C46 STEAM ENGINE of engine is shown in fig. 9. It is very com- pact, light, and moderately economical, and excels in simplicity. The feathering paddle wheel is made with floats or buckets variable in position, and so adjusted by the feathering mechanism that less power is expended in ob- FIG. 9. Oscillating Engine and feathering Paddle Wheel. lique action, raising or pushing downward the water impinged upon, than with the ordinary radial wheel, in which the floats are rigidly attached to the arms. The usual arrangement is such that the feathering wheel has the same action upon the water as a radial wheel of double diameter. This reduction of the diam- eter of the wheel, while retaining maximum effectiveness, permits a high speed of engine, and therefore less weight, volume, and cost. The smaller wheel boxes, by offering less re- sistance to the wind, retard the progress of the vessel less than those of radial wheels. The feathering of the paddle is produced by the use of a rod, E D, fig. 10, which connects an eccentric strap, E F, secured to the vessel, with the short arm A D, by which the paddle is turned upon the pin A. is the centre of the paddle wheel, and B is one of the arms. Circular hoops, or bands, connect all of the arms, each of which carries a float. They are all thus tied togeth- er, forming a very firm and powerful combina- tion to resist external forces. Inclined engines are sometimes used for driving paddle wheels. In these the steam cylinder lies in an inclined position, and its connecting rod directly connects the crank with the cross head. The condenser and air pump usually lie beneath the cross-head guides, and are worked by a bell crank driven by links on 10. each side the connecting rod, attached to the cross head. Such engines are used to some extent in Europe, and they have been adopted in the United States navy for side- wheel gun- boats. They are also used on the ferry boats plying between New York and Brooklyn. Paddle wheels should be immersed usually not more than one third the radius of the wheel for sea-going vessels, and on rivers they are frequently not immersed more than one sixth or one eighth. In the first case the loss by oblique action is about 5 per cent., in the last case about 10 per cent. A loss of 20 or 25 per cent, of the total power applied to the wheel is frequently caused by slip. Many forms of engines have been used for driving the screw, but they are now almost invariably of one type. The ordinary screw engine is direct-acting. Two engines are placed side by side, with cranks on the shaft at an angle of 90 degrees with each other. In merchant steamers the steam cylinders are usually ver- tical and directly over the crank pins, to which the cross heads are coupled. The condenser is placed behind the engine frame, or, where a jet condenser is used, the frame itself is some- times made hollow and serves as a condenser. The air pump is worked by a beam connected by links with the cross head. The general ar- rangement is like that shown in figs. 13 and 14. For naval purposes such a form is objection- able, since its height is so great that it would be exposed to injury by shot. In naval engi- neering the cylinder is placed horizontally, as in fig. 11, which is a sectional view, represent- ing a horizontal, direct-acting naval screw en- gine, with jet condenser and double-acting air and circulating pumps. A is the steam cylin- der, B the piston, which is connected to the crank pin by the piston rod D and connecting rod E. F is the cross-head guide. The eccen- trics G operate the valve, which is of the " three-ported variety," by a Stephenson link. FIG. 11. Horizontal direct-acting Naval Screw Engine. Reversing is effected by the hand wheel C, which by means of a gear m and a rack Jt ele- vates and depresses the link, and thus reverses the valve. As shown in the sketch, this valve is so constructed that, when in precisely the middle of its path, it covers both steam ports as well as the exhaust port. When it is moved to the right, the forward steam port is opened
