and, by
some personal experiments of Mr. Witz's, to which we shall shortly
advert, and whence there results a sensibly equivalent production for a
motor of 100 indicated h.p., corresponding to a power of 75 effective
horses.

Before establishing, with Mr. Witz, a comparison of the two systems in
pressure, steam or gas, let us state in a few words in what the latter
consists, the steam engine and the boiler that supplies it being so well
known that no description is necessary.

The Dowson gas generator does not differ essentially from the numerous
generators devised during recent years for the manufacture of gaseous
combustibles, the use of which is so often convenient. The motor that it
supplies is the most powerful single cylinder one that has hitherto been
constructed. It is of 100 indicated h.p., and its normal angular
velocity is 100 revolutions per minute. On trial it has yielded 112
indicated h.p., and 76.8 effective h.p., corresponding to an organic
rendering of 69 per cent. This motor, elaborated by Messrs.
Delamare-Bouteville & Malandin, of Rouen, operates by compression and in
four periods, according to the Beau de Rochas cycle. We give the aspect
of it in Fig. 3. In the first period the mixture of air and gas is
sucked in, in the second it is compressed, in the third it is ignited,
and in the fourth the products of combustion are expelled.

[Illustration: FIG. 2.--SIMPLEX MOTOR, DOWSON GENERATOR OF 100 INDICATED
H.P.--ELEVATION AND PLAN.

A, cylinder; B, gas conduit; C, rubber pockets; D. gasometer; E,
purifier; F, scrubber; G, hydraulic main; H, cooling pipe; I steam
injector; K, steam boiler and superheater; L, gas generator; M, charger;
N, discharge of the motor.]

Ignition is effected electrically by a series of sparks playing between
two platinum points in the slide valve, and this permits of regulating
the instant of ignition through the edges of the orifices. The angular
velocity is regulated by a Watt's governor, which secures an isochronism
of the motion independently of the charge.

The setting in motion of so powerful an engine is effected very easily
by means of an arrangement that permits of introducing into the
cylinder, while the piston is in the center of the stroke, a mixture of
air and gas whose pressure is sufficient at the arrival to expel the
inert products. After this the ignition takes place, and the explosion
is sufficient to set the motor in motion.

The trials made by Mr. Wi