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OX PRIME-MOYEES.

CHAP. VI.

ON THE PROPERTIES OF STEAM.

Before considering the application of the steam-engine as aprime-mover, it may be interesting to know something of theproperties of steam by which it is moved, in regard to pressure,temperature, and density, as ascertained by various philosopherssince the days of Newcomen and Watt. Of late years a greatchange has gradually taken place in the system of working thesteam-engine. At the time of the introduction of the doubleacting engine of Watt, the makers of engines never dreamed ofemploying steam at a greater pressure than 10 lbs. on the squareinch, and up to 1840 that was the maximum pressure at whiclisteam-engines were worked, with the exception of a few con-structed on Wolf’s principle of double cylinders, where the steamis first admitted to the piston of the smaller cylinder at a pres-sure of 30 to 40 lbs. per square inch, and after having performedits office there, is allowed to expand into the second cylinder ofthree or four times greater capacity, and thus to unite its forcewith that of the small cylinder, as it moved from one extremityof the stroke to the other. To work this description of enginewith high-pressure steam, it was necessary to proportion thestrength of the parts of the engine as well as the boiler to amuch greater extent of pressure than in the double-acting engineof Watt. Hence it was soon found that the waggon form for thelatter, as employed by Watt, was not calculated to resist a pres-sure exceeding 10 or 12 lbs. per square inch without the intro-duction of numerous wrouglit-iron stays to retain it in form.To raise steam for the compound engine such a boiler was whollyinadequate, and a series of small boilers, with hemisphericalends, were introduced in its stead wherever steam of high-pressure was required.