*A.*—The screw steamer Fairy, if enlarged
to three times the size while retaining the same form,
would have twenty-seven times the capacity, nine times
the sectional area, and nine times the power.
The length of such a vessel would be 434 feet; her
breadth 63 feet 4-1/2 inches; her draught of water
16-1/2 feet; her area of immersed section 729 square
feet; and her nominal power 1080 horses. Now
as the lengths of the Fairy and of the new vessel
are in the proportion of 1 to 3, the speeds will be
in the proportion of the square root of 1 to the square
root of 3; or, in other words, the speed of the large
vessel will be 1.73 times greater than the speed of
the small vessel. If therefore the speed of the
Fairy be 13 knots, the speed of the new vessel will
be 22.49 knots, although the proportion of power to
sectional area, which is supposed to be the measure
of the resistance, is in both cases precisely the
same. If the speed of the Fairy herself had to
be increased to 22.29 knots, the power would have to
be increased in the proportion of the cube of 13 to
the cube of 22.49, or 5.2 times, which makes the power
necessary to propel the Fairy at that speed equal
to 624 nominal horses power.

STRUCTURE AND OPERATION OF PADDLE WHEELS.

551. *Q.*—Will you describe the configuration
and mode of action of the paddle wheels in general
use?

*A.*—There are two kinds of paddle
wheels in extensive use, the one being the ordinary
radial wheel, in which the floats are fixed on arms
radiating from the centre; and the other the feathering
wheel, in which each float is hung upon a centre,
and is so governed by suitable mechanism as to be
always kept in nearly the vertical position. In
the radial wheel there is some loss of power from
oblique action, whereas in the feathering wheel there
is little or no loss from this cause; but in every
kind of paddle there is a loss of power from the recession
of the water from the float boards, or the *slip*
as it is commonly called; and this loss is the necessary
condition of the resistance for the propulsion of the
vessel being created in a fluid. The slip is
expressed by the difference between the speed of the
wheel and the speed of the vessel, and the larger this
difference is the greater the loss of power from slip
must be—the consumption of steam in the
engine being proportionate to the velocity of the
wheel, and the useful effect being proportionate to
the speed of the ship.

552. *Q.*—The resistance necessary
for propulsion will not be situated at the circumference
of the wheel?