water heater, was then constructed. This boiler is about 4 1/2 feet
wide at the bottom, 8 feet long and 6 feet high. It weighs, with the
casing, the dome, and the smoke stack and connections, a little less
than 1,000 lbs. The water first passes through a system of small
tubes--1/4 inch in diameter and 1/60 inch thick--which were placed at
the top of the boiler and immediately over the large tubes.... This
feed-water heater is found to be very effective. It utilises the heat
of the products of combustion after they have passed through the boiler
proper and greatly reduces their temperature, while the feed-water
enters the boiler at a temperature of about 250 F. A forced circulation
is maintained in the boiler, the feed-water entering through a spring
valve, the spring valve being adjusted in such a manner that the
pressure on the water is always 30 lbs. per square inch in excess of
the boiler pressure. This fall of 30 lbs. in pressure acts upon the
surrounding hot water which has already passed through the tubes, and
drives it down through a vertical outside tube, thus ensuring a positive
and rapid circulation through all the tubes. This apparatus is found to
act extremely well.'

Thus Maxim, who with this engine as power for his large aeroplane
achieved free flight once, as a matter of experiment, though for what
distance or time the machine was actually off the ground is matter for
debate, since it only got free by tearing up the rails which were to
have held it down in the experiment. Here, however, was a steam engine
which was practicable for use in the air, obviously, and only the rapid
success of the internal combustion engine prevented the steam-producing
type from being developed toward perfection.

The first designers of internal combustion engines, knowing nothing
of the petrol of these days, constructed their examples with a view to
using gas as fuel. As far back as 1872 Herr Paul Haenlein obtained a
speed of about 10 miles an hour with a balloon propelled by an internal
combustion engine, of which the fuel was gas obtained from the balloon
itself. The engine in this case was of the Lenoir type, developing
some 6 horse-power, and, obviously, Haenlein's flights were purely
experimental and of short duration, since he used the gas that sustained
him and decreased the lifting power of his balloon with every stroke of
the piston of his engine. No further progress appears to have been made
with the gas-consuming t