safe as possible, yet, though
both safety and economy of running have been improved, much remains to
be done. Safety depends largely on engine reliability, fire prevention
and the capacity of the machine to land in small spaces.

Though neither roads nor rails have to be laid and aircraft possess the
great advantages of mobility and point to point transit, the initiation
and maintenance of an air service is a very complex and costly matter.
The utilization of converted war machines is no longer sufficient and
those specially designed for commercial work are beginning to make their
appearance. Such are the Handley Page W.8, the Vickers, the D.H.18 and
34, and the Bristol 10-seater.

The first two are twin-engine and the last three single-engine machines.
Opinions differ as to the relative advantages of the twin and
single-engine type. The first and running costs of the single engine are
lower, but the twin has greater power and carrying capacity, while most
pilots prefer to have a surplus of power over and above that required
for normal flight. For these reasons, and because of the psychological
effect on insurance companies and on passengers, the twin engine will
probably remain in use for large commercial machines, until long-lived
and economic engines of more than 500 horse-power are available. On the
other hand, where extra power is not required, the twin-engine is not
safer than the single-engine machine; no existing twin-engine commercial
aeroplane can maintain its height and land safely with only one engine
running. Experiments have been made, especially in Germany, on the
multi-engined machine with all the engines in the fuselage, but its
advantages have so far been counterbalanced by loss of efficiency due to
transmission gearing and shaft drives to the propellers and the
vibration and weight of the gearing.

High-powered engines are very expensive to run and every effort has
therefore to be made by aerodynamic efficiency to carry more useful load
with less horse-power. Improvement is being made in this direction; thus
the D.H.18 carries eight passengers at 56 horse-power per passenger, the
D.H.32 is designed for the same number at 45 horse-power each, and the
D.H.34 for ten passengers at 45 horse-power each.

The two best German commercial machines, the Junkers and the Fokker,
have a comparatively low horse-power and a low fuel load, but greater
attention has been paid to the design of the machines, which ar