536 THE POPULAR SCIENCE MONTHLY. 



York to Queenstown in a week, making their passages with wonder- 

 ful 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 driv- 

 ing 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 help- 

 ing greatly to overcome the troubles arising from boiler-incrustation, 

 which had checked the rise in steam-pi-essure 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 de- 

 termined 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. Per- 

 kins, 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-in- 

 formed 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 reevaporation, large quantities of heat into the condenser, with- 

 out deriving any power from it. 



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



