The recess for the bolt heads is covered with a thin plate having the curve of the flange, so that the flanges and the boss form a section of a sphere. This method of construction is a little more expensive than exposed flanges and bolts, which, however, render the boss a huge churn. With the high revolutions at which these screws work, a spherical boss is extremely desirable, but, of course, the details need not be exactly as shown in the illustration. The conical tail is fitted to prevent loss with eddies behind the flat end of the boss, and is particularly valuable with the screws of high speed ships. The light hood shown on the stern bracket is for the purpose of preventing eddies behind the boss of the stern bracket, and to save the resistance of the flat face of the screw boss. The edges of the blades are cast sharp, instead of being rounded at the back, with a small radius, as in the usual practice--the object of the sharp edge being the diminution of the edge resistance. The driving key extends the whole length of the boss, and the tapered shaft fits throughout its length. [Illustration: FIG. 1.] These points of detail have been features of all Admiralty screws for some years. The frictional resistance of screw propellers is always a fruitful source of inefficiency. With a given screw, the loss due to friction may be taken to vary approximately as the square of the speed. This is not to say that the frictional resistance is greater in proportion to the thrust at high than at low speeds. The blades of screws for any speed should be as smooth and clean as possible, but for high speed screws the absolute saving of friction may be considerable with an improvement of the surface. There is no permanent advantage in polishing the blades. No doubt there is some advantage for a little time, and, probably, better results may thereby be secured on trial, but the blades soon become rough, and shell fish and weed appear to grow as rapidly on recently polished blades as on an ordinary surface. These screws are of gun metal. They were fitted to the ships in the condition in which they left the foundry. It appears that within certain limits mere shape of blade does not affect the efficiency of the screw, but, with a given number of blades and a given disk, the possible variations in the form or distribution of a given area are such that different results may be realized. The shapes of the blades of these propellers are shown in Figs. 2, 3, and 4. It will be seen the shapes are not exactly the same for all the screws, but the differences do not call for much remark. [Illustration: FIG. 2., FIG. 3. & FIG. 4.]