Many of the readers of this volume will no doubt have often been struck by the enormous rate of speed which a porpoise exhibits as he goes through the water. Those who spend their time crossing the ocean are familiar with the sight of these creatures saucily playing about the bows of a fast liner as she goes tearing through the water. It has been calculated that it would require no less than 15 horse-power to obtain the twenty miles an hour at which these animals can travel for long periods at a time. The explanation is that in their skins there is a wonderful system of glands, which exude oil and so minimise the influence of skin-friction. Remembering this, mechanical attempts have even been made quite recently to obtain a steel plate which would allow the oil to exude under pressure from the inside of the vessel’s bows. Possibly, nowadays, every engineer has his own formula for determining the amount of horse-power essential for a given speed. All sorts of sliding scales and devices have been invented for this purpose, and the ideal shape of the modern propeller has still to be ascertained. It is a well-known fact that when a vessel moves through the sea she sets the water itself in motion, so that some of it actually travels with the ship; but Naval Constructor D. W. Taylor, of the United States Navy, found by experiment in 1908 that when a ship progresses the flow of the water is down forward, and then it passes under the ship, coming up again aft. Practically we can sum up the resistance which a ship has to encounter under three heads. First of all, there is the skin resistance already mentioned, which, of course, varies with the amount of wetted surface. Then after the ship has passed through the water there ensues an impeding eddy at the stern, as the reader must often have observed. Finally, there is the resistance caused by wave-making, which for vessels propelled at high speeds is an important consideration, but varies according to the design of the ship and her pace.