CONSTRUCTIONS

OF THE

TRIGONOMETRICAL CANON.

A Trigonometrical Canon is a Table, which, beginning from one second or one minute, orderly expresses the lengths that every sine, tangent, and secant have, in respect of the radius, which is supposed unity; and is conceived to be divided into 10000000 or more decimal parts. And so the sine, tangent, or secant of an arc, may be had by the help of this table; and, contrariwise, a sine, tangent, or secant being given, we may find the arc it expresses. Take notice that in the following tract, R signifies the radius, Sa sine, Cos. a cosine, T a tangent, and Cot. a cotangent; also ACq signifies the square of the right line AC; and the marks or characters, +, -, -, :,::, and ✅✔✅ are severally used to signify addition, subtraction, equality, proportionality, and the extraction of the square root. Again, when a line is drawn over the sum or difference of two quantities, then that sum or difference is to be considered as one quantity.

The two sides of any right angled triangle being given, Fig. 28. the other side is also given.

For (by 47. 1.) ACq=ABq+BCq, and ACq-BCq ABq, and interchangeably ACq-ABq= BCq. Whence, by the extraction of the square root, there is given ACABq+BCq; and AB= ACq-BCq; and BC= ACq—ABq.

PROP. II. PROB.

Fig. 29. The sine DE of the arc BD, and the radius CD, being given, to find the cosine DF.30

Fig. 29.

The radius CD, and the sine DE, being given in the right angled triangle CDE, there will be given (by the last Prop.)✓ CDq—DEq=(CE=) DF.

PROP. III. PROB.

The sine DE of any arc DB being given, to find DM or
BM, the sine of half the arc.

DE being given, CE (by the last Prop.) will be given, and accordingly EB, which is the difference between the cosine and radius. Therefore DE, EB, being given, in the right angled triangle DBE, there will be given DB, whose half DM is the sine of the arc DL the arc BD.

PROP. IV. PROB.

Fig. 29. The sine BM of the arc BL being given, to find the sine of double that arc.

Fig. 30.

The sine BM being given, there will be given (by Prop. 2.) the cosine CM. But the triangles CBM, DBE, are equiangular, because the angles at E and M are right angles, and the angle at B common: wherefore (by 4. 6.) we have CB: CM:: (BD, or) 2 BM : DE. Whence, since the three first terms of this analogy are given, the fourth also, which is the sine of the arc DB, will be known.

COR. Hence CB: 2 CM:: BD: 2 DE; that is, the radius is to double the cosine of one half of the arc DB, as the subtense of the arc DB is to the subtense of double that arc. Also CB: 2 CM:: (2 BM: 2 DE: :) BM: DE:: CB: CM. Wherefore the sine of an arc, and the sine of its double, being given, the cosine of the arc itself is given.

PROP. V. PROB.

The sines of two arcs, BD, FD, being given, to find FI, the sine of the sum, as likewise EL, the sine of their difference. Let the radius CD be drawn, and then CO is the

cosine of the arc FD, which accordingly is given, and draw OP through O parallel to DK; also let OM, GE, be drawn parallel to CB. Then because the triangles CDK, COP, CHI, FOH, FOM, are equiangular; in the first place, CD: DK:: CO: OP, which, consequently, is known. Also we have CD: CK:: FO: FM; and so, likewise, this will be known. But because FO=EO, then will FM-MG-ON; and so OP+FM=FI=sine of the sum of the arcs; and OP -FM: that is, OP-ON-EL-sine of the difference of the arcs: which were to be found.

COR. Because the differences of the arcs BE, BD, BF, are equal, the arc BD is an arithmetical mean between the arcs BE, BF.

PROP. VI. THEOR.

The same things being supposed, the radius is to double Fig. so. the cosine of the mean arc, as the sine of the difference is to the difference of the sines of the extremes.

For we have CD: CK:: FO: FM; whence by doubling the consequents, CD: 2 CK :: FO: (2 FM, or) to FG, which is the difference of the sines EL, FI.

Q. E. D.

COR. 1. If the arc BD be 60 degrees, the difference of the sines FI, EL, will be equal to the sine FO of the difference. For, in this case, CK is the sine of 30 degrees; the double whereof is equal to the radius (by 15. 4.); and so, since CD-2 CK, we shall have FO=FG. And, consequently, if the two arcs BE, BF, be equidistant from the arc of 60 degrees, the difference of the sines will be equal to the sine of the difference FD.

COR. 2. Hence, if the sines of all arcs distant from one another by a given interval, be given, from the beginning of a quadrant to 60 degrees, the other sines may be found by one' addition only. For the sine of 61 degrees the sine of 59 degrees+the sine of one degree; and the sine of 62 degrees the sine of 58 degrees+the sine of two degrees. Also, the sine of 63 degrees the sine of 57 degrees + the sine of 3 degrees, and so on.

COR. 3. If the sines of all arcs, from the beginning of a quadrant to any part of a quadrant, distant from each other by a given interval, be given, thence we may find the sines of all arcs to the double of that part. For

Fig. 31.

example: let all the sines to 15 degrees be given; then, by the preceding analogy, all the sines to 30 degrees may be found. For the radius is to double the cosine of 15 degrees, as the sine of 1 degree is to the difference of the sines of 14 degrees, and 16 degrees: so, also, is the sine of 3 degrees to the difference between the sines of 12 and 18 degrees; and so on continually, until you come to the sine of 30 degrees.

After the same manner, as the radius is to double the cosine of 30 degrees, or to double the sine of 60 degrees, so is the sine of 1 degree to the difference of the sines of 29 and 31 degrees:: sine 2 degrees to the difference of the sines of 28 and 32 degrees :: sine 3 degrees to the difference of the sines of 27 and 33 degrees. But, in this case, the radius is to double the cosine of 30 degrees, as 1 to

3.

For (see the figure for Prop. 15. Book IV. of the Elements) the angle BGC-60 degrees, as the arc BC, its measure, is a sixth part of the whole circumference; and the straight line BC-R. Hence it is evident that the sine of 30 degrees is equal to half the radius; and therefore, by Prop. 2. the cosine of 30 degrees= R2_✓ 3R2 and its double√ 3R2=R × √ 3. ✔ R2

2

2

Consequently, radius is to double the cosine of 30°::
R: RX 3:1:3.

And, accordingly, if the sines of the distances from the arc of 30 degrees, be multiplied by 3, the differences of the sines will be had.

So, likewise, may the sines of the minutes in the beginning of the quadrant be found, by having the sines and cosines of one and two minutes, given. For, as the radius is to double the cosine of 2′ :: sine 1': difference of the sines of 1' and 3':: sine 2′ : difference of the sines of O' and 4'; that is, to the sine of 4'. Andre so, the sines of the first four minutes being given, we may thereby find the sines of the others to 8', and from thence to 16', and so on.

PROP. VII. THEOR.

In small arcs, the sines and tangents of the same arcs are nearly to one another, in a ratio of equality.

For, because the triangles CED, CBG, are equiangular, CE: CB:: ED: BG. But as the point Ex

approaches B, EB will vanish in respect of the arc BD;
whence CE will become nearly equal to CB, and so
ED will be also nearly equal to BG. If EB be less
than the
part of the radius, then the dif-
ference between the sine and the tangent will be also

1 10000000

1

less than the 10000000 part of the tangent.

COR. Since any arc is less than the tangent, and greater than its sine, and the sine and tangent of a very small arc are nearly equal; it follows, that the arc will be nearly equal to its sine: and so in very small arcs, it will be, as arc is to arc, so is sine to sine.

PROP. VIII. PROB.

To find the sine of the arc of one minute.

The side of a hexagon inscribed in a circle, that is, the subtense of 60 degrees, is equal to the radius (by Coroll. 15th of the 4th); and so the half of the radius will be the sine of the arc of 30 degrees. Wherefore the sine of the arc of 30 degrees being given, the sine of the arc of 15 degrees may be found (by Prop. 3.). Also the sine of the arc of 15 degrees being given (by the same Prop.) we may have the sine of 7 degrees 30 minutes. So, likewise, can we find the sine of the half of this, viz. 3 degrees 45 minutes; and so on, until 12 bisections being made, we come to an arc of 522, 443, 03*, 455, whose cosine is nearly equal to the radius; in which case (as is manifest from Prop. 7.) arcs are proportional to their sines: and so, as the arc of 522, 443, 034, 455, is to an arc of one minute, so will the sine before found be to the sine of an arc of one minute, which therefore will be given. And when the sine of one minute is found, then (by Prop. 2. and 4.) the sine and cosine of two minutes will be had.

If the angle BAC, being in the periphery of a circle, be Fig. 32. bisected by the right line AD, and if AB be produced until DE AD meets it in E; then will CE=AB.

In the quadrilateral figure ABDC (by 22. 3.) the angles B and DCA are equal to two right angles= DČE-DCA (by 13. 1.): whence the angle BDCE.