so that they caused the minimum drag, or by lifting the screw into a well. The _Novelty_ lifted hers on deck over the quarter by means of davits. This arrangement will also be seen in the illustration. This idea is now obsolete, since sails are but rarely employed as auxiliaries. [Illustration: STERN OF THE “ARCHIMEDES.” _From the Model in the Victoria and Albert Museum._] [Illustration: THE “NOVELTY” (1839). _From the Model in the Victoria and Albert Museum._] Now the introduction of the propeller was not so simple an event as the reader might imagine. Ordinarily, one is tempted to argue that it was merely a case of putting the power aft instead of at either side, as in the use of the paddle-wheels. But, in fact, the introduction of the screw opened up a new set of problems connected with ship design. In the early days the design of a ship’s stern, both in the sailing ship and the steamer, was badly neglected. Later on the improved lines of the clipper sailing ships certainly did much to improve matters. I referred at the beginning of the previous chapter to the manner in which a vessel going ahead moves the water in which she floats, and how the eddies round the stern impede her advance. Now when a propeller revolves, much of its power is, even nowadays, wasted by what is called “slip”--that is to say, by the yielding of the water so that the screw does not progress to the full extent of its “pitch.” (The “pitch” of a propeller is the amount of distance which is represented by one whole turn of the thread. We could measure, for instance, the “pitch” of a corkscrew by the distance which it would penetrate in a cork.) Even after years of experiments and improvements the wake at the end of a steamship tends to reduce the speed of the water past the propeller, but when first the screw experiments were conducted the design of the afterbody of a ship’s hull was so carelessly considered that the “slip” of the propeller was considerable. There is also to be taken into account the fact that by the rounding in of the “stream lines” at the stern the vessel receives a pressure which helps her forward. When, however, a propeller is added to a ship and set in motion it disturbs this helping-forward movement, and in a ship fitted with only a single screw this disturbance is even greater than in a twin-screw steamer, because the latter has her propellers placed well out, away from the hull. We need not here pursue the subject further; it is enough now to show that every improvement in the steamship began a new chapter of problems, introduced difficulties that could never have been anticipated, which time and patience alone can solve satisfactorily.