Page:Popular Science Monthly Volume 24.djvu/550

534 of only eleven tons, boiler, water, engine, condenser, propeller, and shaft included.

The special feature of the boat is the enormous power developed per hundredweight of propelling machinery. The boilers evaporate eighteen pounds of water per hour per square foot of heating surface, and 1·20 pounds of coal per square foot of grate-surface. This is fully six times the amount of water and coal usually dealt with per square foot of surface in furnace and boiler. Such a forced combustion precludes all thought of economy, yet a one horse-power is secured at full speed with an expenditure of three and a half pounds of coal. The forced draught is secured by maintaining in the stoke-hole an air-pressure corresponding to a column of water six inches high; this renders the stoke-hole quite cool and comfortable.

One ton of coal will last for a run of 100 miles at a ten-knot speed. A speed of twenty-two and a half knots has been secured in trials lasting three hours. This is a speed of 2,250 feet a minute, or thirty-seven and a half feet a second, and seems almost incredible.

But, remarkable and important as these results are in the phase of steam-engineering, these little vessels have revealed in their performances under speed-trials facts of equal importance to another department. The speeds attained are high even for large steam-vessels, but enormously high for such small vessels. It is found that passing the ten and twelve knot point, which bears about the same ratio to these little boats that eighteen knots an hour does to large steamers, the ratio of resistance to the speed decreases, and at the fifteen-knot point it is about the 3-power, at the eighteen-knot point about the 3-power, and sometimes at the twenty-two-knot point is as low as the 1-power of the speed.

Effort has been frequently made to utilize steam at much higher pressures than I have mentioned, but, owing to the solvent nature of steam or water at a high temperature, the results have not been satisfactory; among many difficulties encountered was that of lubricating the cylinders.

Loftus Perkins, an English engine-builder of prominence, is devoting much time to the use of steam at about five hundred pounds pressure, and with some success. Unfortunately, the gain to be anticipated from the use of these exceedingly high pressures does not seem to be very great on trial. The Anthracite, a small steamer fitted with engines and boilers specially adapted to the utilization of steam at five hundred pounds pressure, was more wasteful than many steamers using steam at one hundred pounds. However, here is a wide field and one that promises well.

Should the same change of law as to the resistance increasing as the square of the speed be found to hold good in large steamers as in the little torpedo-boats, we shall most of us live to see locomotive speeds at sea. There is now building in this country an engine which