will be easily found when the
telescope is directed towards the place indicated.

It will be noticed that this method does not require the time to be
exactly known. All we have to do is to note the moment at which the sun
passes the point of fastening of the two rods, and to take our 1h. 42m.
from that moment.

This method, it may be noticed in passing, may be applied to give
naked-eye observations of Mercury at proper seasons (given in the
almanac). By a little ingenuity it may be applied as well to morning as
to evening observations, the sun's passage of the cross-rod being taken
on one morning and Mercury's on the next, so many minutes _before_ the
hour of the first observation. In this way several views of Mercury may
be obtained during the year.

Such methods may appear very insignificant to the systematic observer
with the equatorial, but that they are effective I can assert from my
own experience. Similar methods may be applied to determine from the
position of a known object, that of any neighbouring unknown object even
at night. The cross-rod must be shifted (or else two cross-rods used)
when the unknown _precedes_ the known object. If two cross-rods are