en the strut or spar suffers in strength much more than one might
think at first sight; and, if it ever gets a tendency to bend, it is
likely to collapse at that point.

4. The wood must have a good, clear grain with no cross-grain, knots,
or shakes. Such blemishes produce weak places and, if a tendency to bend
appears, then it may collapse at such a point.

5. The struts, spars, etc., must be properly bedded into their sockets
or fittings. To begin with, they must be of good pushing or gentle
tapping fit. They must never be driven in with a heavy hammer. Then
again, a strut must bed well down all over its cross-sectional area
as illustrated above; otherwise the stress of compression will not be
evenly disposed about the centre of strength, and that may produce a
bending stress. The bottom of the strut or spar should be covered
with some sort of paint, bedded into the socket or fitting, and then
withdrawn to see if the paint has stuck all over the bed.

6. The atmosphere is sometimes much damper than at other times, and this
causes wood to expand and contract appreciably. This would not matter
but for the fact that it does not expand and contract uniformly, but
becomes unsymmetrical, i.e., distorted. I have already explained
the danger of that in condition 2. This should be minimized by WELL
VARNISHING THE WOOD to keep the moisture out of it.

FUNCTION OF INTERPLANE STRUTS.--These struts have to keep the lifting
surfaces or "planes" apart, but this is only part of their work. They
must keep the planes apart, so that the latter are in their correct
attitude. That is only so when the spars of the bottom plane are
parallel with those of the top plane. Also, the chord of the top plane
must be parallel with the chord of the bottom plane. If that is not so,
then one plane will not have the same angle of incidence as the other
one. At first sight one might think that all that is necessary is to cut
all the struts to be the same length, but that is not the case.

Sometimes, as illustrated above, the rear spar is not so thick as the
main spar, and it is then necessary to make up for that difference by
making the rear struts correspondingly longer. If that is not done, then
the top and bottom chords will not be parallel, and the top and bottom
planes will have different angles of incidence. Also, the sockets or
fittings, or even the spars upon which they are placed, sometimes
vary in thickness owing to faulty manufactur