rvals to
enable it to carry a number of diminutive wheels upon which these long
vanes are mounted, and a very strong connection must be made between
these wheels and the shaft of the screw. Inasmuch as a high speed of
rotation is to be maintained, the pitch of the screw in the water is
set so as to offer but slight opposition to the water at each turn.
The immense speed attained is thus due, not to the actual power with
which the water is struck by the screw at each revolution, but to the
extraordinary rapidity with which the shaft rotates.

The twin screw, with which the best and safest of modern steam-ships
are all fitted, will soon develop into what may be called "the twin
stern". Each screw requires a separate set of engines and the main
object of the duplication is to lessen the risk of the vessel being
left helpless in case of accident to one or other. The advisability of
placing each engine and shafting in a separate water-tight compartment
has therefore been seen. At this point there presents itself for
consideration the advisability of separating the two screws by as wide
a distance as may be convenient and placing the rudder between the
two. Practically, therefore, it will be found best to build out a
steel framework from each side of the stern for holding the bearings
of each screw in connection with the twin water-tight compartments
holding the shafting; and thus will be evolved what will practically
represent a twin, or double, stern.

In the case of the turbine steamer several of the forms of screw which
were first proposed when that type of propeller was invented will
again come up for examination, notably the Archimedean screw, wound
round a fairly long piece of shafting. The larger the circular area of
this screw is the less will be the risk of "smashing" the water, or of
losing hold of it entirely in rough weather. With twin screws of the
large Archimedean type the propelling apparatus of a turbine steamer
will--if the screws are left open--be objected to on the ground of
liability to foul or to get broken in crowded fairways. Hence will
arise a demand for accommodation for each screw in a tube forming part
of the lower hull itself and open at the side for the taking in of
water, while the stern part is equally free. In this way there is
evolved a kind of compromise between the two principles of marine
propulsion, by a screw and by a jet of water thrown to sternward. The
water-jet is already very suc