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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A D has a socket at its extremity, to admit a steel needle,
which is fastened by a clamp E. This last contrivance is
simple but valuable. The fine point of the compasses is
soon destroyed by continued use, and to renew it by
grinding reduces the length of the leg, and in course of
time renders the instrument worthless; whereas, a fresh
needle can be introduced into the socket as often as is
necessary, and a constant delicacy of point maintained.

Uig. 3.) {Fig. 4.) (.Fig. 5.)

Bow compasses can also be had carrying a pencil-leg.
T hey differ from those previously described only in having
a holder for the reception of a thin short pencil, which is
held tight by the screw F (Fig. 4). These have also the
joint C, the needle-point E, and box-screw joint A. Bows
to carry a pencil are seldom included in the drawing-case;
but they, and most, if not all, of the other instruments,
can be purchased separately. The pencil-leg is certainly a
very useful aid to the draughtsman, since there are many
occasions where it is desirable to get all the parts of a
drawing inserted with the lead, before making them per-
manent ; and arcs and circles of small radii are not readily
described with the larger compasses, supposing them to be
properly adapted for sweeping curves of greater magnitude.

Another sort of Bows, named Spring-Bow Compasses
(Fig. 5), though limited in their application to small curves
and circles, are very delicate and exact instruments, so far
as their range extends. They are in principle identical
with the Spring Dividers, which we have already described,
and one leg is provided with a holder for a pencil or pen.
The advantages of this construction can be appreciated
only by those who know the difficulty of securing a small
radius, with perfect exactness, by compasses that are ex-
tended and closed in the ordinary manner ; and who have
experienced the mortification of seeing an otherwise fine
(.Rawing marred and disfigured by small curves or circles,
described with a radius deviating from truth in an error
of perhaps not more than a hair-breadth, yet failing in
one instance to reach the point of junction, and in another
passing beyond it.

Directors, or Triangular Compasses.—This instrument

is used for taking three angular points at once, or for
laying down correctly a third point with relation to other
two. One form of construction is that of an ordinary pair
of compasses, with an additional leg attached by a universal
joint; and another contrivance, much recommended for
simplicity and facility in its use, is a solid plate of three
arms, each arm carrying a movable limb, into which a
short-pointed needle is inserted at right angles. In using
the first, the compasses are opened, and two points taken,
and the additional leg is extended in any direction to take
up the third point; the management of the second is
equally easy, the needle-points are successively adjusted
to the angles by the flexure of the movable limbs. With
either instrument, the draughtsman is saved the tedious
process of constructing triangles, and determining the rela-
tive position of neighbouring points in his drawing.

Proportional Compasses.—These are used for the en-
largement or reduction of drawings. The simplest form is
that named wholes ancl halves, which may be described
as two bars pointed at each extremity, and working trans-
versely on a box-screw joint, and forming, as it were, two
compasses, the legs of the one being twice the length of those
of the other. If any distance be taken between the points
of the longer legs, half that distance will be contained at
the other end. The application of the instrument to the
reducing or enlarging any drawing one-half,
is sufficiently obvious. The proportional com-
passes, properly so called, is a more compli-
cated contrivance, and admits of more varied
application. Its form and general construction
are seen in the annexed engraving (Fig. 6).
It is in principle the same as the wholes and
halves, with this difference, that the screw-
joint C passes through slides moving in the
slots of the bars, and admits of the centre
being adjusted for various relative propor-
tions between the openings A B and D E.
Different sets of numbers are engraved on
the outer faces of the bars, and by these the
required proportions are obtained. The in-
strument must be closed for adjustment, and
the nut C loosened ; the slide is then moved
in the groove, until a mark across it, named
the index, coincides with the number re-
quired ; which done, the nut is tightened again.

The scales usually engraved on these compasses are
named Lines, Circles, Planes, and Solids.

The scale of lines is numbered from 1 to 10, and the
index of the slide being brought to any one of these divi-
sions, the distance D E will measure A B in that propor-
tion. Thus, if the index be set to 4, D E will be contained
four times in A B.

The line of circles extends from 1 to 20, and the index
being set to (say) 10, D E will be the tenth part of the
circumference of the circle, whose radius is A B.

The line of planes, or squares, determines the proportion
of similar areas. Thus, if the index is placed at 3, and
the side of any one square be taken by A B from a scale
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