kept as
high as possible. The process of combustion is usually complex. First,
in the case of coal, close to the fire-bars complete combustion of the
red hot carbon takes place, and the heat so developed distills the
volatile hydrocarbons and moisture in the upper layers of the fuel.
The inflammable gases ignite on or near the surface of the fuel, if
there be a sufficient supply of air, and burn with a bright flame for
a considerable distance around the boiler. If the layer of fuel be
thin, the carbonic acid formed in the first instance passes through
the fuel and mixes with the other gases. If, however, the layer of
fuel be thick, and the supply of air through the bars insufficient,
the carbonic acid is decomposed by the red hot coke, and twice the
volume of carbonic oxide is produced, and this, making its way through
the fuel, burns with a pale blue flame on the surface, the result, as
far as evolution of heat is concerned, being the same as if the
intermediate decomposition of carbonic acid had not taken place. This
property of coal has been taken advantage of by the late Sir W.
Siemens in his gas producer, where the supply of air is purposely
limited, in order that neither the hydrocarbons separated by
distillation, nor the carbonic oxide formed in the thick layer of
fuel, may be consumed in the producer, but remain in the form of crude
gas, to be utilized in his regenerative furnaces.

[Illustration: THE GENERATION OF STEAM. Fig 3.]

[Illustration: THE GENERATION OF STEAM. Fig 4.]

[Illustration: THE GENERATION OF STEAM. Fig 5.]

[Illustration: THE GENERATION OF STEAM. Fig 6.]

[Illustration: THE GENERATION OF STEAM. Fig 7.]

_(To be continued.)_

* * * * *

[Continued from SUPPLEMENT No. 437, page 6970.]

PLANETARY WHEEL-TRAINS.

By Prof. C.W. MACCORD, Sc.D.

II.

[Illustration: PLANETARY WHEEL TRAINS. Fig. 14]

It has already been shown that the rotations of all the wheels of a
planetary train, relatively to the train-arm, are the same when the
arm is in motion as they would be if it were fixed. Now, in Fig. 14,
let A be the first and F the last wheel of an _incomplete_ train, that
is, one having but one sun-wheel. As before, let these be so connected
by intermediate gearing that, when T is stationary, a rotation of A
through _m_ degrees shall drive F through _n_ degrees: and also as
before, let T in the same time move through _a_ degrees. Then, if _m'_
rep