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.

XV. A Diameter of a Circle is a Right Line drawn by the Center thereof, and ending at the Circumference, on either side dividing the Circle into two equal Parts.

So the Line ABC in the former Figure, is the Diameter thereof, because it passeth from the Point A on the one side, and passeth also by the Point B, which is the Center of the Circle; and moreover, it divideth the Circle into two equal parts, namely, AEC being on one side of the Diameter, equal to AFC on the other. And this Observation was first made by Thales Milesius; For, saith he, if a Line drawn by the Center of any Circle do not divide it equally, all the Lines drawn from the Center of that Circle, from the Circumference, cannot be equal.