ress is resisted, the speed of the engine diminishes
nearly in the proportion of the diminished speed of ship, it happens that
in a screw vessel this is not so,--at least to an equal extent,--but the
engines work with nearly the same rate of speed as if no increase of
resistance had been encountered by the ship. It follows from this
circumstance, that whereas in paddle vessels the consumption of steam, and
therefore of fuel, per hour is materially diminished when head winds occur,
in screw vessels a similar diminution in the consumption of steam and fuel
does not take place.

573. _Q._--But perhaps under such circumstances the speed of the screw
vessel will be the greater of the two?

_A._--No; the speed of the two vessels will be the same, unless the
strength of the head wind be so great as to bring the vessels nearly to a
state of rest, and on that supposition the screw vessel will have the
advantage. Such cases occur very rarely in practice; and in the case of the
ordinary resistances imposed by head winds, the speed of the screw and
paddle vessel will be the same, but the screw vessel will consume most
coals.

574. _Q._--What is the cause of this peculiarity?

_A._--The cause is, that when the screw is so proportioned in its length as
to be most suitable for propelling vessels in calms, it is too short to be
suitable for propelling vessels which encounter a very heavy resistance. It
follows, therefore, that if it is prevented from pursuing its spiral course
in the water, it will displace the water to a certain extent laterally, in
the manner it does if the engine be set on when the vessel is at anchor;
and a part of the engine power is thus wasted in producing a useless
disturbance of the water, which in paddle vessels is not expended at all.

575. _Q._--If a screw and paddle vessel of the same mould and power be tied
stern to stern, will not the screw vessel preponderate and tow the paddle
vessel astern against the whole force of her engines?

_A._--Yes, that will be so.

576. _Q._--And seeing that the vessels are of the same mould and power, so
that neither can derive an advantage from a variation in that condition,
does not the preponderance of the screw vessel show that the screw must be
the most powerful propeller?

_A._---No, it does not.

577. _Q._--Seeing that the vessels are the same in all respects except as
regards the propellers, and that one of them exhibits a superiority, does
not this circum