Stevens and his son were crossing the Hudson in this boat on one occasion when the boiler, which was constructed of small tubes, gave way, and the next boiler was constructed with the tubes placed vertically. The engine was kept going for a fortnight or three weeks in the latter part of the summer of 1804, the boat making excursions for two or three miles up and down the river, and for a short distance he could get a speed out of it of seven or eight miles an hour. Stevens’ early experiments with the screw propeller taught him that a vessel driven by only one screw has a tendency to move in a circle. This tendency is displayed in single-screw vessels to the present day. As is well known, a vessel driven by a right-handed screw will deflect slightly to the left, and a vessel driven by a left-handed screw will have a tendency to turn to the right. The explanation given of this peculiarity in the Stevens’ boat by Dr. P. Jones, who was superintendent of the United States Patent Office up to the date of its reorganisation under the law of 1836, in the _Journal of the Franklin Institute_ for 1838, is that this tendency was due to the lessened resistance, as the vanes of the propeller rose towards the surface, in consequence of the greater ease with which the water was removed out of the way. Consequently Stevens overcame this difficulty by using two such wheels placed side by side and revolving in reverse directions. The original screw-engine is still in existence in the Museum of the Stevens Institute at Hoboken, New Jersey. The original boat, of course, has long since disappeared. A replica of it was tried with the old engine on the Hudson in October 1844, and attained a speed of eight miles an hour. One great difficulty which early steamers had to contend with was that of boiler pressure. It should be remembered that the five distinct means Stevens proposed in connection with his screw propeller were: 1. The short four-bladed screw propeller. 2. The use of steam of high pressure. 3. The multitubular boiler. 4. The quick-moving engine connected directly to the propeller shaft. 5. Twin screws. Not one of these means was applied to steam-ships until about forty years later, but all have contributed since their adoption to the success of the ocean navigation of the present day. Stevens’ plan for working twin screws by a single cylinder is the most simple that could be devised. When the screw propeller came into use this plan was revived both in America and in Europe, and was known in France as the “Etoile” engine.