Shortly after his arrival, or in 1840, a prize was offered by the Mechanics' Institute of New York for the best plan of a steam fire-engine. With his previous experience in London, Ericsson easily carried off the palm and was awarded the prize. He further occupied himself with the introduction of propellers on boats engaged in the inland navigation of the United States, with the design and construction of the United States steam frigate "Princeton," with the development of the compound principle in the steam-engine, then in 1851 with his hot-air ship "Ericsson," or ship propelled by hot-air or caloric engines, as they were then termed, and later with caloric engines in smaller sizes for stationary purposes, of which several thousand were sold during the next succeeding years. In the work of introducing his propellers good progress was made, especially in boats built for use on the Great Lakes, so that by 1844, when the U.S.S. "Princeton" went into commission, there were in use some twenty-five vessels with the screw-propeller as a means of propulsion. The project of building a vessel for the American Navy, the purpose which had most strongly attracted Ericsson to the United States, suffered long delay in connection with the arrangements between Captain Stockton and the naval authorities at Washington. At length, in 1841, Captain Stockton was authorized to proceed with the construction of a screw steam frigate of about one thousand tons. This was the U.S.S. "Princeton," which marks an epoch as the first screw vessel-of-war. She was followed by the French "Pomone" in 1843, and the English "Amphion" in 1844, for the equipment of which Ericsson's agent in England, Count Von Rosen, received commissions from the French and English governments respectively. The "Princeton" was completed in due time and was equipped with two 12-inch wrought-iron guns, one brought by Ericsson from England and one designed and built under the direction of Captain Stockton. At the trials of the ship in 1844 the latter gun exploded, killing the Secretaries of State and of the Navy, besides other prominent visitors on board, and wounding several others. This terrible disaster threw an entirely undeserved stigma upon the ship herself and upon Ericsson's work, and it was not until many years after that his name was entirely free from some kind of reproach in connection with the "Princeton" and the deplorable results of the accident on board.