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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42

ENGINEER AND MACHINIST’S DBA WING-BOOK/

of Figs. 3 and 6; and, having joined the contiguous points,
and the corresponding angles of the upper and lower sur-
face, we obtain the complete vertical projection of the
prism in its doubly-inclined position.

Construction of the Conic Sections.—Plate III.

Let it be required in the first place, to represent a cone
in elevation and plan. Figs. 1 and 2.

The horizontal projection of the cone is simply a circle,
described from the centre S', of a diameter equal to that
of the base. Its elevation is an isosceles triangle, ob-
tained by drawing tangents A' A, B' B, perpendicular to,
and intersecting the ground line; then set off upon the
centre line the height C S, and join S A, S B. These lines
are called the exterior generatrices of the cone.

Figs. 1 and 2.—Given the projections of a cone, and
the direction of a plane X X, cutting it perpendicularly
to the vertical, and obliquely to the horizontal plane;
required to find, first, the horizontal projection of this
section; and, secondly, the outline of the ellipse thus
formed.

Through the vertex of the cone, draw a line S E to any
point within the base A B ; this line is to be regarded as
the vertical projection of a generatrix of the cone, and the
point e, where it intersects the line X X, is the projection
of that point on the surface of the solid, where the cutting-
plane actually passes through the generatrix E S. The
point e may be projected upon the plan by letting fall a
perpendicular from E, cutting the circumference of the
base in E', and joining E' S'; then another perpendicular
let fall from e will intersect E' S' in a point e', which will
be the horizontal projection of a point in the curve
required. By drawing another line, such as S D, and
projecting its point of intersection d with the cutting
plane, to d', a second point in the curve is obtained ; and
so on for any required number of points.

The exterior generatrices A S and B S, being both
projected upon the line A' B', the extreme limits of the
curve sought will be at the points a’ and b', on that line,
which are the projections of the points of intersection
a and b, of the cutting plane with the outlines of the cone.
And, since the line a b' will obviously divide the curve
symmetrically into two equal parts, the points /', g, h',
&c., will be readily obtained by setting off above that
line, and on their respective perpendiculars, the distances
d'd?, e e2, &c. A sufficient number of points having thus
been determined, the curve drawn through them (which
will be found to be an ellipse), will be the outline of the
section required.

This curve may be obtained by another, and perhaps
simpler method, depending on the principle that all sec-
tions of a cone by planes parallel to the base are circles.
Thus, let the line F G represent a cutting plane ; the sec-
tion which it makes with the cone will be denoted, on
the horizontal projection, by a circle drawn from the
centre S' with a radius equal to half the line F G ; and,
by projecting the point of intersection H, of the horizontal
and oblique planes, by a perpendicular II IT', and noting

where this line cuts the circle above referred to, we obtain
two points IF and I' in the curve required. By a similar
construction, as exemplified in our drawings, any number
of additional points may be found.

As the projection obtained by the preceding methods
exhibits the section as fore-shortened, and not in its true
dimensions, we shall now proceed to the consideration of
the second question proposed. Let the cutting plane X X
be conceived to turn upon the point b, so as to coincide
with the vertical line b h, and (to avoid confusion of lines),
let b k be transferred to a b', which will represent, as
before, the extreme limits of the curve required. Now,
taking any point, such as d, it is obvious that, in this new
position of the cutting plane, it will be represented by d2,
and that, if we make the further supposition that the
cutting plane were turned upon a' b', as an axis, till it
should be parallel to the vertical plane, the point which
had been projected at d2 would then have described round
a1 b' an arc of a circle whose radius is the distance d'd2,
Fig. 2. This distance, therefore, being set off at d' and/',
on each side of a b', gives two points in the curve sought.
By a similar mode of operation any number of points may
be obtained, through wdiich, if we draw a curve, it will
be an ellipse, of the true form and dimensions of the sec-
tion. Or, having determined the major and minor axes,
the student may, if he deems it preferable, construct the
ellipse by any of the methods already given.

Figs. 3 and 4.—To find the horizontal projection, and
actual outline of the section of a cone, made by a plane
Y Y parallel to one side, or generatrix, and perpendi-
cular to the vertical plane.

By following the second method laid down in the pre-
ceding problem, we may readily obtain any number of
points, as F', G', J', K, &c., in the curve representing the
horizontal projection of the section specified; a simple
reference to the lines in the drawings will sufficiently
explain the application to the present case. We may only
remark that the horizontal plane passing through M gives
only one point M' (which is the vertex of the curve sought),
because the circle which denotes the section that it makes
-with the cone is a tangent to the given plane.

In order to determine the actual outline of this curve,
let us suppose the plane Y Y to turn, as upon a pivot at
M, until it has assumed the position M B, and transfer
M B parallel to itself, to M' B'. The point F will thus
have first described the arc F E till it reaches the point
E, which is then projected to E2; so that, if we conceive
the given plane, now represented by M' B', to turn upon
that line as an axis, until it assumes a position parallel to
the vertical plane, we shall find that the point E2, which
is distant from the axis M' B' by the distance F' S', Fig. 4,
will now be projected to F', Fig. 3. The same distance
F' S' set off on the other side of the axis M' B', gives ano-
ther point G' in the curve required, which is that called
the parabola.

Figs. 5 and 6.—To dravj the vertical projection of the
section of a cone made by a plane parallel to its axis
and to the vertical plane.
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