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SECTION XVII. 
OF PLANE TRIGONOMETRY. 
DEFINITIONS. 
1. "SILANE Trigonometry is the art whereby, hav- 
I ing 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 peri 
pheries of circles, but also that 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, See. Any part of the periphery is called an 
arch, and is measured by the number of degrees and 
minutes, &c. it contains. 
3. The difference of any arch from 90 degrees, or a 
quadrant, is called its complement, and its difference 
from ISO degrees, or a semi-circle its supplement. 
. 4. A chord, or sub 
tense, is a right line 
drawn from one ex 
tremity of an arch to 
the other; thus BE 
is the chord or sub 
tense of the arch 
BAE, or BDE. 
5. The sine (or 
right sine) of an arch 
is a right line drawn 
from one extremity 
of the arch perpen 
dicular to the diame 
ter passing through the other extremity: thus BF is 
the sine of the arch AB, or BD. 
R