But it would be tedious to mark the successive steps true one with the other by measuring each step, because one step would require to be pencilled in before the next could be marked. To avoid this the centre line 1, Figure 134, is first marked, and the arcs

Side View

End View

Fig. 134.

1

for the steps are then marked as shown. Centre lines. are also necessary to show the alignment of one part to another; thus in Figure 135 is a cube with a hole

Ө

passing through it.

Fig. 135.

The dotted lines in the side view show that the hole passes clear through the piece and is a parallel one, while the centre line, being central to the outline throughout the piece, shows that the hole is equidistant, all through, from the walls of the piece.

The pencil lines for this piece would be marked as in Figure 136, line 1 representing the centre line from which all the arcs are marked. It will be noted that

the length of the piece is marked by arcs which occur, because being a cube the set of the compasses for arcs 2, 3, 4 and 5 will answer without altering to mark arcs 6 and 7.

If the hole in the piece were a taper or conical one, it would be denoted by the dotted lines, as in Figure 137, and that the taper is central to the body is shown

он

Fig. 137.

by these dotted lines being equidistant from the centre line.

Suppose one of the sides to be tapered, as is the side A, in Figure 138, and that the hole is not central, and both facts will be shown by the centre lines 1 and 2 in the figure. The measurement of face A would be marked from A to line B at each end, but the distance the hole was out of the centre would be

marked by the distance between the centre line 2 and the edge C of the piece.

If the hole did not pass entirely through the piece, the dotted lines would show it, as in Figure 139.

Ө

Fig. 139.

The designations of the views of a piece of work depend upon the position in which the piece stands,

when in place upon the machine of which it form a part. Thus in Figure 140 is a lever, and if its shaft stood horizontal when the piece is in place in the

machine, the view given is an end one, but suppose that the shaft stood vertical, and the same view becomes a plan or top view.

In Figure 142 is a view of a lever which is a side view if the lever stands horizontal, and lever B hangs down, or a plan view if the shaft stands horizontal, but lever B stands also horizontal. We may take the same drawing and turn it around on the paper as in Figure 143, and it becomes a side view if the shaft stands vertical, and a plan view if the shaft stands horizontal and arm D vertical above it.

In a side or an end view, the piece that projects highest in the drawing is highest when upon the

A

B

Fig. 143.

machine; also in a side elevation the piece that is at the highest point in the drawing extends farthest upward when the piece is on the machine. But in a plan or top view the height of vertical pieces is not shown, as appears in the case of arm D in Figure 143.

In either of the levers, Figures 142 or 143, all the dimensions could be marked if an additional view given, but this will not be the case if an eye

were