made of the sun's
altitude: but those ways and methods are of less use, for it is superfluous
to try to find in winding and roundabout ways what can be more readily and
as accurately found in a shorter one. For the whole art is in the proper
use of the instruments by which the sun's place is expeditiously and
quickly taken (since it does not remain stationary, but moves on): for
either the hand trembles or the sight is dim, or the instrument makes an
error. Besides, to observe the altitude on both sides of the meridian is
just as expeditious as to observe on one side only and at the same time to
find the elevation of the pole. And he who can take one altitude by the
instrument can also take another; but if the one altitude be uncertain,
then all the labour with the globe, numbers, sines and triangles is lost;
nevertheless those exercises of ingenious mathematicians are to be
commended. It is easy for anyone, if he stand on land, to learn the
variation by accurate observations and suitable instruments, especially in
a nearly upright sphere; but on the sea, on account of the motion and the
restlessness of the waters, exact experiments in degrees and minutes cannot
be made: and with the usual instruments scarcely within the third or even
the halt of a rumbe, especially in a higher latitude; hence so many false
and bad records of the observations of navigators. We have, however, taken
care for the finding of the deviation by a sufficiently convenient and
ready instrument, by means of the rising of certain stars, by the rising or
setting of the sun, and in northern regions by the Pole Star: for the
variation is learned with greater certainty even by the skilful with an
instrument which is at once simple and less sensitive to the waves of the
sea. Its construction is as follows.

[228]Let an instrument be made of the form of a true and meridional
mariners' compass of at least one foot in diameter (with a versorium which
is either nude or provided with a cardboard circle): let the limb be
divided into four quadrants, and each quadrant into 90 degrees. The movable
compass-box (as is usual in the nautical instrument) is to be balanced
below by a heavy weight of sixteen pounds. On the margin of the suspended
compass-box, where opposite quadrants begin, let a half-ring rising in an
angular frame in the middle be raised (with the feet of the half-ring fixed
on either side in holes in the margin) so that the top of the frame may be