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to escape into the funnel of the engine to increase the draught, an expedient which was so largely responsible for Stephenson's success with his locomotive twenty years later, and which retains its utility in the case of the most highly developed modern locomotive.

Trevithick was, however, entirely subordinated by the great influence of Watt, and the use of high pressure was in consequence discountenanced by the leading mechanical engineers of England for some decades. Meantime, in America, the initiative of Evans led to a much earlier general use of high-pressure steam. In due course, however, the advantages of steam under high pressure became evident to engineers everywhere, and its conquest was finally complete.

The essential feature of super-heated steam is that it contains, as the name implies, an excess of heat beyond the quantity necessary to produce mere vaporization, and that the amount of water represented in this vapor is not the maximum possible under given conditions. In other words, the vapor is not saturated. It has been already explained that the amount of vapor that can be taken up in a given space under a given pressure varies with the temperature of the space. Under normal conditions, when a closed space exists above a liquid, evaporation occurs from the surface of the liquid until the space is saturated, and no further evaporation can occur so long as the temperature and pressure are unchanged. If now the same space is heated to a higher degree, more vapor will be taken up until again the point of saturation is attained. But, obviously, if the space were disconnected with the liquid, and

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OLD IDEAS AND NEW APPLIED TO BOILER CONSTRUCTION.

The lower figure shows Robert Trevethick's famous boiler, used in operating his locomotive about the year 1804. The original is preserved in the South Kensington Museum, London. The upper figure shows a modern tubular boiler, by way of contrast.

then heated, it would acquire a capacity to take up more vapor, and so long as this capacity was latent, the vapor present would exist in a super-heated condition.

It will be understood from what has been said before, that with all accessions of heat, the expansive power of the vapor is increased,-its molecules becoming increasingly active; hence one of the very obvious advantages of super-heated steam for the purpose of pushing a piston. There are other advantages, however, which are not at first sight so apparent, having to do with the properties of condensation. To understand these, we must pay heed for a few moments to the changes that take place in steam itself in the course of its passage through the cylinder, where it performs its work upon the piston.

Many of these changes were not fully understood by the earlier experimenters, including Watt. Indeed the theory of the steam engine, or rather the general theory of the heat engine, was not worked out until the year 1824, when the Frenchman Carnot took the subject in hand, and performed a series of classical experiments, which led to a nearly complete theoretical exposition of the subject. It remained, however, for the students of thermo-dynamics, about the middle of the nineteenth century, with Clausius and Rankine at their head, to perfect the theory of the steam engine, and the general subject of the mutual relations of heat and mechanical work.

We are not here concerned with any elaboration of details, but merely with a few of the essential principles which enter practically into the operation of the steam

engine. It appears, then, that when steam enters the cylinder and begins to thrust back the piston of the steam engine, a portion of the steam is immediately condensed on the walls of the cylinder, owing to the fact that previous condensation of steam has cooled these walls to a certain extent. We have already pointed out that Watt endeavored in his earlier experiments to overcome this difficulty, by equalizing the temperature of the cylinder walls to the greatest practicable extent.

Notwithstanding his efforts, however, and those of numberless later experimenters, it still remains true that under ordinary conditions, particularly if steam enters the cylinder at the saturation point, a very considerable condensation occurs. Indeed this may amount to from thirty to fifty per cent. of the entire bulk of water contained in the quantity of steam that enters the cylinder. This condensation obviously militates against the expansive or working power of the steam. But now as the steam expands, pushing forward the cylinder, it becomes correspondingly rarefied, and immediately a portion of the condensed steam becomes again vaporized, and in so doing it takes up a certain amount of heat and renders it latent. This disadvantageous cycle of molecular transformations is very much modified in the case of super-heated steam, for the obvious reason that such steam may be very much below the saturation point, and hence requires a very much greater lowering of temperature in order to produce condensation of any portion of its mass. Without elaborating details, it suffices to note that in all highly efficient modern engines, steam is employed at a rela

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