y the stress of compression. The wood between the
centre and bottom lines is then in compression; and the nearer the
centre of the circle, the greater the strain, and consequently the
greater the compression.

It then follows that there is neither tension nor compression, _i.e._,
no stress, at the centre line, and that the wood immediately surrounding
it is under considerably less stress than the wood farther away. This
being so, the wood in the centre may be hollowed out without unduly
weakening struts and spars. In this way 25 to 33 per cent. is saved in
the weight of wood in an aeroplane.

The strength of wood is in its fibres, which should, as far as possible,
run without break from one end of a strut or spar to the other end. A
point to remember is that the outside fibres, being farthest removed
from the centre line, are doing by far the greatest work.

SHEAR STRESS is such that, when material collapses under it, one part
slides over the other. Example: all the locking pins.

[Illustration]

Some of the bolts are also in shear or "sideways" stress, owing to lugs
under their heads and from which wires are taken. Such a wire, exerting
a sideways pull upon a bolt, tries to break it in such a way as to make
one piece of the bolt slide over the other piece.

TORSION.--This is a twisting stress compounded of compression, tension,
and shear stresses. Example: the propeller shaft.

NATURE OF WOOD UNDER STRESS.--Wood, for its weight, takes the stress
of compression far better than any other stress. For instance: a
walking-stick of less than 1 lb. in weight will, if kept perfectly
straight, probably stand up to a compression stress of a ton or more
before crushing; whereas, if the same stick is put under a bending
stress, it will probably collapse to a stress of not more than about
50 lb. That is a very great difference, and, since weight is of the
greatest importance, the design of an aeroplane is always such as to, as
far as possible, keep the various wooden parts of its construction in
direct compression. Weight being of such vital importance, and designers
all trying to outdo each other in saving weight, it follows that the
factor of safety is rather low in an aeroplane. The parts in direct
compression will, however, take the stresses safely provided the
following conditions are carefully observed.

CONDITIONS TO BE OBSERVED:

1. _All the spars and struts must be perfectly straight._

[Illustration]

The