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 The gravity system is a comparatively recent development, the earliest steam-heating systems being generally auxiliary to a steam-power plant. In these the steam was taken direct from the boiler supplying the engine, and the return water was run through a steam trap into an open tank, from which the water supply was taken for the boilers. This method is still employed in many old plants.

Exhaust steam heating.—The greater economy in high-pressure steam for engines, however, gradually increased the boiler pressure used for steam power, and with this increase in pressure it became difficult to heat a building directly from the same boiler that supplied steam to the engines, as steam heating at high pressure was found unsatisfactory for many reasons, principally on account of the very high temperature of the radiators and the liability to leaks and the increased danger from water-hammer. And furthermore, the same desire for greater economy which had increased engine pressures drew the attention of steam users to the value of the latent heat in exhaust steam for heating purposes.

A brief study of the steam engine shows us that not much over 12 per cent. of the heat energy supplied to an engine is transformed into mechanical work, and by far the major part of the wasted heat escapes in the latent heat of the exhaust steam. This heat, though it has been thus far impossible to transform it into mechanical energy, is readily available for heating purposes; but a generation ago, when it was first proposed to use exhaust steam for heating, the problem involved the then serious question of back pressure on the engines. Heating systems at that time were built to accommodate the high pressure then in use and with what would now be called very small pipes, and admitting exhaust steam into such a system required a considerable pressure on the exhaust side of the engine to force steam through the piping and radiators and the water of condensation out through the returns.

The back pressure necessary frequently amounted to 10 or 15 pounds per square inch, and certainly made a decided reduction in the economy of the engine. It at once became a question whether the saving by using exhaust steam exceeded the loss on account of the back pressure on the engine. If the back pressure was very high in comparison to the mean pressure in the engine cylinder there might be difficulties in the practical operation of the engine; but as far as the theoretical consideration of the coal