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Sin B : sin A :: sin AC : sin BC,
(24.); the affection of BC is un-
less than 180°, (10.).
CD perpendicular to AB; then
From the angle required, c, draw
CD perpendicular to AB.
quired, draw CD perpendicular to
(AC-BC): tan * E; ihen, if
In the foregoing table, the rules are given for ascertaining the af, fection of the arch or angle found, whenever it can be done: Most of these rules are contained in this one rule, which is of general application, viz. that when the thing found is either a tangent or a cosine, and of the tangenis or cosines employed in the coniputation of it, either one or three belong to obtuse angles, the angle found is also obtuse. This rule is particularly to be attended to in cases 5. and 7. where it removes part of the ambiguity.
It may be necessary to remark with respect to the 11th ease, that the segments of the base computed there are those cut off by the nearest perpendicular; and also, that when the sum of the sides is less than 180°, the least segment is adjacent to the least side of the triangle: otherwise to the greatest (17.)
The last table may also be conveniently expressed in the following manner, denoting the side opposite to the angle A, by a, to B by 6, and to C by c; and also the segments of the base, or of opposite angle, by x and y.
NAPIER'S RULES OF THE CIRCULAR PARTS.
use in Spherical Trigonometry, by reducing all the theorems employed in the solution of right angled triangles to two. These two are not new propositions, but are merely enunciations, which, by help of a particular arrangement and classification of the parts of a triangle, include all the six propositions, with their corollaries, which have been demonstrated above from the 18th to the 23d inclusive. They are perhaps the happiest example of artificial memory that is known.
I. If in a spherical triangle, we set aside the right angle, and consider
only the five remaining parts of the triangle, viz. The three sides and the two oblique angles, then the two sides which contain the right angle, and the complements of the other three, namely, of the two angles and the hypotenuse, are called the Circular Parts. Thus, in the triangle ABC right angled at A, the circular parts are
AC, AB with the complements of B, BC, and C. These parts are called circular; because when they are named in the natural order of their succession they go round the triangle.
II. When of the five circular parts any one is taken, for the middle part,
then of the remaining four, the two which are immediately adjacent to it, on the right and left, are called the adjacent parts; and the other two, each of which is separated from the middle by an adjaçent part, are called opposite parts.