cylinder (position IV). On the upward or compression stroke the valve continues in its rotation and covers the inlet port C and permits proper compression of the charge which is fired in the usual manner (position II). After the explosion the piston on its downward stroke revolves the shaft with its connecting gears and thus rotates the valves until the opening B uncovers the exhaust port D, through which the burnt gases are forced by the returning piston (position III). This motor is too new to judge of its ultimate future or general adoption, but it possesses many advantages over either the poppetvalve or sleeve-valve types, and where it has been used it has given most highly satisfactory results. The exhaust opening being a trifle larger than the inlet, allows a full and perfect scavenging of the burnt gas, while the uniform and accurate opening of the inlet and exhaust ports is fully equal to the same results obtained in the Knight motor. The valves being of bronze form their own bearing surface on the iron of the cylinders, while the constant rotary motion and the fact that they are always firmly seated prevent the formation of carbon deposits. Of course there is some wear of the valves against their seats, but adjustment for taking up this small amount of abrasion is provided. There is comparatively little friction and the valves are provided with ball bearings at the upper end of the stems, while the action of the spiral or helical gears is positive, accurate, and practically noiseless. The shaft gear, vertical valve driving rod, and valve gears are all enclosed, giving the motor a very neat and clean appearance, and yet all are readily accessible. Although here

tofore used principally as a marine engine, yet there is no reason why the rotary-valve motor should not give equally good results in stationary or vehicle use, and automobiles are now being equipped with this motor which will doubtless prove most successful.

Another rotary-valve engine which has recently

Fig. 34.-Offset Cylinder Motor

appeared on the market is the Russell motor manufactured by The Silent Valve Co. of America. In this engine the rotating valves are conical in form and through a special arrangement of driving yokes and springs the valves automatically adjust themselves for

wear or expansion. Doubtless many other designs of rotary- and slide-valve motors will soon appear, for the present tendency of design is to improve upon the old poppet-valve mechanism. For general use, however, it is doubtful if this type of valve can be greatly improved upon.

Four-cycle motors vary considerably in design and arrangement of parts with different makers. These motors, as well as those of the two-cycle type, are often made with the cylinders offset (Fig. 34), a system which is supposed to overcome the tendency to a dead centre and which is used to a considerable extent on vehicle motors but has never become general on marine or stationary engines, but is a special feature of the wellknown Ferro engines. Motors with the cylinders horizontal and opposed or opposite are also widely used, and this arrangement has many advantages over the vertical-cylinder motors and has been adopted by many makers of marine, vehicle, and stationary engines. These motors are very compact, light, and powerful, and the balanced method of construction gives a minimum vibration owing to the impulse from the opposite pistons overcoming the jar and rotary tendency of ordinary motors (Fig. 35). For small vehicles such motors are excellent and in many classes of boats they are far more convenient than the regular vertical engines. Their operation, as well as that of certain "V-shaped" models, is practically the same as in the ordinary fourcycle motor, but in the opposed-cylinder type the two cylinders may be adjusted so that the explosion in the two cylinders occurs alternately, thus giving an impulse

for every revolution as in a two-cycle motor. Multiplecylinder, four-cycle motors are made in from two to six or more cylinders, but in the four-cylinder machine

the cranks are set at an angle of 180 degrees and are commonly constructed with the two central cranks close together without a central bearing, or with a very small