aterial difference to the flying of the machine, but quite
bad enough to throw out the angle of incidence, which cannot be
corrected at such a place.

If the angle is wrong, it should then be corrected as follows:

If it is too great, then the rear spar must be warped up until it is
right, and this is done by slackening _all_ the wires going to the top
of the strut, and then tightening _all_ the wires going to the bottom
of the strut.

If the angle is too small, then slacken _all_ the wires going to the
bottom of the strut, and tighten _all_ the wires going to the top of the
strut, until the correct adjustment is secured.

Never attempt to adjust the angle by warping the main spar.

The set measurement, which is of course stated in the aeroplane's
specifications, should be accurate to 1/16 inch.

LATERAL DIHEDRAL ANGLE.--One method of securing this is as follows,
and this method will, at the same time, secure the correct angle of
incidence:

[Illustration: FRONT ELEVATION and PLAN.]

The strings, drawn very tight, must be taken over both the main and
rear spars of the top surface. They must run between points on the
spars just inside the outer struts. The set measurement (which should
be accurate to 1/16 inch or less) is then from the strings down to four
points on the main and rear spars of the centre-section surface. These
points should be just inside the four centre-section struts; that is
to say, as far as possible away from the centre of the centre-section.
Do not attempt to take the set measurement near the centre of the
centre-section.

The strings should be as tight as possible, and, if it can be arranged,
the best way to accomplish that is as shown in the above illustration,
_i.e._, by weighting the strings down to the spars by means of weights
and tying their ends to struts. This will give a tight and motionless
string.

However carefully the above adjustment is made, there is sure to be some
slight error. This is of no great importance, provided it is divided
equally between the left- and right-hand wings. In order to make sure
of this, certain check measurements should be taken as follows:

Each bay must be diagonally measured, and such measurements must be the
same to within 1/16 inch on each side of the aeroplane. As a rule such
diagonal measurements are taken from the bottom socket of one strut to
the top socket of another strut, but this is bad practice, because of
possible inaccuracies due