of the fire, exposing the structure as a whole to much less strain than befalls a fire-tube boiler. A water-tube boiler has also a much larger draft area than its rival. The sole reason why a fire-tube boiler retains its hold of the market is that it is simpler and cheaper to manufacture than its vastly more efficient competitor.

As remarkable as this adoption and improvement of the sectional boiler, was John Stevens' modification of the screw propeller. He thus describes it in addressing Robert Hare, Junior, of Philadelphia, on November 16, 1805:

". . . To the extremity of an axis passing nearly in a horizontal direction through the stem of the boat, are fixed a number of arms with wings like those of a windmill or smokejack. These arms may be readily adjusted, so that the most advantageous obliquity of their angle may be attained after a few trials. The principle of an oblique stroke is the same as in the scull-but the continuity of movement in the wings gives them greatly the advantage over the alternation in the sculls, both in the loss of time and in the resistance of the fluid to change of motion. Besides that, this change of motion must give to the boat a wriggling movement, with a tendency to lift and lower, by turns, the stern of the boat. The sculls would also be liable to be affected by the swells in rough water, and, like the paddles I had thought of using, would be an awkward appendage to the stern of a boat. The consideration which determined me, when I saw you last, to try the paddles was merely to avoid the necessity of giving the boat a draught of water too great for passing the overslough near Albany, but this objection to the use of wheels I expect to obviate by an increase in their number and a consequent diminution of their diameter. Indeed, it is absolutely necessary to have at least two revolving in opposite directions to prevent the tendency to rotation which a single wheel gives to a boat.

"Since you were here I have made a fair experiment on the wheel compared with oars. Two men were placed at two cranks by which a wheel in the stern of the boat was turned; with a stopwatch the time of passing over a given distance was precisely ascertained. After making a suffi

[From a photograph of the rebuilt boat containing the original machinery.]

cient number of trials the wheel was taken off and the same men were furnished with oars. The result of repeated trials was a few seconds in favor of the wheel. It is unnecessary to observe that the wheel must have worked to much disadvantage. The proper angle of obliquity was not attended to, besides the wings were made with a flat surface, whereas a certain curve was necessary. And in order to give a due immersion to the wheel, its axis was inclined 30 to 40 degrees below the horizontal line. The machinery, too, was put up in a very coarse manner. One important consideration in favor of these wheels is the facility with which they can be defended from all external injury by placing them in the stern. My foreman promises to have the engines going in the boat in about two weeks from this time." *

Colonel Stevens for six years, ending with 1806, sought to establish steam navigation by the screw propeller, endeavoring to introduce (1) the short four-bladed screw, (2) steam at high pressure, (3) multitubular boilers, (4) quick-moving engines directly connected to propeller shafts, (5) twin screws.

* Francis B. Stevens, grand-nephew of Colonel John Stevens, in the Stevens Indicator, April, 1893, said:

"Colonel Stevens considered himself the inventor of the screw propeller. He was mistaken. It was proposed by the mathematician, Daniel Bernouilli, in 1752. It is described by David Bushnell in a letter to Thomas Jefferson, in 1787, giving an account of his submarine boat, in which a screw propeller, worked by hand, was used. The same idea was afterward suggested by Franklin, Watt, Paucton, and others. Prior to 1802 the screw propeller was twice distinctly patented in England: first, by William Lyttleton, in 1794; second, by Edward Shorter, in 1800."

John Bourne, in his "Treatise on the Screw Propeller," London, 1867, mentions a prior patent, that of Joseph Bramah, issued May 9, 1785. His propeller was "a wheel with inclined fans or wings, similar to the fly of a smokejack, or the vertical sails of a windmill. This wheel was to be fixed on the spindle of a rotary engine, and might be wholly under water, where it could be turned round either way, causing a ship to be forced forward or backward, as the inclination of the fans or wings might determine.”

Forty years had to elapse before these elements of success were adopted in ocean navigation. At the time of Colonel Stevens' experiments there were no competent workmen in America to construct the boilers and engines he planned. He had, therefore, to fall back upon the paddle-wheel as a propeller. with its slow-moving engine, whose boilers carried steam at only two or three pounds above atmospheric pressure.

Speed soon became a prime consideration in steamboating. At first Colonel John Stevens bestowed his attention wholly upon his motive power and machinery, giving little heed to the hulls of his vessels. In improving their lines, his son and associate, Robert, effected a notable advance. At first his father's steamers were little else than boxes with pointed ends. In the New Philadelphia, Robert Stevens introduced a false bow, long and sharp, which parted the water with a new facility. At once this vessel bounded forward at thirteen and a half miles an hour, a marvelous speed for that period, and even to-day a goodly pace. When the designer asked his shipbuilders, Brown & Bell, to construct this bow, they declined from fear of public ridicule. Mr. Bell said: "That bow will be called 'Bell's nose,' and I will be a general laughing-stock." So Robert Stevens had to build the bow himself, with anything but laughter at the result. The New Philadelphia inaugurated a day line between Albany and New York. No predecessor of hers had ever run fast enough to complete a trip betwixt dawn and dusk. With her, too, began models which, in clipper sailers and steamers, won new records in speed. Of equal importance with the steamboats plying between the metropolis and the capital of New York, were the steam ferries which joined New York City with the shores of New Jersey and Long Island. Until 1810 only comfortless rowboats or pirogues offered a passage across the North and East Rivers. First as an im