The compleat surveyor containing the whole art of surveying of land by the plain table, theodolite, circumferentor, and peractor ... : together with the taking of all manner of heights and distances, either by William Leybourn.

CHAP. VI. Of the Protractor.

A Protractor is an Instrument by which you may Protract or lay down upon paper or otherwise, the true symetry or proportion of any field, having made observation of the sides and angles thereof by some of the Instruments be∣fore described. This Instrument consisteth of two parts, the one is a Semicircle divided into degrees, as is the frame of the Table, and the other is a Scale divided into equall parts, the Semicircle be∣ing to lay down the angles, and the Scale to plot the sides. This Instrument ought to be made of a piece of thin brasse well polished, the edges thereof being very smooth, and the Scale thereof, namely, the right angled Parallelogram, or long square containing in length from A to B about 4 Inches and three quarters, and in breadth from A to C about one and a halfe. Let the two ends of the Scale,

[illustration]

namely, the sides AC and BD be divided into equall parts of 16 or 20 in an Inch, and let the side CD be divided according to a Scale of 10 or 12 in an Inch.

The Scale being thus divided, on the middle of the line AB, as at H, describe the Semicircle EGF, which divide into two Qua∣drants in the point G, by help of the perpendicular HG: then di∣vide each of those Quadrants into 90 equall parts called degrees, so shall the whole Semicircle contain 180 degrees, which must be numbered by 10, 20, 30, 40, &c. to 180, from E by G to F, and the same way also from 180 to 360, as you see done in the Figure, the numbers of the first Semicircle from 00 to 180 being for the East side of the Protractor, and the other numbers from 180 to 360 for the West side.

Now you are to note, that the line AB alwayes representeth the Meridian line, and is somtimes noted with the letters S and N, for South and North, but then it is necessary that the Protractor be divided on either side the plate, which this double numbering a∣voydeth: for the line AB being taken for the Meridian in gene∣rall, the Semicircle of the Protractor may be turned any way (either upward or downward) and so one Semicircle being divided will be sufficient; yet if any man be desirous, he may have it made accor∣ding to his own fancie, but this manner of numbering (in my opini∣on) is the best, it being most agreeable to your Instruments.

To use with this Protractor in protracting, you must provide a fine needle, put into a piece of Box or Ivory neatly turned, this will serve to fix in your center, note your degrees, and for other uses in drawing your Plot, and is called a Protracting pin.