or at rest, and whether moving at a high or a low velocity through the water.  Now to achieve an equal speed during calms in two vessels of the same model, there must be the same amount of propelling thrust in each; and this thrust, whatever be its amount, cannot afterward vary if a uniform pressure of steam be maintained.  The thrusts, therefore, caused by their respective propelling instruments, when a screw and paddle vessel are tied stern to stern, must be the same as at other times; and as at other times those thrusts are equal, so must they be when the vessels are set in the antagonism supposed.

578. Q.—­How comes it then that the screw vessel preponderates?

A.—­Not by virtue of a larger thrust exerted by the screw in pressing forward the shaft and with it the vessel, but by the gravitation against the stern of the wave of water which the screw raises by its rapid rotation.  This wave will only be raised very high when the progress of the vessel through the water is nearly arrested, at which time the centrifugal action of the screw is very great; and the vessel under such circumstances is forced forward partly by the thrust of the screw, and partly by the hydrostatic pressure of the protuberance of water which the centrifugal action of the screw raises up at the stern.

579. Q.—­Can you state any facts in corroboration of this view?

A.—­The screw vessel will not preponderate if a screw and paddle vessel be tied bow to bow and the engines of each be then reversed.  In, some screw vessels the amount of thrust actually exerted by the screw under all its varying circumstances, has been ascertained by the application of a dynamometer to the end of the shaft.  By this instrument—­which is formed by a combination of levers like a weighing machine for carts—­a thrust or pressure of several tons can be measured by the application of a small weight; and it has been found, by repeated experiment with the dynamometer, that the thrust of the screw in a screw vessel when towing a paddle vessel against the whole force of her engines, is just the same as it is when the two vessels are maintaining an equal speed in calms.  The preponderance of the screw vessel must, therefore, be imputable to some other agency than to a superior thrust of the screw, which is found by experiment not to exist.

580. Q.—­Has the dynamometer been applied to paddle vessels?

A.—­It has not been applied to the vessels themselves, as in the case of screw vessels, but it has been employed on shore to ascertain the amount of tractive force that a paddle vessel can exert on a rope.

581. Q.—­Have any experiments been made to determine the comparative performances of screw and paddle vessels at sea?

A.—­Yes, numerous experiments; of which the best known are probably those made on the screw steamer Rattler and the paddle steamer Alecto, each vessel of the same model, size, and power,—­each vessel being of about 800 tons burden and 200 horses power.  Subsequently another set of experiments with the same object was made with the Niger screw steamer and the Basilisk paddle steamer, both vessels being of about 1000 tons burden and 400 horses power.  The general results which were obtained in the course of these experiments are those which have been already recited.