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Through C, and each point of division, let a chord be drawn, and let the lengths of these chords be accurately laid off on a scale: such a scale is called a scale of chords. In the figure, the chords are drawn for every ten degrees.

The scale of chords being once constructed, the radius of the circle from which the chords were obtained, is known; for, the chord marked 60 is always equal to the radius of the circle. A scale of chords is generally laid down on the scales which belong to cases of mathematical instruments, and is marked cнo.

To lay off, at a given point of a line, with the scale of chords, an angle equal to a given angle.

Let AB be the line, and A the given point.


Take from the scale the chord of 60 degrees, and with this radius and the point A as a centre, describe the arc BC. Then A take from the scale the chord of the given angle, say 30 degrees, and with this line as a radius, and B as a centre, describe an arc cutting BC in C. Through A and C draw the line AC, and BAC will be the required angle.

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24. This instrument is used to lay down, or protract angles. may also be used to measure angles included between lines

It consists of a brass semicircle ABC divided to half degrees. The degrees are numbered from 0 to 180, both ways; that is, from A to B and from B to A. The divisions, in the figure, are only made to degrees. There is a small notch at the middle of the diameter AB, which indicates the centre of the protractor.

To lay off an angle with a Protractor.

Place the diameter AB on the line, so that the centre shall fall on the angular point. Then count the degrees contained in the given angle from A towards B, or from B towards A and mark the extremity of the arc with a pin. Remove the protractor, and draw a line through the point so marked and the angular point: this line will make with the given line the required angle.


100 110 120 130 140 150 160 170 180 190

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

25. The sector is an instrument generally made of ivory or brass. It consists of two arms, or sides, which open by turning round a joint at their common extremity.

There are several scales laid down on the sector: those, however, which are chiefly used in drawing lines and angles, are, the scale of chords already described, and the scale of equal parts now to be explained.

On each arm of the sector, there is a diagonal line that passes through the point about which the arms turn: these diagonal lines are divided into equal parts.

On the sectors which belong to the cases of English instruments, the diagonal lines are designated by the letter L, and numbered from the centre of the sector, 1, 2, 3, 4, 5, 6, 7,

to cases of French instruments, they are designated, "Les parties egales," and numbered, 10, 20, 30, 40, &c. to 200. On the English sectors there are 20 equal divisions between either two of the lines numbered 1, 2, 3, &c., so that, there are 200 equal parts on the scale.

The advantage of the sectoral scale of equal parts, is this— When it is proposed to draw a line upon paper, on such a scale that any number of parts of the line, 40 for example, shall be represented by one inch on the paper, or by any part of an inch, take the inch, or part of the inch from the scale of inches on the sector: then, placing one foot of the dividers at 40 on one arm of the sector, open the sector until the other foot reaches to the corresponding number on the other arm: then lay the sector on the table without varying the angle.

Now, if we regard the lines on the sector as the sides of a triangle, of which the line 40 measured across, is the base, it is plain, that if any other line be likewise measured across the angle of the sector, the bases of the triangles, so formed, will be proportional to their sides. Therefore, if we extend the dividers from 50 to 50, this distance will represent a line of 50, to the given scale: and similarly for other lines.

Let it now be required to lay down a line of sixty-seven feet, to a scale of twenty feet to the inch.

Take one inch from the scale of inches: then place one foot of the dividers at the twentieth division, and open the sector until the dividers will just reach the twentieth division on the other arm: the sector is then set to the proper angle; after which the required distance to be laid down on the paper, is found, by extending the dividers from the sixty-seventh division on one arm, to the sixty-seventh division on the other.


26. This is a scale of two feet in length, on the faces of which a variety of scales are marked. The face on which the divisions of inches are made, contains, however, all the scales necessary for laying down lines and angles. These are, the scale of equal parts, the diagonal scale of equal parts, and the



At a given point in a given straight line, to erect a perpendicular to the line.

27. Let A be the given point, and BC the given line. From A lay off any two distances AB and AC equal to each other. Then, from the points B and C, as centres, with a radius greater than BA, describe two arcs intersecting each other in D: draw AD, and it will be the perpendicular required.



From a given point without a straight line, to let fall a perpen

dicular on the line.

28. Let A be the given point and BD the given line.

From the point A as a centre, with a radius sufficiently great, describe an arc cutting the line BD in the two points B and D: then mark a point E, equally distant from the points B and D, and draw ᏁᎬ : ᏁᎬ will be the perpendicular required.




At a point, in a given line, to make an angle equal to a given angle..

29. Let A be the given point, AE

the given line, and IKL the given angle.


From the vertex K, as a centre, K with any radius, describe the arc IL, terminating in the two sides of the angle. From the point A as a centre, with a distance AE equal to KI, describe the arc ED; then take the chord LI, with which, from the point E as a centre, describe an arc cutting the indefinite arc DE, in D; draw AD, and the angle EAD will be equal to the given angle K.


To divide a given angle, or a given arc, into two equal parts.

30. Let C be the given angle, and AEB

the arc which measures it.

From the points A and B as centres, de- A\ scribe with the same radius two arcs cutting each other in D: through D and the centre

C draw CD: the angle ACE will be equal

to the angle ECB, and the arc AE to the arc EB.



Through a given point to draw a parallel to a given line.

31. Let A be the given point, and BC the given line.

From A as a centre, with a radius

greater than the shortest distance from



A to BC, describe the indefinite arc ED: from the point E as a centre, with the same radius, describe the arc AF; make ED AF, and draw AD: then will AD be the parallel required.


Two angles of a triangle being given, to find the third.

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CEH equal to the other: the remaining angle HEF will be the third angle required.


To lay down, on paper, a line of a given length, so that any number of its parts shall correspond to the unit of the scale.

33. Suppose that the given line were 75 feet in length, and it were required to draw it on paper, on a scale of 25 feet to

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