On the Use of this Book.

as D Z, that is the distance between the Sun and the zenith or head-point. If one then adds to that the northerly declination D E 10 degrees, that is together Z E 30 degrees, the distance between the Equinoctial and the head-point, and equal to P A the elevation of the Pole.

Likewise, if the Sun has southerly declination, and is elevated from the horizon 50 degrees as in S, one shall not find 50 degrees on the staff for B S, but 40 degrees for S Z, the distance between the Sun and the head-point. If one then subtracts S E, the southerly declination, from S Z 40, there remains E Z 30 degrees for the distance between the Equinoctial and the head-point, equal to P A, the altitude of the Pole.

That one slider near the sight is movable, that happens for two reasons: the first, so that one could slide it inward or outward, according to what fits the use and sliding of the large cross. The second, so that one may set it inward a lot or a little (as the situation may require) in high water or low water, to be able to measure better with it. For at low water, very long instruments are not as well used as those that are somewhat shorter.

This aforementioned quadrant is extremely suitable for measuring the altitude very surely on solid land, without using the horizon or sea-rim. To do that, one lays the staff on a table or some other flat place perfectly level, with the eye-end straight toward the south, and with the other end (where the slanted slider stands) toward the north, raised on two even wooden blocks, so much that the long cross may slide back and forth without hindrance. When the Sun then comes exactly in the south (that is, when the staff is illuminated equally on both sides), one then slides the long cross back and forth until the shadow of the ear on the small cross falls exactly between the two parallel lines on the slanted cross. That having been done, one finds then next to the long cross on the staff the certain altitude of the Sun. Having that, one obtains from it, according to the foregoing instruction, the altitude of the Pole or the latitude of the place where the measurement was taken.

What more one could say about the use of this instrument can be sufficiently understood from what has been said.