e accuracy of _both_ its edges must then be
tested with a thread.

The first right angle made (for the corner of the form) must also be
tested by raising a perpendicular, with a radius of the compasses not
less than 6 inches and with a needle-pointed pencil, and by the
subjoined formula and no other.

From a given point in a given straight line to raise a perpendicular.
Let A B be the given straight line (this must be the _long_ side of the
form, and the point B must be one corner of the base-line): it is
required to raise from the point B a line perpendicular to the line A B.

[Illustration: FIG. 71.]

(1) Prolong the line A B at least 6 inches beyond B (if there is not
room on the paper, it must be pinned on to a smooth board, and a piece
of paper pinned on, so as to meet the edge of it, and continue it to the
required distance).

(2) With the centre B (the compass leg being in all cases placed with
absolute accuracy, using a lens if necessary to place it) describe the
circle C D E.

(3) With the centres C and E, and with a radius of not less than 9
inches, describe arcs intersecting at F and G.

(4) Join F G.

Then, if the work has been correctly done, the line F G will _pass
through the point_ B, and be perpendicular to the line A B. If it does
not do so, the work is incorrect, and must be repeated.

When the base and the springing-line are drawn on the form, the form
must be accurately measured from the bottom upwards, and _every foot
marked on both sides_. Such markings to be in fine pencil-line, and
to be drawn from the sides of the form to the extreme margin of the
paper, and you are not to trust your eye by laying the lath flat down
and ticking off opposite the inch-marks, but you are to stand the lath
on its edge, so that the inch-marks actually meet the paper, and then
tick opposite to them.

Also if there are any bars in the window to be observed, the places of
these must be marked, and it must be made quite clear whether the mark
is the middle of the bar or its edge; and all this marking must be done
lightly, but very carefully, with a needle-pointed pencil.

In every case where the forms are set out from templates, the accuracy
of the templates must be verified, and in the event of the base not
being at right angles with the side, a true horizontal must be made from
the corner which is higher than the other (the one therefore which has
the obtuse angle) and marked within the untrue line; and