John Ericsson in 1836 patented a propeller consisting of two drums from which projected seven helical blades connected by an external hoop. The blades were inclined in opposite directions, thus forming a double screw propeller, the propellers being placed immediately behind the rudder, which had the usual “shark’s mouth” to allow of steering. The shafts were made so that one passed through the other, the outer one being tubular. The drums revolved in opposite directions, that nearer the sternpost moving at a slightly faster rate than the after drum. This method of arranging the propellers was adopted with a view to avoiding the loss caused by the motion imparted to the water by the single screw, but it was found that the trouble caused by the contrivance was not worth the results obtained. Another drawback was that the extra friction induced by one shaft operating within the other was so great that the contrivance was practically useless where a high speed was desired. The steamer _Francis B. Ogden_ was tried with this type of propeller in 1837, and towed the American sailing ship _Toronto_, of 630 tons burden, on the Thames at the rate of five miles an hour. The _Francis B. Ogden_ was about double the tonnage and power of Smith’s boat, being 45 feet long and having a high-pressure two-cylinder engine giving the propellers about 30 revolutions per minute. Ericsson’s next experiment was with the _Robert F. Stockton_, which was built by Laird at Birkenhead in 1838. She was 63 feet long and of 33 tons, and had engines of 30 horse-power. Prior to this his screw boat towed the Admiralty barge with my Lords of the Admiralty on board on the Thames, but the effort to convince them of the practicability of the method was doomed to failure, since they had previously decided that as the power was applied at the stern the vessel would not steer. [Illustration: MODEL OF THE “NOVELTY.” BUILT 1839.] The _Robert F. Stockton_ crossed the Atlantic under canvas in 1839, and after one of the screws had been removed as useless, she was employed for a quarter of a century as a single-screw tugboat on the Delaware, under the name of the _New Jersey_. Commodore Robert F. Stockton in that year induced Ericsson to resign his office in London as superintending engineer of the Eastern Counties Railway and go to the United States. Several vessels were fitted with his propellers for river and inland waters navigation in America.