LET ABC be a rectilineal angle, if about the point B as a centre, and with any distance BA, a circle be described, meeting BA, BC, the straight lines including the angle ABC in A, C; the angle ABC will be to four right angles, as the arch AC to the whole circumference.

Produce AB till it meet the circle again in F, and through B draw DE perpendicular to AB meeting the circle in D, E.

By 33. 6. Elem. the angle ABC is to a right angle ABD, as the arch AC to the arch AD: and quadrupling the consequents the angle ABC, will be to four right angles, as the arch AC to four times the arch AD, or to the whole circumference.

LEMMA II. FIG. 2.

LET ABC be a plane rectilineal angle as before: about B as a centre with any two distances BD, BA, let two circles be described meeting BA, BC in D, E, A, C; the arch AC will be to the whole circumference of which it is an arch, as the arch DE is to the whole circumference of which it is an arch.

By Lemma 1. the arch AC is to the whole circumference of which it is an arch, as the angle ABC is to four right angles; and by the same Lemma 1. the arch DE, is to the whole circumference of which it is an arch, as the angle ABC is to four right angles; therefore the arch AC is to the whole circumference of which it is an arch, as the arch DE to the whole circumference of which it is an arch.

DEFINITIONS, FIG. 3.

1.

LET ABC be a plane rectilineal angle; if about B as a centre, with BA any distance, a circle ACF be described meeting BA, BC in A, C; the arch AC is called the measure of the angle ABC.

II.

The circumference of a 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, &c. And as many degrees, minutes, seconds, &c. as are contained in any arch, of so many degrees, minutes, seconds, &c. is the angle, of which that arch is the measure, said to be.

COR. Whatever be the radius of the circle of which the measure of a given angle is an arch, that arch will contain the same number of degrees, minutes, seconds, &c. as is manifest from Lemma 2.

III.

Let AB be produced till it meet the circle again in F, the angle CBF, which, together with ABC, is equal to two right angles, is called the Supplement of the angle ABC.

IV.

A straight line CD drawn through C, one of the extremities of the arch AC perpendicular upon the diameter passing through the other extremity A, is called the Sine of the arch AC, or of the angle ABC, of which it is the measure.

COR. The Sine of a quadrant, or of a right angle, is equal to the radius.

V

The segment DA of the diameter passing through A, one extremity of the arch AC between the sine CD, and that extremity, is called the Versed Sine of the arch AC, or angle ABC.

VI.

A straight line AE touching the circle at A, one extremity of the arch AC, and meeting the diameter BC passing through the other extremity C in E, is called the Tangent of the arch AC; or of the angle ABC.

VII.

The straight line BE between the centre and the extremity of the tangent AE is called the Secant of the arch AC, or angle ABC.

COR. to def. 4. 6. 7. the sine, tangent, and secant of any angle ABC, are likewise the sine, tangent and secant of its supplement CBF.

It is manifest from def. 4. that CD is the sine of the angle CBF. Let CB be produced till it meet the circle again in G; and it is manifest that AE is the tangent, and BE the secant, of the angle ABG or EBF from def. 6, 7.

COR. to def. 4. 5. 6. 7. The sine, versed sine, tangent, and secant, of any arch which is the measure of any given angle ABC, is to the sine, versed sine, tangent, and secant, of any other arch which is the measure of the same angle, as the radius of the first is to the radius of the second. Let AC, MN be measures of the angle ABC, according to def. 1. CD the sine, DA the versed sine, AE the tangent and BE the secant of the arch AC, according to def. 4. 5. 6. 7. and NO the sine, OM the versed sine, MP the tangent and BP the secant of the arch MN, according to the same definitions. Since CD, NO, AE, MP are parallel, CD is to NO as the radius CB to the radius NB, and AE to MP as AB to BM, and BC or BA to BD as BN or BM to BO; and, by conversion, DA to MO as AB to MB. Hence the corollary is manifest; therefore, if the radius be supposed to be divided into any given number of equal parts, the sine, versed sine, tangent, and secant of any given angle, will each contain a given number of these parts; and, by trigonometrical tables, the length of the sine, versed sine, tangent, and secant of any angle may be found in parts of which the radius contains a given number; and, vice versa, anumber expressing the length of the sine, versed sine, tangent, and secant being given, the angle of which it is the sine, versed sine, tangent, and secant may be found.

VIII.

Fig. 3. The difference of an angle from a right angle is called the complement of that angle. Thus, if BH be drawn perpendicular to AB the angle CBH will be the complement of the angle ABC, or of CBF.

IX.

Let HK be the tangent, CL or DB, which is equal to it, the sine, and BK the secant of CBH, the complement of ABC, according to def. 4. 6. 7. HK is called the co-tangent, BD the co-sine, and BK the co-secant of the angle ABC.

COR. 1. The radius is a mean proportional between the tangent, and co-tangent.

For, since HK, BA are parallel, the angles HKB, ABC will be equal, and the angles KHB, BAE, are right; therefore the triangles BAE, KHB are similar, and therefore AE isto AB, as BH or BA to HK.

COR. 2. The radius is a mean proportional between the co-sine and secant of any angle ABC.

Since CD, AE are parallel, BD is to BC or BA, as BA to BE