ed by the central crank or eccentric,
that the entering and emerging floats have a direction intermediate between
a radius and a vertical line.

560. _Q._--Can you give any practical rules for proportioning paddle
wheels?

_A._--A common rule for the pitch of the floats is to allow one float for
every foot of diameter of the wheel; but in the case of fast vessels a
pitch of 2-1/2 feet, or even less, appears preferable, as a close pitch
occasions less vibration. If the floats be put too close, however, the
water will not escape freely from between them, and if set too far apart
the stroke of the entering paddle will occasion an inconvenient amount of
vibratory motion, and there will also be some loss of power. To find the
proper area of a single float:--divide the number of actual horses power of
both engines by the diameter of the wheel in feet; the quotient is the area
of one paddle board in square feet proper for sea going vessels, and the
area multiplied by 0.6 will give the length of the float in feet. In very
sharp vessels, which offer less resistance in passing through the water,
the area of paddle board is usually one-fourth less than the above
proportion, and the proper length of the float may in such case be found by
multiplying the area by 0.7. In sea going vessels about four floats are
usually immersed, and in river steamers only one or two floats. There is
more slip in the latter case, but there is also more engine power exerted
in the propulsion of the ship, from the greater speed of engine thus
rendered possible.

561. _Q._--Then is it beneficial to use small floats?

_A._--Quite the contrary. If to permit a greater speed of the engine the
floats be diminished in area instead of being raised out of the water, no
appreciable accession to the speed of the vessel will be obtained; whereas
there will be an increased speed of vessel if the accelerated speed of the
engine be caused by diminishing the diameter of the wheels. In vessels
intended to be fast, therefore, it is expedient to make the wheels small,
so as to enable the engine to work with a high velocity; and it is
expedient to make such wheels of the feathering kind, to obviate loss of
power from oblique action. In no wheel must the rolling circle fall below
the water line, else the entering and emerging floats will carry masses of
water before them. The slip is usually equal to about one-fourth of the
velocity of the centre of pressure in well propo