The geometrical square, with the use thereof in plain and spherical trigonometrie chiefly intended for the more easie finding of the hour and azimuth / by Samuel Foster ...

A DESCRIPTION OF THE SQUARE.

THE whole superficies is divided into four lesser Squares, by the Diame∣ters FG and HI.

Each of the 4 Semidiameters EF, EH, EG, EI, are divided as the lines of Sines upon the Sector, the Semidiameters being the whole Sine, And through the parts of each Semi∣diameter are drawn right lines per∣pendicular thereunto, quite over the face of the whole Square every 10^th, 5^th, &c. are to be distinguished from the rest, for the more easie and speedy account.

Upon the limb are inserted several Scales, for several uses. The edges of these Scales bordering close upon the sides of the inner Square, that it may be discerned which lines and parts of the Scales doe butt one upon the other.

On the sides AB, AC, are inscribed Scales of equal parts, the whole being divided into 10, and subdivided as quantity will give leave. The parts are numbred by 1. 2. 3. 4. 5. 6. 7. 8. 9. 10, and may stand, either for 1. 2. 3, &c. or for 10. 20. 30, &c. or for 100. 200. 300, &c. as occasion re∣quireth.

To the lines BI and CG, are annexed Scales of right sines whose beginning is at B and C, and the end at I and G, num∣bred by 10. 20, &c. to 90.

The other parts ID, GD, have Scales of Tangents from 0 gr. to 45 gr. numbred either from G to D, and so to I. Or from I to D, and so to G, or rather both wayes, by 10, 20, 30, &c. till 90, and these divisions have respect to the Cen∣ter E.

Lastly, a Scale of larger Tangents, lying behind these last named, Parallel to the sides BD, CD, beginning 0 gr. from B and C, and so proceeding to 45 gr. in D and ending in 90 gr. at C and B, accounted both wayes. These have respect to their Center A.

[illustration]

〈 in non-Latin alphabet 〉〈 in non-Latin alphabet 〉

There is further added a threed and plummet, which is to be used in every practice, and must be in length equal to the lines AF, and FG. And if the threed be found inconveni∣ent in practice, because it will take up the use of both hands, there may instead of it, be used a little Bowe, the threed of it being at the least equal to AD, which will perform the office of the other threed by the help of one hand only, or a straight ruler may serve, if it be thought convenient for that purpose.

If the Square be applyed to the observation of Angles, it

may be fitted thereto one of these two wayes, Either by placing two sights upon the side of the Square, one upon the Center A, the other upon the line AB, which issueth out of the Center A. And a running sight contrived upon the utter edge of the Instrument to move from B to C by D forward, and so from C to B by D, backward again; Or else if this be thought inconvenient, or not feasible because of the sights turn∣ing over at the Angle D, then this moveable sight may goe onely upon one of the sides BD or CD. And for that pur∣pose the sight at A, is to stand precisely upon the Center, and both the sides AB, AG must have sights there fixed, as pre∣cisely, upon their lines that come from A.