e aeroplane to-day, one of the structural
triumphs of the world in its lightness and its strength, has a factor
of safety which is sufficient to meet, and to withstand, not merely
ordinary strains, but any such abnormal stresses as it may
encounter--and which may be many times greater than the strains of
normal flight.

The aviator knows also that his engine, as it gives him power to
combat successfully his treacherous enemy, the wind, represents the
fruit of many tests and of many failures, and of the spending of
hundreds of thousands of pounds. Many of its defects have revealed
themselves, and been rectified; it is no longer light where it should
have weight of metal, nor weak where it should be strong. So far as
any piece of mechanism can be made reliable, consisting as it does of
a large number of delicate parts, operating at high speed, the
aeroplane motor has been made reliable. But, so long as one motor is
used, there must always, as we have said, remain a risk of breakdown.
It is for this reason that, thanks largely to the stimulus of the
war--which has created a practical demand for such machines--aeroplanes
are now being built, and flown with success, which are fitted with
duplicate motors. With such machines, which give us a first insight as
to the aircraft of the future, engine failure begins to lose its
perils--particularly in regard to war. More than once during the great
campaign, when flying a single-engine machine, an aviator has found
his motor fail him, and has been obliged to land on hostile soil; with
the result that he has been made prisoner. But with dual-engine
machines it has been found that, when one motor has failed
mechanically, or has been put out of action by shrapnel, the remaining
unit has been sufficient--though the machine has flown naturally at a
reduced rate--to enable the pilot to regain his own lines.

In peace flying, too, as well as in war, the multiple-engined
aeroplane brings a new factor of safety. If one of his motors fails,
and he is over country which offers no suitable landing-place, the
pilot with a duplicate power-plant need not be concerned. His
remaining unit or units will carry him on. There are problems with
duplicate engines which remain to be solved--problems of a technical
nature--which involve general efficiency, transmission gear, and the
number and the placing of propellers; but already, though this new
stride in aviation is in its earliest infancy, results th