e squares of equal size giving one a unit of measurement
by which all parts can be scaled.

[Illustration: Diagram III.

A DEVICE FOR ENABLING STUDENTS TO OBSERVE APPEARANCES AS A FLAT SUBJECT]

[Sidenote: Fixing Positions of Salient Points]

Vertical and horizontal lines are also of the utmost importance in that
first consideration for setting out a drawing, namely the fixing of
salient points, and getting their relative Positions. Fig. Z, on page
87 [Transcribers Note: Diagram IV], will illustrate what is meant. Let
A B C D E be assumed to be points of some importance in an object you
wish to draw. Unaided, the placing of these points would be a matter of
considerable difficulty. But if you assume a vertical line drawn from A,
the positions of B, C, D, and E can be observed in relation to it by
noting the height and length of horizontal lines drawn from them to this
vertical line. This vertical can be drawn by holding a plumb line at
arm's length (closing one eye, of course) and bringing it to a position
where it will cover the point A on your subject. The position of the
other points on either side of this vertical line can then be observed.
Or a knitting-needle can be held vertically before you at arm's length,
giving you a line passing through point A. The advantage of the needle
is that comparative measurements can be taken with it.

[Illustration: Diagram IV.

SHOWING THREE PRINCIPLES OF CONSTRUCTION USED IN OBSERVING FIG. X,
MASSES; FIG. Y, CURVES; FIG. Z, POSITION OF POINTS]

In measuring comparative distances the needle should always be held at
arm's length and the eye kept in one position during the operation; and,
whether held vertically or horizontally, always kept in a vertical
plane, that is, either straight up and down, or across at right angles
to the line of your vision. If these things are not carefully observed,
your comparisons will not be true. The method employed is to run the
thumb-nail up the needle until the distance from the point so reached to
the top exactly corresponds with the distance on the object you wish to
measure. Having this carefully noted on your needle, without moving the
position of your eye, you can move your outstretched arm and compare it
with other distances on the object. #It is never advisable to compare
other than vertical and horizontal measurements.# In our diagram the
points were drawn at random and do not come in any obvious mathematical
relationship, and this