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.

SECT. I. How to take the Breadth of a River.

SUppose you were to take the Breadth of a River, as I have at Crocken-Pill, which runs betwixt Glocester-shire and Somerset-shire, and found the breadth of the Water upon a Spring-Tide 40 Perch or a Furlong; you must do it thus. Being on the River∣side as in the former Figure at E, there set your Compass; Observe some mark on the other side of the Water, as at D; then set a mark at E, and go square-wise either to the right-hand, or to the left from these 2 marks, so far, until you spie the mark D on the other side the Water doth justly make an Angle of 45 degr. with the mark E; and this will be when you come to F; then measure carefully F E, the distance of the 2 Stations, and that shall be equal to the breadth of the River: so that if FE be 10:20:30:40:50: or 100 Poles, or Yards, or Feet, the breadth is the same. The like may be done by any other Angle, as if you go to G, and make an Angle of 26 degr. 30 min. in D; then is the distance GE twice the breadth; but ever if you can get an Angle of 45 degr. for that is the best and readiest Angle to find out such a distance; therefore if you can, use no other.

And the like way of Working you may do at Sea, if you gain the Sight of any Cape, Head-Land, or Island, set it by your Compass when you see it, without altering your Course, make an Angle of 45 degr. And by your Plain-Scale if you have kept a good account of your Way by the same Rules as before, you shall have the true distance of your Ship from the first Place, or Cape, or Head-Land, or Island whatsoever: Or you may get the Slope-side D F or D G if you measure it with your Compasses, and apply it to the same Scale of equal parts by which you put down the distance E F or E G. Thus you may find the distance from the Ship, to any Cape; These are made so plain by the Rules before-going, that it need no further precept.