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. XXX. How to make an Ʋniversal Dial on a Globe; and to cover it, if it be required.

A Globe, saith Euclid, is made by the turning about of a Semicircle, keeping the Diameter fixed. This Dial, if Universal, will want the aid of a Magneti∣cal Needle to set it, and it must move on an Axis in an Horizon, as the usual Globes do; whose Aequator let be divided into 24 Hours, the proportion of the Day Natural.

You may see the Figure on the top of the Dial in the Title, but that you cannot see the two Poles, and the Semicircle, and the Horizontal Circle.

You may imagine this Globe set to the Elevation of the Pole, as that is, with two Gnomons of the length of the Suns greatest Declination, proportion∣ed to the Poles Circle, with the 24 Hours, according to the 24 Meridians, and serves for a North and South Polar Dial.

But in the Meridian let be placed the 12 a Clock Line; then turn the Semi∣circle till it cast no shadow: then doth it cross the Hours, which Hours are drawn from the Pole to each of the 24 Divisions, as before.

If you desire to cover the Globe, and make other Inventions thereon, first learn here to cover it exactly. With a Pair of Compasses bowed towards the Points (like a Pair of Calapers the Gunners use) measure the Diameter of the Globe you intend to cover; which being known, find the Circumference thus.

Multiply the Diameter by 22, and divide the Product by 7, and you have your desire.

Let the Circumference found be the Line EF, which divide into 12 Equal parts; draw the Parallel AB and CD, at the distance of three of those Parts from E to A and from F to C; then by the outward Bulks of those Arches draw the Line AB and CD.

And to divide the Circumference in∣to 12 parts, as our Example is, work thus.

[illustration] geometrical diagram

Set your Compasses in E, and make the Arch FC: The Compasses so open∣ed set again in F, and make Arch E A; then draw the Line from A to F, and from E to C. Then your Com∣passes opened at any distance, prick down one part less on both those slant∣ing Lines, than you intend to divide thereon; which is here 11, because we would divide the Line EF into 12: Then draw Lines from each Division to his opposite, that cuts the Line EF in the parts or Division. —But to proceed, It is Mr. Morgan's Conceit, page 116. Continue the Circumference at length to G and H, numbring from E towards G 12 of those Equal Parts, and from F towards H as many, which shall be the Center for each Arch; so those Quarters so cut out, shall exact∣ly cover the Globe, whose Circumfe∣rence is equal to EF.

Thus have you a glance of the Ma∣thematicks, striking at one thing through the side of another: For here one Fi∣gure is made for several Operations, to save the Press the charge of Fi∣gures.