d 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.

582. _Q._--Will you recapitulate some of the main incidents of these
trials?

_A._--I may first state some of the chief dimensions of the vessels. The
Rattler is 176 feet 6 inches long, 32 feet 8-1/2 inches broad, 888 tons
burden, 200 horses power, and has an area of immersed midship section of
380 square feet at a draught of water of 11 feet 5-1/2 inches. The Alecto
is of the same dimensions in every respect, except that she is only of 800
tons burden, the difference in this particular being wholly owing to the
Rattler having been drawn out about 15 feet at the stern, to leave abundant
room for the application of the