Page:Popular Science Monthly Volume 12.djvu/554

536 York to Queenstown in a week, making their passages with wonderful regularity, and developing their power at far less cost than the old side-wheel steamers. This increased economy is due, not only to the use of a more efficient propelling instrument, but, perhaps in even a higher degree, to the economy which has followed as a consequence of the accompanying changes in structure of the steam-engine driving it.

101. The earliest clays of screw propulsion witnessed the use of steam of ten or fifteen pounds' pressure, in a geared engine using jet condensation, and giving a horse-power at an expense of, perhaps, seven or eight pounds of coal per hour.

A little later came direct-acting engines with jet condensation, and steam at twenty pounds' pressure, costing about five or six pounds per horse-power per hour. The steam-pressure rose a little higher with the use of greater expansion, and the economy of fuel was further increased. The introduction of the surface-condenser, which began to be generally adopted some ten or fifteen years ago, brought down the cost of power to between three and four pounds in the better class of engines.

At about the same time, this change to surface-condensation helping greatly to overcome the troubles arising from boiler-incrustation, which had checked the rise in steam-pressure above about twenty-five pounds, and, it being at the same time learned by engineers that the deposit of the scale and sulphate of lime in the marine boiler was determined by temperature rather than by the degree of concentration, and that all the lime entering the boiler was deposited at the pressure just mentioned, a sudden advance took place.

Careful design, good workmanship, and skillful management, made the surface-condenser an efficient apparatus, and, the dangers of incrustation being thus lessened, the movement toward higher pressures recommenced and progressed so rapidly that, now, seventy-five pounds per square inch is very usual, and two hundred and fifty pounds has been attained in marine engines built by the Messrs. Perkins, who are said to have reached the remarkable economy of a horse-power for each pound of combustible consumed in the boiler.

102. These high pressures, and the greater expansion of the steam, have, in turn, produced another revolution in engine-construction.

It has at last become generally known, as was seen by well-informed and scientific engineers long ago, that one of the most serious losses of heat, and consequently of power, in the steam-engine, when expansion is carried to a considerable extent, occurs in consequence of condensation and the deposition of moisture upon the interior of the cylinder, which moisture, when the exhaust takes place, carries, by its reëvaporation, large quantities of heat into the condenser, without deriving any power from it.

The steam-jacket furnishes one means of reducing the amount of