Most people fail to find much resemblance, if any at all, between that comparatively small-looking two or three-bladed thing that drives the steamship through the water at the rate of twenty miles an hour, and what is commonly known as a screw; but the discrepancy is easy of explanation. Archimedes, who is credited with the invention of the screw as a mechanical lever, little dreamed of the uses to which it was to be turned two thousand years later. He is said to have employed the screw in launching a large ship, pushing it into the water as is now done by hydraulic appliances. By changing his fulcrum and making the screw a part of the ship, the modern engineer has only reversed the mode of applying propelling power; the principle is the same. The effect produced by the screw in propelling a ship will be best understood by supposing an ordinary screw of large dimensions to be revolving rapidly in a trough full of water. It would then send the water away from it with great force; but as action and reaction are equal it would be itself, at the same time, urged in the opposite direction with exactly the same degree of force. If we suppose it, then, to be fixed in a ship, the ship will be pushed forward with the same force that is exerted by the screw in pushing back against the water. If the screw is made to revolve in the opposite direction, the converse of this takes place, and the ship is pushed backwards by the reaction of the screw.[11] The idea has long occupied the attention of inventive genius. As far back as 1746, at least, the capabilities of the screw as a motive power for ships have been tested by experiments. In 1770 James Watt, who had so much to do with perfecting the steam-engine, suggested the use of screw-propellers. In 1815 Trevethick took out a patent for one. Woodcroft did the same in 1826; but it was not until ten years later that its utility was successfully demonstrated.