power and of unsuitable design, and her coal capacity too small to give her a useful radius of action. For the attainment of all the properties specified it was subsequently calculated and shown that a much larger displacement was necessary. Just as Fitch had discovered and Fulton had discerned, increase in scale reduced many of the difficulties encountered in designing heavily weighted steam vessels. Hence the success of the _Terrible_. In the case of the _Terrible_, a large paddlewheel frigate of 1,850 tons and 800 horse-power built in 1845, it was clear that an increase of size had given a partial solution to the problem of designing a war-vessel with heavy and spacious propelling machinery, with adequate armament, and with full sail-power and all the properties of a sailing ship. Still the steam war-vessel was not satisfactory. Her machinery usurped the weight and space required for armament, her cumbrous paddlewheels were far too exposed to damage by shot or shell. And how to surmount these difficulties and reconcile the conflicting requirements of artillery and motive power, was a problem which cost the country years of unsuccessful experiments and millions of money. “It was,” said Dahlgren, “the riddle of the day.” § The problem was solved by the adoption of the screw propeller. Since Archimedes’ day the screw had been known in the form of a pump, and in two familiar objects--the smoke-jack and the windmill--the principle of the driven screw had been for centuries widely employed. In connection with ship propulsion the screw appears to have been tried at an early date, like the Marquis of Worcester’s water-wheel, in the form of a mill. Among the machines and inventions approved by the Royal Academy of Sciences of Paris between the years 1727 and 1731 is one described as a screw, suspended in a framework between two boats, which when acted upon by the current was intended to warp the vessels upstream, the motion of the screw being transmitted to a winch barrel on which a tow-rope was wound. But so far as is known no attempt had been made at this date to use the screw directly as a propeller. In 1768 its use in this form was suggested in a work entitled _Théorie de la Vis d’Archimede_.[152] And shortly after, as we have already seen, Bramah in England and Bushnell in America had patented, and the latter had actually put into use, the screw as a means of propelling vessels through water. We have seen, too, that Fulton successfully adapted the screw propeller, on a small scale, in one of his experimental steamboats. Sporadic attempts were made in the early days of the nineteenth century both in this country and in America to drive ships by means of screws, both manually and by the medium of steam, some of which were attended with a certain measure of success.[153] Yet some time was to elapse before screw propulsion gained recognition. Doubt as to the efficiency of a screw’s action, ignorance as to the shape