An interesting and noteworthy fact in connection with these propellers is the wide differences in the pitches and revolutions, though the products of the two do not greatly vary. Such differences are extremely rare in the mercantile marine for similar speeds, but in war ships they are inseparable from the conditions of the engine design. As a general rule, with (revolutions × pitch) a constant, an increase of revolutions and the consequent decrease of pitch allow a diminution of disk and of blade area--other modifying conditions, such as the thrust, slip, number, and pattern of blades, being the same. The screws for E and F are interesting, because, with practically the same speeds and slips, there is a considerable difference in the revolutions. It will be observed that F is a vessel of finer form and a little less displacement than E, and, therefore, has less resistance. Although E has the greater resistance and the screw the smaller pitch/diameter, the higher revolutions permit the use of a smaller screw. But from this example the influence of the high revolutions in diminishing the size of screw does not appear so great as some empirical rules would indicate. The screws for A and B are also worthy of attention. Although the ship A has a much greater resistance than B, the screw of the former is much the smaller, both in the blade area and the disk. A's screws, however, in addition to 22 per cent. more revolutions than B, have a much larger slip, and the blades have rather a fuller form at the tips. Compared with the practice in the mercantile marine, the revolutions of these screws are very high, and from the foregoing remarks it may appear that much larger screws would be required for a merchant ship than for a war ship of the same displacement and speed. There would, however, be several items favorable to the use of small screws. For a given displacement the resistance would be less in the mercantile ship, and with the lower revolutions the proportion of blade area to the disk could be increased without impairing the efficiency. Thus in passing from the war vessel to a merchant ship of the same displacement, there are the lower revolutions favorable to a larger screw, but, on the other hand, the smaller resistance, larger proportion of blade area, and the coarser pitch, are favorable to a diminution of the screw. The ship B has a very large screw at 88 revolutions, but the tips are very narrow. If the blade were as dotted for a diameter of 16 ft., the same work could be done with the same revolutions, but with a little coarser pitch and a little more slip.