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 no means thorough agreement as to whether single or twin screws have the greater propulsive efficiency.  What is required to form a sound judgment upon the whole question is a series of examples of twin and single screw vessels, each of which is known to be fitted with the most suitable propeller for the type of vessel and speed; and until this information is available, little can be said upon the subject with any certainty.  So far the following large passenger steamers, particulars of which are given in table II., have been fitted with twin screws.  It appears t be a current opinion that the twin screw arrangement necessitates a greater weight of machinery.  This is not necessarily so, however; on the contrary, the opportunity is offered for reducing the weight of all that part of the machinery of which the weight relatively to power is inversely proportional to the revolutions for a given power.  This can be reduced in the proportion of 1 to the square root of 2, that is 71 per cent. of its weight in the single screw engine; for since approximately the same total disk area is required in both cases with similar proportioned propellers, the twins will work at a greater speed of revolution than the single screw.  From a commercial point of view there ought to be little disagreement as to the advantage of twin screws, so long as the loss of space incurred by the necessity for double tunnels is not important; and for the larger passenger vessels now built for ocean service the disadvantage should not be great.  Besides their superiority in the matter of immunity from total breakdown, and in greatly diminished weight of machinery, they also offer the opportunity of reducing to some extent the cost of machinery.  A slightly greater engine room staff is necessary; but this seems of little importance compared with the foregoing advantages.

TABLE II.—­PASSENGER STEAMERS FITTED WITH TWIN SCREWS.

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+--------+-------+
| Length    |     |     Cylinders,    | Boiler |Indi-  |
| between   |     |  two sets in all  |pressure|cated  |
Vessels.     | perpen-   |Beam.|      cases.       |  per   |horse- |
| diculars. |     |-----------+-------| square |power. |
|           |     |Diameters. |Stroke.|  inch. |       |
-----------------+-----------+-----+-----------+-------+----

----+-------+
|   Feet.   |Feet.| Inches.   |Inches.| Lb.    |       |
City of Paris.   |\          |     |           |       |        |       |
| }  525    | 631/4 |45, 71, 113|  60   |  150   |20,000 |
City of New York.|/          |     |           |       |        |       |
-----------------+-----------+-----+-----------+-------+----

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