perpendicular to the plane of the compass; on its top let a rule sixteen
digits in length be fastened at its middle on a joint like a balance beam,
so that it may move, as it were, about a central axis. At the ends of the
rule there are small plates with holes, {173} [Illustration] {174} through
which we can observe the sun or stars. The variation is best observed and
expeditiously by this instrument at the equinoxes by the rising or setting
sun. But even when the sun is in other parts of the zodiack, the deviation
becomes known when we have the altitude of the pole: that being known, one
can learn the amplitude on the Horizon and the distance from the true east
both of the sun and of the following fixed stars by means of a globe, or
tables, or an instrument. Then the variation readily becomes known by
counting from the true east the degrees and minutes of the amplitude at
rising. Observe the preceding star of the three in the Belt of Orion as
soon as it appears on the horizon; direct the instrument toward it and
observe the versorium, for since the star has its rising in the true east
about one degree toward the south, it can be seen how much the versorium is
distant from the meridian, account being taken of that one degree. You will
also be able to observe the arctick pole star when it is on the meridian,
or at its greatest distance from the meridian of about three degrees (the
pole star is distant 2 deg. 55 min. from the pole, according to the
observations of Tycho Brahe), and by the instrument you will learn the
variation (if the star be not on the meridian) by adding or subtracting,
_secundum artem_, the proper reduction [_prostaphaeresis_][229] of the
star's distance from the meridian. You will find when the pole star is on
the meridian by knowing the sun's place and the hour of the night: for this
a practised observer will easily perceive without great error by the
visible inclination of the constellation: for we do not take notice of a
few minutes, as do some who, when they toil to track the minutes of degrees
at sea, are in error by a nearly whole rumbe. A practised observer will, in
the rising of sun or stars, allow something for refraction, so that he may
be able to use a more exact calculation.

Bright and conspicuous stars[230] which are
_not far distant from the equator which it
will be useful to observe at their rising and
setting: the amplitude at the Horizon on
rising being known from the altitude of the