PLANE TRIGONOMETRY.

DEFINITIONS.

1. PLANE Trigonometry is the art whereby, having given any three parts of a plane triangle (except the three angles), the rest are determined. In order to which, it is not only requisite that the peripheries of circles, but also certain right lines in and about the circle be supposed divided into some assigned number of equal parts.

2. The periphery of every circle is supposed to be divided into 360 equal parts, called degrees; and each degree into 60 equal parts, called minutes; and each minute into 60 equal parts, called seconds, or second minutes, &c.

3. Any part AB (fig. 1.) of the periphery of the circle is called an arch, and is said to be the measure of the angle ACB at the centre, which it subtends.

Note. The degrees, minutes, seconds, &c. contained in any arch, or angle, are written in this manner, 50° 18′ 35′′, which signifies that the given arch, or angle, contains 50 degrees, 18 minutes, and 35 seconds.