ction with the inspection and
testing of aircraft parts, particularly in the case of metal, was the
experimental application of X-ray photography, which showed up latent
defects, both in the material and in manufacture, which would otherwise
have passed unnoticed. This method was also used to test the penetration
of glue into the wood on each side of joints, so giving a measure of the
strength; and for the effect of 'doping' the wings, dope being a film
(of cellulose acetate dissolved in acetone with other chemicals)
applied to the covering of wings and bodies to render the linen taut and
weatherproof, besides giving it a smooth surface for the lessening of
'skin friction' when passing rapidly through the air.

An important result of this experimental work was that it in many cases
enabled designers to produce aeroplane parts from less costly material
than had previously been considered necessary, without impairing the
strength. It may be mentioned that it was found undesirable to use
welded joints on aircraft in any part where the material is subjectto
a tensile or bending load, owing to the danger resulting from bad
workmanship causing the material to become brittle--an effect which
cannot be discovered except by cutting through the weld, which, of
course, involves a test to destruction. Written, as it has been, in
August, 1920, it is impossible in this chapter to give any conception of
how the developments of War will be applied to commercial aeroplanes,
as few truly commercial machines have yet been designed, and even those
still show distinct traces of the survival of war mentality. When,
however, the inevitable recasting of ideas arrives, it will become
evident, whatever the apparent modification in the relative importance
of different aspects of design, that enormous advances were made under
the impetus of War which have left an indelible mark on progress.

We have, during the seventeen years since aeroplanes first took the air,
seen them grow from tentative experimental structures of unknown and
unknowable performance to highly scientific products, of which not
only the performances (in speed, load-carrying capacity, and climb) are
known, but of which the precise strength and degree of stability can be
forecast with some accuracy on the drawing board. For the rest, with
the future lies--apart from some revolutionary change in fundamental
design--the steady development of a now well-tried and well-found
engineer