iles long, distances of
greater length are calculated, until at length stretches 100 miles long,
or even more, can be accomplished. It is thus possible to find the
length of a long line running due north and south.

So far the work has been merely that of the terrestrial surveyor. The
distance thus ascertained is handed over to the astronomer to deduce
from it the dimensions of the earth. The astronomer fixes his
observatory at the northern end of the long line, and proceeds to
determine his latitude by observation. There are various ways by which
this can be accomplished. They will be found fully described in works on
practical astronomy. We shall here only indicate in a very brief manner
the principle on which such observations are to be made.

Everyone ought to be familiar with the Pole Star, which, though by no
means the most brilliant, is probably the most important star in the
whole heavens. In these latitudes we are accustomed to find the Pole
Star at a considerable elevation, and there we can invariably find it,
always in the same place in the northern sky. But suppose we start on a
voyage to the southern hemisphere: as we approach the equator we find,
night after night, the Pole Star coming closer to the horizon. At the
equator it is on the horizon; while if we cross the line, we find on
entering the southern hemisphere that this useful celestial body has
become invisible. This is in itself sufficient to show us that the earth
cannot be the flat surface that untutored experience seems to indicate.

On the other hand, a traveller leaving England for Norway observes that
the Pole Star is every night higher in the heavens than he has been
accustomed to see it. If he extend his journey farther north, the same
object will gradually rise higher and higher, until at length, when
approaching the pole of the earth, the Pole Star is high up over his
head. We are thus led to perceive that the higher our latitude, the
higher, in general, is the elevation of the Pole Star. But we cannot use
precise language until we replace the twinkling point by the pole of the
heavens itself. The pole of the heavens is near the Pole Star, which
itself revolves around the pole of the heavens, as all the other stars
do, once every day. The circle described by the Pole Star is, however,
so small that, unless we give it special attention, the motion will not
be perceived. The true pole is not a visible point, but it is capable of
being accu