The paddle-wheels of the _Clermont_ were on the sides of the boat about midship. As the wheel turned, about half of it was in the water and about half was out. There were engineers, even in Fulton's day who did not believe the wheels ought to be on the sides of the boat. Look at waterfowl, they said, look at the graceful swan; its feet do not work at its sides, half under the water and half out. Every animal that swims propels itself from behind, and its propellers are entirely under the water. So, thought these engineers, the paddle-wheel of a boat should be placed behind, and should be entirely covered by the water. John Stevens, an engineer of Hoboken, New Jersey, in 1805 built a steamboat according to this notion (Fig. 18). A close inspection of the wheel of the boat would show that it is spiral- or screw-like in shape. Stevens' boat made a trial trip on the Hudson and worked well; but after Fulton's great success the little steamer with its spiral-shaped wheel in the rear was soon forgotten. The idea of a screw-propeller, however, was not lost. It was taken up by John Ericsson, a Swedish engineer, who, in 1839, built, in an English shipyard for an American captain, the first screw-propeller that crossed the Atlantic--the _Robert F. Stockton_. This was the last step in the development of the boat. Since 1839 there has been marvelous progress in ship-building, but the progress has consisted in improving upon the invention of Ericsson rather than in making new discoveries. With the screw-propeller in its present form we may close our story of the boat. The homely log propelled by rude paddles has become the magnificent floating palace. FOOTNOTE: [18] A spirited account of life on a Roman galley is found in Wallace's "Ben Hur." [Illustration: THE ADRIATIC AT SEA.] THE CLOCK "Tic-tac! tic-tac! go the wheels of time. We cannot stop them; they will not stop themselves." Time passing is life passing and the measurement of time is the measurement of life itself. How important then that our chronometers, or time measures, be accurate and faithful! It is said that a slight error in a general's watch caused the overthrow of Napoleon at Waterloo and thus changed the history of the world. Because of its great importance the measurement of time has always been a subject of deep human interest and the story of the clock begins with the history of primeval man.