We have now to consider how this energy is transformed. When the steam in the cylin der of an engine performs work by pushing the piston against a resistance, that work robs the steam of a portion of its heat, hence the steam is condensed. Theoretically, it requires two and a half pounds of steam, saturated, to supply one horse power of work each hour; but practically, from 5 to 25 times that amount is required to pass through an engine in order to secure this result.
This is due to the loss of the energy of the steam, in giving up its heat to the walls of the cylinder, and to the immense portion of the steam which acts only as backing. In the best quadruple engines of to-day less than one-fifth of the energy of the steam is con verted into actual work, and in the best non condensing engines only one-tenth. The waste is enormous, but the abundance of the supply in part compensates for it.
According to the 1914 census of manufac tures the number of steam engines and steam turbines employed in manufacturing in the United States was 137,176, and the total horse power developed was 15,681,688 or 69.5
per cent of all forms of power in the country. A large amount of steam is used, in addition to this, for heating purposes. Its value in this respect is due to its being able to carry more heat for a given weight than any other substance, and when it has given up its heat to drop out of the way, by condensation, and make way for a fresh supply. Steam is also used extensively in extracting processes,, It is often injected directly into a liquid, bub bling through it and carrying over certain con stituents to be released in the condensing chamber. In other cases, especially those of liquids boiling at a lower temperature than water, coils of steam pipes supply the mild de gree of heat needed for a gentle distillation. Consult Goodenough, G. A., 'Properties of Steam and Ammonia' (New York 1915) ; Hausbrand, E., and Wright, A. C., 'Evaporat ing, and Cooling Apparatus' (London 1916) ; Holmes, U. T., 'Experimental Engineering' (Annapolis 1911).