Figure 118.

[Illustration: Fig. 117.]

Figure 119 represents a key drawn in perspective, so that all the
dimensions may be marked on one view. Perspective sketches may be used
for single pieces, as they denote the shape of the piece more clearly to
the eye. On account of the skill required in their production, they are
not, however, used in mechanical drawing, except as in the case of
Patent-Office or similar drawings, where the form and construction
rather than the dimension is the information sought to be conveyed.

[Illustration: Fig. 118.]

[Illustration: Fig. 119.]

CHAPTER VI.

_THE ARRANGEMENT OF DIFFERENT VIEWS._

THE DIFFERENT VIEWS OF A MECHANICAL DRAWING.

The word _elevation_, as applied to mechanical drawing, means simply a
view; hence a side elevation is a side view, or an end elevation is an
end view.

The word _plan_ is employed in place of the word top; hence a plan view
is a top view, or a view looking down upon the top of the piece.

A _general_ view means a view showing the machine put together or
assembled, while a detail drawing is one containing a detail, as a part
of the machine or a single piece disconnected from the other parts of
the whole machine.

It is obviously desirable in a mechanical drawing to present the piece
of work in as few views as possible, but in all cases there must be a
sufficient number to permit of the dimensions in every necessary
direction to be marked on the drawing. Suppose, then, that in Figure 120
we have to represent a solid cylinder, whose length equals its diameter,
and it is obvious that both the diameter and length may be marked in the
one view given; hence, a second view, such as shown by the circle in
Figure 121, is unnecessary, except it be to distinguish the body from a
cube, in which the one view would also be sufficient whereon to mark
all the dimensions necessary to enable the piece to be made. It happens,
however, that a cube and a cylinder are the only two figures upon which
all the dimensions can be marked on one view of the piece, and as
cylindrical pieces are much more common in machine work than cubes are,
it is taken for granted that, where the pieces are cylindrical, but one
view shall be used, and that where they are cubes either two views shall
be given, or where they are square a cross shall be marked upon the
parts that are square; thus, in Figure 122, is shown a cross formed by
the lines A B across the face of the drawi