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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GEOMETRICAL CONSTRUCTIONS—DRAWING OF ELEMENTARY FORMS.

19

For example, vertical lines are perpendicular to horizontal
lines; and the edge of the blade of a T square is perpen-
dicular to the edge of the stock.

(Fig. 31.) (Fig. 32.)

Right Angle. Acute Augle.

(Fig. S3.)

An acute angle {Fig. 32), is less than a right angle.

An obtuse angle {Fig. 33) is greater than a right angle.

A plane triangle, or simply a triangle, is a flat surface
bounded by three straight lines or
sides. When the sides are equal
{Fig. 34), the triangle is equila-
teral; when one of the angles is a
right angle, the triangle is right-
angled.

An angle is designated by a letter A
at the apex, as, A {Fig. 34), or by ^ Equaateral Triangle.

the letters used to denote its sides, as B A C.

For convenience, any one side of a triangle may be
distinguished as the base, the remaining two sides being
properly the sides.

A quadrilateral figure is a surface bounded by four
straight lines. When the opposite sides are parallel, it is
called a parallelogram {Figs. 35, 36, 37); if it have right

(Fig. 31.)

c

(Fig. 3G.)

Rectangle.

Square.

(Fg. 37.)

Parallelogram.

angles, it is a rectangle {Fig. 35); if all the sides also be
equal, it is a square {Fig. 36).

A diagonal is a straight line joining two opposing
angles of a figure.

Plane figures of more than four sides are called poly-
gons. When the sides are equal, they are regular poly-
gons ; of which {Figs. 38-41) are examples, annexed to
which are their respective designations.

Pentagon, five sides.

A circle {Fig. 42) is a plane figure bounded by one
continuous curve line, bed, called the circumference, and

$*9- 42•) (Fig. 43.) (Fig. 44.)

is such that the circumference is at all points equally
distant from the centre, a, within it. Any straight line,

b a, or b c {Fig. 43), drawn from the centre to the cir-
cumference, is termed a radius ; and a line a b {Fig. 44),
drawn through the centre and terminated by the circum-
ference, is a diameter. The magnitude of a circle is
usually designated in terms of its diameter; the radius is
half the diameter.

An arc of a circle is any part of the circumference, as
ced {Fig. 45).

A chord of a circle is a straight line joining the extre-
mities of an arc, as c cl {Fig. 45).

A sector of a circle is the space or area enclosed by two
radii, as abc {Fig. 43). When the radii are at right
angles, the space is called also a quadrant, as it is one-
fourth of a circle.

A segment of a circle is the space cut off by a chord, as
the space ced {Fig. 45).

(Fg. 45.) (Fg. 40.) (Fig. 47.)

O)

A semicircle, or half-circle, is one of the two equal
parts into which a circle is divided by a diameter, as in
{Fig. 44).

A tangent to a circle or other curve line, is a straight
line which touches it, meeting it only at one point, as a b
{Fig. 46), touching at c.

Circles are concentric when they have the same centre,
as in {Fig. 47); if they have different centres, they are ec-
centric, as in (Fig. 48); where also they are shown to touch
at their circumferences, or to be tangential to each other.

(Fig. 48.) (Fig. 49.) (Fig. 50.)

A triangular or other figure with a greater number of
sides, is inscribed in a circle, or circumscribed by it,
when all its angles touch the circumference, as in {Fig. 49).

Inversely, a circle is inscribed in a straight-sided figure,
when it touches all the sides {Fig. 50).

All regular polygons may be inscribed in circles, and
circles may be inscribed in polygons; hence the facility
with which polygons may be constructed.

Of solids. As surfaces are bounded by lines, and derive
their names from the character and disposition of these
lines—so solids are bounded by surfaces, and also derive
their titles therefrom.

A prism is a solid of which the ends are equal, similar,
and parallel straight-sided figures, and of which the other
sides are parallelograms. When all the sides are squares,
it is called a cube {Fig. 51).

A pyramid is a solid having a straight-sided base,
and triangular sides terminating in one point or vertex
{Fig. 52).

Prisms and pyramids are distinguished as triangular,
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