thread, by altering the size and shape of the
screw's centre, and not less by an ingenious
contrivance for " feathering" the blades, and
diminishing or inreasing their pitch or slope
at will —was greatly added to the value of
screw machinery.

The merchant service and public companies
have equally availed themselves of the invention ;
and, at the present time, some of the
largest ships afloat are screw-propelled.
Indeed, so marked are the advantages of the screw
over the paddle, that there is little doubt but
that the former will eventually be superseded,
except in navigating shallow water; and that
a paddle steamer across the ocean will, twenty
years hence, be as rare an object as a stage
coach on the high roads of Britain.

Having thus sketched the progress of
Screw Steam Navigation, a short space will
suffice for an explanation of what this screw
consists, how placed, and in what its great
advantages reside. The reader will no doubt
gladly be spared a treatise on the resistance
of fluid bodies, on the true pitch and
disc of screw propellers, on positive and
negative slip, or centrifugal action. It may be
enough to say, that the screw-propellers now
most commonly in use are what are termed
double-threaded, of about one-sixteenth of a
convolution; in plainer language, they consist
of two twisted iron blades fixed upon a shaft
revolving beneath the water, at the stern.
This shaft is surrounded by a stuffing-box
with hemp packing, to keep the aperture in
the ship's stern watertight; its extremity is
set in a socket attached to the rudder-post.
The screw itself revolves in that part of the
stern of the ship called the deadwood, in
which a suitably sized hole is cut to admit
of its working. It is the thrust, or forward
pressure of the blades, or sections of the
screw threads, which is effective in propelling
the ship.

Numerous trials as to the relative qualities
of the paddle and the screw have resulted in
a most complete demonstration of the
superiority of the latter as an auxiliary power to
vessels under canvas. For long sea-voyages
in which calms, light airs, or fair breezes are
looked for, a screw ship of fifteen hundred
tons and three hundred horse power,
would be preferable in point of speed and
economical working to a paddle steamer of
the same size and of three times the horse
power. It has been clearly shown that a screw
steamer makes as much way under canvas
and with half steam on, as without sails and
with her whole steam power applied. Indeed,
wherever sails can be used at all, the
advantages of the screw appear most clearly: even
in sailing close-hauled to the wind, a vessel
by the aid of the screw may be propelled four
knots, when previously only making one knot
an hour.

Experiment has demonstrated that an
auxiliary screw-power sufficient to propel a
ship not more than a mile or a mile and a
half an hour, when brought to the aid of the
sails, has in reality added three or four
miles an hour to her speed. This is accounted
for in the following manner: —when the
vessel is propelled by canvas alone, and
at a low rate of sailing, the wind quickly
rebounds from the sails, and forms a sort
of eddy or dead air in their rear, which acts
to an extent adversely; for the sails do not
receive nearly the whole advantage of the
breeze; but, the moment more speed is
imparted by auxiliary power, the sails retain
the wind longer, having more of it, and there
is not the same degree of rebound. In like
manner the sails assist the action of the
screw, by enabling it to work upon a larger
surface of water, and so extend its power.

It is evident, therefore, that except in
running against a head gale, the screw-propelled
ship must have the advantage. In
regard to the original cost and working the
two kinds of steamers, there is an enormous
difference. Calculations show that the relative
expense of the three classes of ships is
as nine for paddle-steamers, to four for
sailing-vessels and three for auxiliary
screw-ships.

Looking to these advantages, it is highly
interesting to examine in what direction
screw steamers fitted on the auxiliary
principle, are most likely to prove of the greatest
utility.

It was a happy circumstance that, coeval
with the extension of the British possessions
in that most remote part of the earth, the
great south land of Australia, the screw
principle should have been brought forward
as a means of economising the use of fuel.
By any of the routes to the colonies of
Australia, the voyage, out and home, of a sailing
vessel has been to the present time a most
tedious and unpleasant affair. It is true
there are Marco Polos, and Flying Dragons,
and Sovereigns of the Seas which have made
rapid passages with sails alone; but we
all know what the old adage tells us about
one swallow not making a summer. An
average taken from the voyages of six
hundred vessels, out and home, in 'thirty-nine
and 'forty-nine, gives one hundred and thirty-four
days as the outward run in the former
year, and one hundred and nineteen days for
the latter; whilst, for the homeward voyage,
they were one hundred and fifty-one and one
hundred and twenty-eight days. In 'forty-
nine, the longest passage made to Port Philip
was one hundred and eighty-six days; the
shortest, one hundred and one days.

This is tedious work; knocking about in
calms, gales of wind, and adverse breezes,
during those one hundred and eighty-six
days, with the biscuit green and wormy,
and the water looking like bad pea-soup,
smelling of stale rum casks and tasting of
logwood and rusty nails. Still it did not
much signify when emigrants were few; when
the homeward-bound with fortunes were stil