MESSRS. EDITORS :—In Vol. XIII, No. 4, ICIENTIFIC AMERICAN., under the heading, 'Inventors to the Rescue!" you have an .rticle in which, after alluding to the loss f the Central America, and the lives of nany of the passengers, you make an earnest /ppeal to inventors to produce pumps that rill not choke, and reliable life-preservers. In he next issue, No. 5, you publish the claim of Sthan Campbell, assignor to Wm. P. Page Lud Edward F. Hodges, for a marine propel-ing apparatus. Having been an eye-witness to the perform-mces of a boat fitted with this propeller, and IOW owned by the proprietors of the patent, md having become familiar with its operation ind capabilities, I believe it will fully meet ;he requirements of your appeal. The ap-Daratus consists of two cylindrical cases, ilaced atliwartships within and upon the bot-;om of the vessel, each case provided with two curved induction passages, which communi-3ate with the water without. Through the axis of each case a shaft ex-:ends, to which two circular plates are attacked eccentrically witljin. An inner cylinder is fitted to revolve upon the circular plates, and to this cylinder is attached a wing )r plate extending downwards between the two water passages. The rotation of the shaft produces an eccentric revolution of the cylinder, which thus becomes a perpetual float or paddle under constant pressure. The volume of displacement, by a case or cylinder of eight feet diameter and eight feet length, at sixty revolutions per minute, is equal to that of a side wheel of twenty-four feet diameter, and nine feet length of paddles, at twenty revolutions per minute, but is more effective, because at the bottom of the vessel. Working "within a resisting radius of four feet, instead of one of twelve feet, it will also allow of a great reduction in the size of machinery, weight of shaftings, and space for the generation of steam. From its position at the bottom and within the ship, it is entirely removed from alldarfger of damage by collision, must always work with perfect uniformity in storm or calm, and offers not the slightest impediment to the progress of a ship while under sail. A valve fitted to the forward induction passages, when closed, cuts off the water communication : anothervalve, opening from the induction passage into the hold, then being opened, each propeller, with a cylinder of the size above mentioned, would relieve the hold of one hundred thousand gallons of water per minute, at sixty revolutions. Here, then, is a pump of great effectiveness, that will not choke. Again, this propeller produces no perceptible agitation of the water on the surface, and is believed to be perfectly adapted to all purposes of towage upon canals. The boat herein mentioned, with an engine of seven horse power, driving a propeller of twenty-eight inches length and diameter of case, makes six knots per hour, and leaves no disturbance of the water more than if driven by wind. The proprietors seem confident that they have a propeller which, in effectiveness, safety from casualties, and security to life and property, is invaluable. Wm. Page, Esq., of Nos. 99 and 101 Commercial street, Boston, will probably be happy to afford any information regarding this invention. P. JOHNSON. Boston, Mass., Nov., 1857.