Another curious form of propulsion, which dates back to the eighteenth century and is still periodically revived by inventors, is the water jet. The idea was to have the engine operate a pump which would drive a stream of water out of the stern of the boat and drive the boat by reaction. The British Government actually built two jet-propelled steamers. One of them, called the _Waterwitch_, was a 1,100-ton vessel and the other, the _Squirt_, was a small torpedo boat. The latter attained a speed of but twelve knots while a sister ship of the same steam power driven by a propeller attained a speed of seventeen knots. The _Waterwitch_ was even less efficient. Some years ago experimental water-jet vessels were built in New York in which a jet only ⅝ inch in diameter with a pressure of 2,500 pounds per square inch was used, but the experiment proved a failure. The propelling force of a jet is the reaction of the stream of water against the orifice from which it issues. The action is just like Hero’s reaction steam turbine referred to on page 143. The propulsion would be the same were the jet discharged in the open air or in a vacuum or against a solid stone wall. WATER AND AIR RESISTANCE It takes very little power to move a boat slowly because the resistance that has to be overcome is merely the parting of the water at the bow and closing in of the water at the stern and the skin friction along the sides of the hull. In addition to this there is a similar resistance offered by the air. At very low speeds the resistances of the water and the air are practically negligible. In perfectly quiet water with no air stirring the pull of a cord will move a ship weighing hundreds of tons, but the motion will be very slow indeed. Unfortunately the speed of a ship does not increase directly in proportion to the power that drives it. Doubling the power does not double the speed. If it takes ten horsepower to drive a vessel at a speed of ten knots it will take not 2 but 2³ or 8 times as much power to drive it at a speed of twenty knots. In other words, the horsepower goes up as the cube of the speed. This is an average condition for ordinary speeds. For very high speeds the horsepower may have to be increased as the 4th and even the 5th power of the speed. The shape of the bow and the stern is of utmost importance. The parting and displacement of the water at the bow and the replacement at the stern produce waves and the forming of these waves represents so much wasted energy. The swell that is kicked up by a steamer is evidence of power uselessly expended. Much of this loss can be overcome by careful design of the ship’s lines. A vessel that kicks up a high bow wave--one that sails with a “bone in its teeth”--may present a very pleasing spectacle and may seem to be traveling at high speed, but the best designed vessel--the one that slips through the water with no fuss--is much more economical of power. It is easy to understand that the bow must be