Now our large screw-steamers are of higher speed than any paddle-steamers on the ocean, and develop their power at far less cost. This increased economy is due not only to the use of a more efficient propelling instrument, and to changes already described, but also, in a great degree, to the economy which has followed as a consequence of other changes in the steam-engine driving it. The earliest days of screw-propulsion witnessed the use of steam of from 5 to 15 pounds pressure, in a geared engine using jet-condensation, and giving a horse-power at an expense of perhaps 7 to 10, or even more, pounds of coal per hour. A little later came direct-acting engines with jet-condensation and steam at 20 pounds pressure, costing about 5 or 6 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 improved. The introduction of the surface-condenser, which began to be generally adopted some ten years ago, brought down the cost of power to from 3 to 4 pounds in the better class of engines. At about the same time, this change to surface-condensation helping greatly to overcome those troubles arising from boiler-incrustation which had prevented the rise of steam-pressure above about 25 pounds per square inch, and as, at the same time, it was learned by engineers that the deposit of lime-scale 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 75 pounds per square inch is very usual, and more than 125 pounds has since been attained. The close of this period was marked by the construction of the most successful types of paddle-steamers, the complete success of transoceanic steam-transportation, the introduction of the screw-propeller and the peculiar engine appropriate to it, and, finally, a general improvement, which had finally become marked both in direction and in rapidity of movement, leading toward the use of higher steam-pressure, greater expansion, lighter and more rapidly-working machinery, and decidedly better design and construction, and the use of better material. The result of these changes was seen in economy of first cost and maintenance, and the ability to attain greater speed, and to assure greater safety to passengers and less risk to cargo.