iven in this book are to be considered only as
approximations to exact truth. All were made with a view, not to some
remote future, but to an arrival within the compass of a few years at
some result in actual flight that could not be gainsaid or mistaken.'

With a series of over thirty rubber-driven models Langley demonstrated
the practicability of opposing curved surfaces to the resistance of the
air in such a way as to achieve flight, in the early nineties of last
century; he then set about finding the motive power which should permit
of the construction of larger machines, up to man-carrying size. The
internal combustion engine was then an unknown quantity, and he had to
turn to steam, finally, as the propulsive energy for his power plant.
The chief problem which faced him was that of the relative weight and
power of his engine; he harked back to the Stringfellow engine of 1868,
which in 1889 came into the possession of the Smithsonian Institution
as a historical curiosity. Rightly or wrongly Langley concluded on
examination that this engine never had developed and never could
develop more than a tenth of the power attributed to it; consequently
he abandoned the idea of copying the Stringfellow design and set about
making his own engine.

How he overcame the various difficulties that faced him and constructed
a steam-engine capable of the task allotted to it forms a story in
itself, too long for recital here. His first power-driven aerodrome
of model size was begun in November of 1891, the scale of construction
being decided with the idea that it should be large enough to carry an
automatic steering apparatus which would render the machine capable of
maintaining a long and steady flight. The actual weight of the first
model far exceeded the theoretical estimate, and Langley found that a
constant increase of weight under the exigencies of construction was a
feature which could never be altogether eliminated. The machine was made
principally of steel, the sustaining surfaces being composed of silk
stretched from a steel tube with wooden attachments. The first engines
were the oscillating type, but were found deficient in power. This led
to the construction of single-acting inverted oscillating engines with
high and low pressure cylinders, and with admission and exhaust ports
to avoid the complication and weight of eccentric and valves. Boiler and
furnace had to be specially designed; an analysis of sustaining surfaces