8o

MODERN STEAM PRACTICE.

nothing but pure water falls to the ground in a gentle shovver. Thus,wherever the steam comes in contact with cold surfaces in its passageto the cylinder, condensation takes place, and it is robbed of anamount of heat, and consequently pressure, thus wasting much valu-able fuel.

The Saturation of steam with watery particles, however, is notentirely due to condensation, as there are various other causes atwork; for instance, when the steam-room in a boiler is not ofsufficient height above the water in the boiler, the violent ebullitionthat goes on has a tendency to surcharge the steam with water.Again, if the steam is taken away from one end of the boiler, insteadof from immediately over the parts where it is generated, the sameresult takes place, the atoms rubbing, too, against one another, inflowing towards one point, has a great tendency to charge the steamwith water. Violent priming, whether from the want of circulationof the water in the boiler, owing to defective construction, or by asudden change of water injected into the boiler, surcharges steamwith water to an aggravated degree. When the boilers, steam-pipes,and cylinders are not properly clothed, condensation takes place, andwatery particles will be mixed with the steam to a large extent, thusreducing its pressure.

Many schemes have been devised to superheat the steam inmarine boilers by the waste heat in the smoke-box, or uptake, withthe view of delivering it into the cylinder in a dry state. Manyof these superheaters have been fitted to boilers defective in con-struction. Some authorities are of opinion that the best place fordrying the steam is in the boiler itself, drawing it from high steam-chests and uptakes, thus the steam is taken away at a greater heightfrom the level of the water in the boiler, while the heated gases fromthe tubes have time to act on the lofty uptake contained in thesteam-chest, and drying the steam sufficiently for all practical pur-poses. In many cases it is. not convenient to form lofty steam-chests, and then other means must be adopted for drying the steam,separate vessels, termed superheaters, being used for that purpose.The steam dried by such contrivances generally receives 80° ofsuperheat above the temperature of the steam in the boiler, this isconsidered, with fine lubricants, a good working temperature, that isto say, steam of 60 Ibs. pressure has 295 0 Fahr., thus the total tem-perature will be 295° -j- 8o° = 375 0 Fahr.; but it should be borne inmind that the best oil or grease must be used as the lubricant, other-