DOI Page / Citation link:
https://doi.org/10.11588/diglit.20658#0020
6
OF DRAWING.
done by hand ; and screw the nut/Yast; then, by turning the adjnster/z, the mark on the slider A- may
be brought exactly to the division required ; and screw fast the nut g. Next open the compasses,
and lift the end e of the screw of the nut f, out of the hole in the nut g; turn the beam round its
pillar a, and put the point e into the hole in the pim«, which is fixed to the under leg : slacken
the screw to the nut^; open the longer legs of the compasses to the distance required, and screw
the nut/' fast : then will the compasses be properly adjusted, and may be used without any in-
convenience, or danger of their changing their position.
The protractor is an instrument used for laying down, or measuring any given angle. There are
two kinds of this instrument, one in the form of a semicircle, and the other form is that of a
parallelogram : the former is, undoubtedly, the most natural; but the latter is often used for con-
venience, as other scales may be placed upon it. The semicircular protractor is divided into 180
equal parts or degrees, the divisions being supposed to be made by right lines drawn from the
center, or mark in the middle; to the circumference (every circle by geometricians being divided
into 3G0 degrees:) the instrument is numbered both from the right, and from the left, for conve-
nience. The inner edge is the diameter of the semicircle. The protractor in the form of a paral-
lelogram, is divided like the semicircular one, and also numbered both ways; the blank side
represents the diameter of the chyle. The side of this instrument applied to the paper is flat,
but that side whereon the degrees are marked is chamferred, or sloped away towards the edge, in
order to mark off the degrees with greater exactness. Some use horn protractors, which are verv
.useful on account of their transparency; but they require more care in keeping-than the others,
'■to.prevent their warping.
The uses of the protractor are principally three : I. to lay down, or describe a given angle ;
Q. to measure a given angle ; and, 3. to erect a perpendicular to a given line. Thus, to lay down
-an angle of GO degrees, a line is to be drawn along the diameter of the instrument to the center
thereof; a small point is then to be made against the number of 60 on the limb of the protractor,
from which point to the center a line is to be drawn, which, with the other line, will form an
angle of 60 degrees. By this example it may be seen how an angle is measured ; viz. by laying
the protractor on the angie in such a manner, that the diameter may coincide with one line of
the angle, the center of the protractor with the angular point: then the other line will point to
•the number of degrees, on the limb of the protractor, which the angle contains. To raise a per-
pendicular to a given line, the given line must coincide with the diameter of the protractor; then
4he line drawn from the HOth degree of the protractor to the center thereof, will be perpendicular
•to the given line ; or the two lines will form a right angle.
The plain scale is usually combined with the protractor, and is an instrument of very general
use. The divisions marked on this instrument are of two kinds ; the one being derived from the
properties of a circle, the other merely the division of straight lines. Of the former there are seven
kinds: 1. the line of chords, marked cuo. ; 2. the line of sines, marked sin.; 3. tangents,
marked tan.; 4. semitangents, marked st. ; 5. secants, marked sec; 6. rhumbs, marked
ru. ; and, 7- longitude, marked lon. The line of secants is often upon the-same line with the
sines, as their graduations do not begin till the sines end. Lines of latitude, and hours, are
now generally omitted, as their chief use was in dialling. Those scales on this instrument taken
from the division of a straight line, are called scales of equal parts; and are of various lengths, for
the convenience of delineating any figure or plan of any required size. They are nothing more
<tha,n small-arbitrary measures, adopted by thedraftsman; each division answering to one foot, vara,
vobain,
OF DRAWING.
done by hand ; and screw the nut/Yast; then, by turning the adjnster/z, the mark on the slider A- may
be brought exactly to the division required ; and screw fast the nut g. Next open the compasses,
and lift the end e of the screw of the nut f, out of the hole in the nut g; turn the beam round its
pillar a, and put the point e into the hole in the pim«, which is fixed to the under leg : slacken
the screw to the nut^; open the longer legs of the compasses to the distance required, and screw
the nut/' fast : then will the compasses be properly adjusted, and may be used without any in-
convenience, or danger of their changing their position.
The protractor is an instrument used for laying down, or measuring any given angle. There are
two kinds of this instrument, one in the form of a semicircle, and the other form is that of a
parallelogram : the former is, undoubtedly, the most natural; but the latter is often used for con-
venience, as other scales may be placed upon it. The semicircular protractor is divided into 180
equal parts or degrees, the divisions being supposed to be made by right lines drawn from the
center, or mark in the middle; to the circumference (every circle by geometricians being divided
into 3G0 degrees:) the instrument is numbered both from the right, and from the left, for conve-
nience. The inner edge is the diameter of the semicircle. The protractor in the form of a paral-
lelogram, is divided like the semicircular one, and also numbered both ways; the blank side
represents the diameter of the chyle. The side of this instrument applied to the paper is flat,
but that side whereon the degrees are marked is chamferred, or sloped away towards the edge, in
order to mark off the degrees with greater exactness. Some use horn protractors, which are verv
.useful on account of their transparency; but they require more care in keeping-than the others,
'■to.prevent their warping.
The uses of the protractor are principally three : I. to lay down, or describe a given angle ;
Q. to measure a given angle ; and, 3. to erect a perpendicular to a given line. Thus, to lay down
-an angle of GO degrees, a line is to be drawn along the diameter of the instrument to the center
thereof; a small point is then to be made against the number of 60 on the limb of the protractor,
from which point to the center a line is to be drawn, which, with the other line, will form an
angle of 60 degrees. By this example it may be seen how an angle is measured ; viz. by laying
the protractor on the angie in such a manner, that the diameter may coincide with one line of
the angle, the center of the protractor with the angular point: then the other line will point to
•the number of degrees, on the limb of the protractor, which the angle contains. To raise a per-
pendicular to a given line, the given line must coincide with the diameter of the protractor; then
4he line drawn from the HOth degree of the protractor to the center thereof, will be perpendicular
•to the given line ; or the two lines will form a right angle.
The plain scale is usually combined with the protractor, and is an instrument of very general
use. The divisions marked on this instrument are of two kinds ; the one being derived from the
properties of a circle, the other merely the division of straight lines. Of the former there are seven
kinds: 1. the line of chords, marked cuo. ; 2. the line of sines, marked sin.; 3. tangents,
marked tan.; 4. semitangents, marked st. ; 5. secants, marked sec; 6. rhumbs, marked
ru. ; and, 7- longitude, marked lon. The line of secants is often upon the-same line with the
sines, as their graduations do not begin till the sines end. Lines of latitude, and hours, are
now generally omitted, as their chief use was in dialling. Those scales on this instrument taken
from the division of a straight line, are called scales of equal parts; and are of various lengths, for
the convenience of delineating any figure or plan of any required size. They are nothing more
<tha,n small-arbitrary measures, adopted by thedraftsman; each division answering to one foot, vara,
vobain,