Armengaud, Jacques Eugène; Leblanc, César Nicolas   [Hrsg.]; Armengaud, Jacques Eugène   [Hrsg.]; Armengaud, Charles   [Hrsg.]
The engineer and machinist's drawing-book: a complete course of instruction for the practical engineer: comprising linear drawing - projections - eccentric curves - the various forms of gearing - reciprocating machinery - sketching and drawing from the machine - projection of shadows - tinting and colouring - and perspective. Illustrated by numerous engravings on wood and steel. Including select details, and complete machines. Forming a progressive series of lessons in drawing, and examples of approved construction — Glasgow, 1855

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CONSTRUCTION AND USE OF DRAWING INSTRUMENTS.

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witli the pen-leg, we require to throw the other leg into a
very oblique position, with the almost certainty of losing
its place, and making a false permanent line. To meet
this difficulty, a brass lengthening bar is provided, which
receives the pen-leg in the one end, and joins to the com-
passes, by a bayonet-fixing, at the other. When thus
lengthened, the instrument wdll command a radius of six
or eight inches with ease and security.

The pencil-leg consists of a tube split through half its
length, with a ring to move up and down, by which a
small short pencil is fixed much on the same principle as
the chalk in a portcrayon. The pen-leg is formed of two
blades of steel, terminating in thin, rounded, and well-
adjusted points. A spring is inserted between the blades,
to separate them ; and they are brought together by a
screw which passes through them, and which is capable of
adjusting the pen for a strong line, or for one as fine as
a hair. In using this leg, the screw is slackened, and ink
inserted between the blades with a quill-pen, or a camel-
hair pencil, according to the nature of the colouring fluid
used ; and the blades are then brought gradually together,
until they will produce a line of the desired quality. Thq/
draughtsman will, of course, try the line on his waste-paper
before he ventures to describe it on his drawing- A third
movable leg, named the Dotting Pen, is sometimes included
in the drawing-case, and though it is an instrument rather
uncertain in its performance, some draughtsmen manage
to employ it with very good effect in the drawing of dotted
lines. It is jointed in the same manner as the pen and
pencil legs, and consists of two blades terminating in a
small revolving wheel E (Fig. 2), which is retained in its
position by the screw D. The one wheel might (Fig_ 2.)
of course be permanently fixed, but usually there
are several given with the pen, to produce dots
of greater or less strength ; the contrivance of
the screw, therefore, admits the ready substitu-
tion of one wheel for another. When this pen is
used, ink must be inserted between the blades
over the wheel; and the latter should be ruq,
several times over the waste-paper, until, by its
revolution, it takes the ink freely, and leaves a
regularly dotted line in its course. It must be
admitted that, with every care, it sometimes fails
in its duty, and leaves blank spaces ; but where
much straight or curved dotted line is required, it will
very much abridge the draughtsman’s labour, and, if it
performs well, will dot with far greater regularity than
the steadiest hand.

The compasses with movable legs have frequently to
describe an entire circle, and an inexperienced hand finds
some difficulty in carrying the traversing leg neatly round
the circumference without the other leg losing position.
Some persons have recommended the movement of the
dividers, that is, to form half the circle in one direction,
and half in a reverse direction ; and this may answer very
well with the pencil-leg, but not with the pen-leg ; since
it is almost impossible, in the latter case, to unite the two
semicircles without leaving marks of junction, which very

much injure the continuity of the line that forms the circle.
This being the case, it is preferable to adopt a method that
shall answer equally well with either leg, and which, by
one continued sweep, shall complete the figure. It is very
desirable to use compasses for circles that have a due rela-
tion to the radii of the circles to be described ; that is to
say, such as will allow both the revolving and fixed leg to
be nearly vertical to the paper, for if the fixed leg is in-
clined obliquely, it is very apt to lose position, or to work
a large unsightly hole in the drawing. When the com-
passes are so adjusted that both legs are vertical, or very
nearly so, it is at once a simple and elegant movement that
carries the traversing point round the circle. Let the fore-
finger rest on the head and the thumb and second finger
on the sides ; commencing the sweep at the top, and towards
the right hand, the second finger becomes disengaged when
a quadrant is described, and the forefinger then winds the
head along the inner part of the thumb, until the point
has performed the entire circuit. It is not always desir-
able to commence the circle at the top, but more frequently
from a point which it is to touch accurately; this, how-
ever, presents no difficult}^ that the method of sweeping
does not meet. The only thing necessary is to place the
fingers and instrument in position, in the first instance,
with reference to the starting-point; and this is readily
done by a slight bending of the wrist. To one familiar
with the use of instruments, these instructions for mani-
pulation may appear unnecessarily minute ; but if he will
place his compasses for the first time in the hands of a
youth, and observe his lack of intuitive dexterity, he will
admit that they are in no degree too minute for a tyro.

Bow Compasses.—In every case of instruments, making
any pretensions to completeness, there is one pair of Bow
Compasses; but we shall advert to several kinds, each
recommended by a peculiar excellence or adaptation to the
draughtman’s purposes. This instrument may be described,
generally, as a small compasses suited for describing arcs
and circles of short radii, and which can be worked with
great facility by the finger and thumb. The most ordinary
construction, and that usually found in the drawing-case,
has the legs, one of which is a pen, moving freely on a
joint, and terminating at the top in a small handle. The
pen-blades are a little longer than the other leg, in order
that the latter may keep its vertical position throughout
a sweep, and not lose its centre. The performance of this
small instrument is very satisfactory; a succession of small
arcs and circles may be entered rapidly and delicately,
without leaving the centres strongly marked by the fixed
point; and this contributes much to the beauty of a
drawing, since nothing is more offensive than to see the
paper studded with small holes, exposing every insertion
of the compasses.

The annexed engraving (Fig. 3) shows an improvement
of the instrument. The vertical position of the pen-leg is
secured by the joint C ; the blades B are closed by the
screw F, which, according as it is tightened or relaxed,
renders the line finer or stronger at pleasure; and the
box-screw A unites the legs and handle firmly. The leg
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