In 1833, or at the period now under consideration, however, the paddle-wheel was the recognized instrument of marine propulsion.  Since the beginning of the century it had been growing in use with the gradual growth in the application of steam, and at this time it held the field alone.  Some years earlier it appears that some of the objections to the paddle-wheel had become plainly apparent to Ericsson, although, occupied with other matters as he was, there was no immediate result.  He apparently recognized that the slow revolutions possible with the paddle-wheel did not favor the improvement of the steam-engine along the lines which have since been followed, and he saw clearly that for warship purposes the engines employed, exposed above the water-line to destruction from the shell of an enemy, were entirely out of the question.  Finally in 1833 and 1834 we find him employed by a carrying company in London to conduct numerous trials with submerged propellers in the London and Birmingham canal.  In an affidavit made in March, 1845, he states that in 1833 his attention was particularly called to the subject of oblique propulsion, and that under his direction propellers of various patterns and embodying these principles were fitted on a canal-boat named the “Francis,” and later in 1834 to another called the “Annatorius.”  Shortly after this, or in 1835, his ideas took more definite form, and he refers to his work in a letter to his friend John Bourne in the following terms:—­

“1835.  Designed a rotary propeller to be actuated by steam-power consisting of a series of segments of a screw attached to a thin broad hoop supported by arms so twisted as also to form part of a screw.  The propeller subsequently applied to the steamship ‘Princeton’ was identical with my said design of 1835.  Even the mode adopted to determine, by geometrical construction, the twist of the blades and arms of the ‘Princeton’s’ and other propellers was identical with my design of the year last mentioned.”

At about this same time, or in 1835, the attention of Mr. F.P.  Smith seems to have been drawn to the subject of the screw-propeller, and we find him taking out a patent for his form, consisting of an elongated helix or spiral of several turns, under date of May 31, 1836.  Ericsson’s patent followed some six weeks later, or on July 13, 1836.  While it thus appears that Ericsson had been studying the problem since 1833 or earlier, according to his own statements, there is no evidence that Smith’s attention was drawn to the matter earlier than 1835.  Delay on Ericsson’s part in the matter of patent gives the earlier date to Smith.  The mere date of a patent, however, is of small moment for our present purposes.  It must be admitted that the modern form of screw-propeller is quite unlike either of these original forms, although they all involve of course the same fundamental principles.  Ericsson’s propeller may properly be called an engineering success, built on sound principles, but improved and largely modified by the results of later experience and research.  Smith’s propeller, while capable of propelling a boat, was the design of an amateur rather than of an engineer, and in comparison with Ericsson’s seemed to show a somewhat less accurate appreciation of the underlying principles upon which the propeller operates.