esults are known. Thus a great advance has been
made on the old method of trial upon the ship itself, which was the
origin of almost every conceivable erroneous view respecting the screw
propeller. The fact was lost sight of that any modification in form,
dimensions, or proportions referred only to that particular combination
of ship and propeller, or to one similar thereto; so something like
chaos was the result. This, however, need not be the case much longer.

In regard to the materials used for propellers, steel has been largely
adopted for both solid and loose-bladed screws; but unless protected in
some way, the tips of the blades are apt to corrode rapidly and become
unserviceable. One of the stronger kinds of bronze is often judiciously
employed for the blades, in conjunction with a steel boss. Where the
first extra expense can be afforded, bronze seems the preferable
material; the castings are of a reliable character, and the metal does
not rapidly corrode; the bronze blades can therefore with safety be made
lighter than steel blades, which favors their springing and
accommodating themselves more readily to the various speeds of the
different parts of the wake. This might be expected to result in some
slight increase of efficiency; of which, however, the writer has never
had the opportunity of satisfactorily determining the exact extent.
Instances can be brought forward where bronze blades have been
substituted for steel or iron with markedly improved results; but in
cases of this kind which the writer has had the opportunity of
analyzing, the whole improvement might be accounted for by the modified
proportions of the screw when in working condition. In other words, both
experiment and practical working alike go to show that, although cast
iron and steel blades as usually proportioned are sufficiently stiff to
retain their form while at work, bronze blades, being made much lighter,
are not; and the result is that the measured or set pitch is less than
that which the blades assume while at work. Some facts relative to this
subject have already been given in a recent paper by the author.

_Twin Screws_.--The great question of twin screw propulsion has been put
to the test upon a large scale in the mercantile marine, or rather in
what would usually be termed the passenger service. While engineers,
however, are prepared to admit its advantages so far as greater security
from total breakdown is concerned, there is by