DOI Page / Citation link:
https://doi.org/10.11588/diglit.20658#0021
OF DRAWING.
chain, mile, &c. according to his pleasure. Consequently his plan, or drawing, will be greater or
less, as the scale he uses contains a less or greater number of divisions in one inch. A variety of
these scales is therefore necessary, to enable him to make his drawing of a given size. There are
generally six of them placed one above the other, and divided into as many equal parts as the length
of the instrument will permit. The numbers placed on the right hand end shew how many parts
each scale contains in one inch. The first of the larger divisions of.every scale, is subdivided into
ten equal parts, from which parts each scale takes its name : thus, if twenty of these'divisions are
equal to one inch, it is called a scale of twenty. If each of these less clivisions;be considered as a
unit, each of the lamer divisions on the same scale is considered as ten : but if the subdivisions
be ranked at ten each, the larger ones will be hundreds, whether they be considered as feet, yards.,
miles, or any other measure. The firstprimary division isoften divided into twelve, to adapt these
scales to feet and inches.
Besides the other plotting scales of equal parts, there is on this instrument a scale of equal*parts
diagonal^ divided, by which extents may be more accurately laid down than by any other scale ;
for this scale consists of three kinds of extents : the whole scale is first divided into ten equal parts ;
the last of these divisions.is then subdivided into ten other equal parts, and by diagonal lines each
of these smaller divisions is again subdivided into ten other equal parts : thus, if the larger divisions
be accounted units, the first subdivisions will be the tenth parts of units, and the second subdivision,
marked by diagonal lines, will behundredth parts of an unit: butif thelargerdivisionsbeconsidered
as hundreds, the first subdivisions^vill represent tens, and the second units. Thus if it be required
to take an extent On this scale of 554 feet, yards, or any other measure ; one foot of the compasses
is to be set on the number 5 of the larger divisions (for the .500), and the other foot of the compasses
extended to the third first subdivision (for the 30), and then both points moved up the scale of the
aforesaid lines, gradually opening the compasses till that point on the diagonal division 3 arrives
to that part of the line which is intersected by the fourth parallel line (for the four units); and.
that extent of the compasses will be. the.required length.
The use of the line of chords is to lay down a .given angle, and consequently to measure one : thus
if it be required to make an angle of 25 degrees, after having drawn a line, or having a given one,
open the compasses to 60 degrees on the line of chords (which is always equal to the radius of
the circle), and .setting one foot of the compasses in the angular point, with the other describe an
arch ; then taking the extent of (25 degrees in the compasses from the line of chords, set that dis-
tance off on the arch just described from the point where it intersects, the line, and to the other
point draw a line from the angular.point; these two lines will, then form an angle of 25 degrees.
And the number of degrees contained in an angle already described, is found in a similar manner;
viz. by opening the compasses to GO degrees on the line of chords, and fr om the angular point as a
center describing an arc, which shall extend from one line of the angle to the other ; the length
of this arc taken in the compasses, and applied to the line of chords, will shew the number of de-
grees the angle contains.
The rhumb is no other than a line of chords, constructed to a quarter of a circle, divided into
eight parts ; a,nd is principally used in navigation for expedition sake,dn laying down the course
of a ship.
The line of longitude is a line divided into 60unequal parts, and'by the help of the line of chords,
serves to shew by inspection the number of equatorial miles on the face of the globe,
in a degree on any given parallel of latitude. The
chain, mile, &c. according to his pleasure. Consequently his plan, or drawing, will be greater or
less, as the scale he uses contains a less or greater number of divisions in one inch. A variety of
these scales is therefore necessary, to enable him to make his drawing of a given size. There are
generally six of them placed one above the other, and divided into as many equal parts as the length
of the instrument will permit. The numbers placed on the right hand end shew how many parts
each scale contains in one inch. The first of the larger divisions of.every scale, is subdivided into
ten equal parts, from which parts each scale takes its name : thus, if twenty of these'divisions are
equal to one inch, it is called a scale of twenty. If each of these less clivisions;be considered as a
unit, each of the lamer divisions on the same scale is considered as ten : but if the subdivisions
be ranked at ten each, the larger ones will be hundreds, whether they be considered as feet, yards.,
miles, or any other measure. The firstprimary division isoften divided into twelve, to adapt these
scales to feet and inches.
Besides the other plotting scales of equal parts, there is on this instrument a scale of equal*parts
diagonal^ divided, by which extents may be more accurately laid down than by any other scale ;
for this scale consists of three kinds of extents : the whole scale is first divided into ten equal parts ;
the last of these divisions.is then subdivided into ten other equal parts, and by diagonal lines each
of these smaller divisions is again subdivided into ten other equal parts : thus, if the larger divisions
be accounted units, the first subdivisions will be the tenth parts of units, and the second subdivision,
marked by diagonal lines, will behundredth parts of an unit: butif thelargerdivisionsbeconsidered
as hundreds, the first subdivisions^vill represent tens, and the second units. Thus if it be required
to take an extent On this scale of 554 feet, yards, or any other measure ; one foot of the compasses
is to be set on the number 5 of the larger divisions (for the .500), and the other foot of the compasses
extended to the third first subdivision (for the 30), and then both points moved up the scale of the
aforesaid lines, gradually opening the compasses till that point on the diagonal division 3 arrives
to that part of the line which is intersected by the fourth parallel line (for the four units); and.
that extent of the compasses will be. the.required length.
The use of the line of chords is to lay down a .given angle, and consequently to measure one : thus
if it be required to make an angle of 25 degrees, after having drawn a line, or having a given one,
open the compasses to 60 degrees on the line of chords (which is always equal to the radius of
the circle), and .setting one foot of the compasses in the angular point, with the other describe an
arch ; then taking the extent of (25 degrees in the compasses from the line of chords, set that dis-
tance off on the arch just described from the point where it intersects, the line, and to the other
point draw a line from the angular.point; these two lines will, then form an angle of 25 degrees.
And the number of degrees contained in an angle already described, is found in a similar manner;
viz. by opening the compasses to GO degrees on the line of chords, and fr om the angular point as a
center describing an arc, which shall extend from one line of the angle to the other ; the length
of this arc taken in the compasses, and applied to the line of chords, will shew the number of de-
grees the angle contains.
The rhumb is no other than a line of chords, constructed to a quarter of a circle, divided into
eight parts ; a,nd is principally used in navigation for expedition sake,dn laying down the course
of a ship.
The line of longitude is a line divided into 60unequal parts, and'by the help of the line of chords,
serves to shew by inspection the number of equatorial miles on the face of the globe,
in a degree on any given parallel of latitude. The