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. VII. How to make a most Ʋseful Protractor.

THis Instrument is always to be made in Brass or Copper, but best in Brass. On the Center C draw the Semicircle BO, and divide it into two 90, or 180 Degrees, as you may see the Figure; and let the sixteen Points of the Compass PP be set in the inward Circle, with the Quarter-Points. And let the

[illustration] diagram of a protractor

[illustration]

[illustration] diagram of a compass

A: Nocturnal:

Index AE be two Diameters and ½ long, and so fitted as the Semidiameter of the Cir∣cle may be the distance from the Center, for the ready setting to any number of De∣grees, or Points and Quarters the Edge thereof, and divide from the Center to the end of the Index into 100 equal Parts, which are accounted sometime Leagues, and rec∣koned by the Surveyors of Land Perches, or any other Denomination of Numbers: You may call it for protraction according as you have occasion to use it.

Let the Index be fastned to the Center with a Brass Rivet, and through the midst of the Rivet there must be a Hole drill'd; you must put the Pin or Needle spoken of in the last Chapter, upon any Point assigned, in any Chart or Protraction whatsoever. You may divide the Edges into equal parts, by which you may make a Meridian-Line on the blank Charts, according to Mercator's or Wright's Projection.

And now you have a necessary Instrum••nt, which will protract any Travis or piece of Land upon Paper, with as much speed as any Instrument I ever yet knew, and readier by much, the use whereof shall be shewn in this Treatise.