MARINE ENGINES.

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densation, we shall notice the disadvantages attending the injectionSystem for the condensing of steam in marine engines. The chiefobjection doubtless arises from the necessity for using a continuoussupply of salt water in the boilers, the salt accumulating to suchan extent that a high degree of heat is required to raise the steam.This accumulation of salt proceeds so rapidly, that it is necessaryto blow off the water in the boiler every two hours or so, the feedfrom the hot well at the same time is turned on, thus the, hot brinebeing blown off, and replaced with a colder fluid, the temperatureof the water in the boiler is greatly reduced, and requires muchvaluable fuel to keep it up to the proper working point. The rapidincrustation that takes place is likewise a serious objection, as thescale formed all round the parts immediately exposed to the actionof the flame is a very bad conductor of heat, and not only impedesthe free transmission of the heat to the water, but in many partsof the boiler it forms to such an extent that rupture of the platestakes place, more especially on the back parts of the furnaces wherethe flame returns through the tubes—necessitating frequent inspec-tion, for the purpose of cleaning the boilers, and removing theincrustations, so as to prevent the plates wearing out too rapidly.In fact, for the high-pressure compound engine System, the injectioncondenser has been discarded, because distilled water is preferableto impure salt water; and with proper precautions we can safelyadopt high-pressure steam with fresh water, and thereby save muchvaluable fuel.

The action of surface condensation may be familiarly illustratedby the well known deposition of moisture on the Windows of acrowded room, due to the cooling surface of the glass. So withthe steam from the cylinder: surface must be provided, and coldmust be applied to that surface, so that with cold on the one side,and the steam impinging against the cold surface on the other, thecaloric is extracted, and water flows down, similar to that on thewindow. Thus when the boilers are provided in the first instancewith pure water, it is used over and over again, with just sufficientwater injected from the sea to meet the waste, and keep the densityof the water in the boiler at about the same as the water in theocean, this being considered in practice very safe. And as waterrequires a large surface in the boiler for the heat to act upon it inraising steam, so in condensing the steam rapidly we must have alarge amount of cold surface for it likewise. The surface condenser