3.  For the same reason the cooling of the agent must be carried to as low a degree as possible.

4.  Matters must be so arranged that the passage of the elastic agent from the higher to the lower temperature must be due to an increase of volume, that is to say, the cooling of the agent must be caused by its rarefaction.

This last proposition indicates the defective information which Carnot possessed.  He knew that expansion of the elastic agent was accompanied by a fall of temperature, but he did not know that that fall was due to the conversion of heat into work.  We should state this clause more correctly by saying that “the cooling of the agent must be caused by the external work it performs.”  In accordance with these propositions, it is immaterial what the heated gases or vapors in the furnace of a boiler may be, provided that they cool by doing external work and, in passing over the boiler surfaces, impart their heat energy to the water.  The temperature of the furnace, it follows, must be 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.