There is little difference, except in detail, between the _Arrow's_ machinery and an ordinary propeller tugboat. Her hull is very light for its strength, and it was so built as to slip easily through the water. She has twin engines, each operating its own shaft and propeller. These are quadruple expansion. The steam, instead of being allowed to escape after doing its work in the first cylinder, is turned into a larger one and then successively into two more, so that all of its expansive power is used. After passing through the four cylinders, the steam is condensed into water again by turning it into pipes around which circulates the cool water in which the vessel floats. The steam thus condensed to water is heated and pumped into the boiler, to be turned into steam, so the water has to do its work many times. All this saves weight and, therefore, power, for the lighter a vessel is the more easily she can be driven. The boilers save weight also by producing steam at the enormous pressure of 400 pounds to the square inch. Steadily maintained pressure means power; the greater the pressure the more the power. It was the inventive skill of Charles D. Mosher, who has built many fast yachts, that enabled him to build engines and boilers of great power in proportion to their weight. It was the ability of the inventor to build boilers and engines of 4,000 horse-power compact and light enough to be carried in a vessel 130 feet long, of 12 feet 6 inches breadth, and 3 feet 6 inches depth, that made it possible for the _Arrow_ to go a mile in one minute and thirty-two seconds. The speed of the wonderful little American boat, however, was not the result of any new invention, but was due to the perfection of old methods. In England, about five years before the _Arrow's_ achievement, a little torpedo-boat, scarcely bigger than a launch, set the whole world talking by travelling at the rate of thirty-nine and three-fourths miles an hour. The little craft seemed to disappear in the white smother of her wake, and those who watched the speed trial marvelled at the railroad speed she made. The _Turbina_--for that was the little record-breaker's name--was propelled by a new kind of engine, and her speed was all the more remarkable on that account. C.A. Parsons, the inventor of the engine, worked out the idea that inventors have been studying for a long time--since 1629, in fact--that is, the rotary principle, or the rolling movement without the up-and-down driving mechanism of the piston.