The mariners magazine, or, Sturmy's mathematical and practical arts containing the description and use of the scale of scales, it being a mathematical ruler, that resolves most mathematical conclusions, and likewise the making and use of the crostaff, quadrant, and the quadrat, nocturnals, and other most useful instruments for all artists and navigators : the art of navigation, resolved geometrically, instrumentally, and by calculation, and by that late excellent invention of logarithms, in the three principal kinds of sailing : with new tables of the longitude and latitude of the most eminent places ... : together with a discourse of the practick part of navigation ..., a new way of surveying land ..., the art of gauging all sorts of vessels ..., the art of dialling by a gnomical scale ... : whereunto is annexed, an abridgment of the penalties and forfeitures, by acts of parliaments appointed, relating to the customs and navigation : also a compendium of fortification, both geometrically and instrumentally / by Capt. Samuel Sturmy.

CHAP. XXVI. How to describe on any Dial the proper Azimuths and Almicantars of the Plane.

FRom any Point of the Gnomon taken at pleasure let fall a Perpendicular upon the Substile; that Perpendicular shall be part of the Axis of the Plane, and shall be reputed Radius to the Horizon of your Plane. The top of this Radius in the Gnomon is called Nodus, because there you must set a Knot, Bead, or Button, or else cut there a Notch in the Gnomon, to give shade; or cut off the Gno∣mon in the place of the Nodus, that the end may give the shadow for those Linea∣ments. Let not your Nodus stand too high above the Plane, for too great a part of the Planes day; nor let it stand too low, for then the Lineaments will run too close together: a mean must be chosen.

At the foot of this Radius take your Center and describe a Circle of the Plane,* 1.1 and divide it into equal Degrees, and from the Center draw Lines through those Degrees infinitely, that is, so far as your Dial Plane will bear; these Lines shall be the Azimuths of the Horizon of the Plane, and shall be numbred from his Meridian or Substile.

Divide any of these Azimuth Lines into Degrees, by Tangents agreeable to the said Radius; and having made a prick at every Degree, through every of these pricks you shall draw parallel Circles, which shall be Almicantars or Parallels of Altitude, to be numbred inwards; so that at the Center be 90 for the Zenith, and from the Center outwards you shall number 80, 70, 60, until you come within 10 or 5 deg. of the Horizon; for the Plane is too narrow to receive its own Horizon, or the Pa∣rallel neer, if the Nodus have any competent Altitude.

Notes

• * 1.1

  Tropicks and other Circles of Declination is shewed in Chap. 3. in the Aequi∣noctial Dial, where they be calculated and drawn by help of a Scale of Equal Parts.