such a result possible?

_A._--It appears to be mainly owing to the centrifugal action of the screw,
which interposes a film or wedge of water between the screw itself and the
water on which the screw reacts. This negative slip, as it is called,
chiefly occurs when the pitch of the screw is less than its diameter, and
when, consequently, the velocity of rotation is greater than if a coarser
pitch had been employed. There is, moreover, in all vessels passing through
the water with any considerable velocity, a current of water following the
vessel, in which current, in the case of a screw vessel, the screw will
revolve; and in certain cases the phenomenon of negative slip may be
imputable in part to the existence of this current.

570. _Q._--Is the screw propeller as effectual an instrument of propulsion
as the radial or feathering paddle?

_A._--In all cases of deep immersion it appears to be quite as effectual as
the radial paddle, indeed, more so; but it is scarcely as effectual as the
feathering paddle, with any amount of immersion, and scarcely as effectual
as the common paddle in the case of light immersions.

COMPARATIVE ADVANTAGES OF PADDLE AND SCREW VESSELS.

571. _Q._--Whether do you consider paddle or screw vessels to be on the
whole the most advantageous?

_A._--That is a large question, and can only receive a qualified answer. In
some cases the use of paddles is indispensable, as, for example, in the
case of river vessels of a limited draught of water, where it would not be
possible to get sufficient depth below the water surface to enable a screw
of a proper diameter to be got in.

572. _Q._--But how does the matter stand in the case of ocean vessels?

_A._--In the case of ocean vessels, it is found that paddle vessels fitted
with the ordinary radial wheels, and screw vessels fitted with the ordinary
screw, are about equally efficient in calms and in fair or beam winds with
light and medium immersions. If the vessels are loaded deeply, however, as
vessels starting on a long voyage and carrying much coal must almost
necessarily be, then the screw has an advantage, since the screw acts in
its best manner when deeply immersed, and the paddles in their worst. When
a screw and paddle vessel, however, of the same model and power are set to
encounter head winds, the paddle vessel it is found has in all cases an
advantage, not in speed, but in economy of fuel. For whereas in a paddle
vessel, when her prog