e. The sudden stoppage of the line jerks the peg, before
referred to, out of the log, thereby allowing the other two fixed cords
to drag it flat and unresisting over the surface of the sea, when the
line is reeled up and put by. The flight of another hour calls for a
repetition of the heaving of the log.

As scientific knowledge advanced, instruments of peculiar and more
complicated form were devised to enable navigators to ascertain more
correctly their position on the surface of the sea; but they did not,
and never will, supersede the method by dead-reckoning--for this reason,
that the latter can be practised at all times, while the former are
useless unless the sun, moon, or stars be visible, which in some
latitudes they are not for many days and weeks, when clouds and fogs
shroud the bright sky from view.

The _Quadrant_ is the chief of those instruments. It is represented on
next page. To give a succinct account of this would take up more space
than we can spare. It may suffice the general reader to say that by
observing the exact position of the sun at noon, or of the moon or a
star, in relation to the horizon, the precise _latitude_ of a ship--that
is, her distance north or south of the equator--is ascertained. The
method of "taking an observation" is complicated, and difficult to
explain and understand. We refer those who are curious on the point to
treatises on navigation.

_Chronometers_ are exceedingly delicate and perfect time-keepers, or
watches, which are very carefully set at the commencement of a voyage.
Thus the _time_ at the _meridian_ whence a vessel starts is kept up
during the voyage. By means of an observation of the sun with the
quadrant, or sextant (a somewhat similar instrument), the true time at
any particular point in the voyage may be ascertained. A _difference_
is found to exist between the time at the spot where the observation is
taken, and the time of the chronometer. A calculation founded on this
difference gives the ship's _longitude_--that is, her distance east or
west of the meridian that passes through Greenwich. That meridian is an
imaginary line drawn round the world longitudinally, and passing through
the north and south poles, as the equator is a line passing round it
latitudinally.

When a ship's latitude and longitude have been ascertained, and a line
drawn through the first parallel to the equator, and another line
through the second parallel to the first meridi