f the seventeenth century, the instrument used was of
this simple kind. It was generally a large quadrant, with one or two
bars moving on a hinge,--to all intents and purposes a huge pair of
compasses. The direction of the sight was fixed by the use of a slit
and a pointer, much as in the ordinary rifle. This instrument was
vastly improved by the use of a telescope, which not only allowed
fainter objects to be seen, but especially enabled the sight to be
accurately directed to the object observed.

The instruments of the pre-telescopic age reached their glory in the
hands of Tycho Brahe. He used magnificent instruments of the simple
"pair of compasses" kind--circles, quadrants, and sextants. These were
for the most part ponderous fixed instruments of little or no use for
the purposes of navigation. But Tycho Brahe's sextant proved the
forerunner of the modern instrument. The general structure is the
same; but the vast improvement of the modern sextant is due, firstly,
to the use of the reflecting mirror, and, secondly, to the use of the
telescope for accurate sighting. These improvements were due to many
scientific men--to William Gascoigne, who first used the telescope,
about 1640; to Robert Hooke, who, in 1660, proposed to apply it to the
quadrant; to Sir Isaac Newton, who designed a reflecting quadrant;[8]
and to John Hadley, who introduced it. The modern sextant is merely a
modification of Newton's or Badley's quadrant, and its present
construction seems to be perfect.

It therefore became possible accurately to determine the position of a
ship at sea as regarded its latitude. But it was quite different as
regarded the longitude that is, the distance of any place from a given
meridian, eastward or westward. In the case of longitude there is no
fixed spot to which reference can be made. The rotation of the earth
makes the existence of such a spot impossible. The question of
longitude is purely a question of TIME. The circuit of the globe, east
and west, is simply represented by twenty-four hours. Each place has
its own time. It is very easy to determine the local time at any spot
by observations made at that spot. But, as time is always changing,
the knowledge of the local time gives no idea of the actual position;
and still less of a moving object--say, of a ship at sea. But if, in
any locality, we know the local time, and also the local time of some
other locality at that moment--say, of the Observat