M. Blundevile his exercises containing sixe treatises, the titles wherof are set down in the next printed page: which treatises are verie necessarie to be read and learned of all yoong gentlemen that haue not bene exercised in such disciplines, and yet are desirous to haue knowledge as well in cosmographie, astronomie, and geographie, as also in the arte of navigation ... To the furtherance of which arte of navigation, the said M. Blundevile speciallie wrote the said treatises and of meere good will doth dedicate the same to all the young gentlemen of this realme.

A briefe description of the Mariners Astrolabe, and the vses thereof. Chap. 20. (Book 20)

THe limbe of the Mariners Astrolabe is traced as you sée with thrée Circles, making two spaces to containe therein the degrées and numbers of al∣titude: and these Circles are deuided by two crosse lines called Diameters, and cutting one another in the very Centre of the Astrolabe into 4. equall parts, of which quarters the vppermost on the left hand towards the Ringle is only marked with degrées and numbers, as you may sée in the figure: In which figure the perpendicu∣lar Diameter signifieth the Zenith, or the line of South and North, at the vpper end whereof is fastened the Ringle or handle. And the other ouerthwart Diameter signifieth the Horizon the one end whereof on the left hande signifieth the East point, and the other end on the right hande the West point. And to this A∣strolabe (as to all other) doth belong a ruler or Diopter, which as you sée hath at ech end a square tablet pearced with two holes, the one greater and the other lesser, the greater to looke through with your eye, to take the altitude of any starre or of the sunne being so darkned by some cloud as though it casteth no shadowe, yet it may be séene with the eye: And the lesser hole is for the sunnes beame to passe thorough when he shineth cleare, and some Astrolabes are deuided in like maner of the Zenith on both sides, and haue two Diopters, whereof the one is pearced with a great hole, and the other with a smaller hole to serue to such purposes as is aboue saide.

The vse of this Astrolabe is onely to take the altitude of the sunne at any time of the day, or of any fixed starre or Planet in the night, in such sort as is before taught in my description of Maister Blagraue his Astrolabe, in the third and fourth Propo∣sition thereof.