ersection of the horizon and aequator; all
those lines aequidistant from the aequator denote the parallels of the sun
and stars; the fiducial line of the rule or alhidade represents the
horizon; and its parts signify the degrees of the horizon, beginning from
the point of setting or of rising. {176}

[Illustration]

Therefore if the fiducial line of the rule be applied to the given latitude
of the place reckoned from either end of that diameter which represents the
axis of the world; and if further the given declination of the sun or of
some star from the aequator (less than the complement of the latitude of
the place) be found on the limb of the instrument; then the intersection of
the parallel drawn from that point of the declination with the horizon, or
with the fiducial line of the rule or alhidade, will indicate for the given
latitude of the place the amplitude at rising of the given star or the sun.

* * * * *

{177} CHAP. XIII.

The observations of variation by seamen vary, for the
_most part, and are uncertain: partly from error and inexperience_,
and the imperfections of the instruments; and partly
_from the sea being seldom so calm that the
shadows or lights can remain quite
steady on the instruments_.

After the variation of the compass had first been noticed, some more
diligent navigators took pains to investigate in various ways the
difference of aspect of the mariners' compass. Yet, to the great detriment
of the nautical art, this has not been done so exactly as it ought to have
been. For either being somewhat ignorant they have not understood any
accurate method or they have used bad and absurd instruments, or else they
merely follow some conjecture arising from an ill-formed opinion as to some
prime meridian or magnetick pole; whilst others again transcribe from
others, and parade these observations as their own; and they who, very
unskilful themselves, first of all committed their observations to writing
are, as by the prerogative of time, held in esteem by others, and their
posterity does not think it safe to differ from them. Hence in long
navigations, especially to the East Indies, the records by the Portuguese
of the deviating compass are seen to be unskilful: for whoever reads their
writings will easily understand that they are in error in very many things,
and do not rightly understand the construction of the Portuguese compass
(the lily of which diverges by