able for superheat, but these obstacles were never sufficient to blind engineers as to the merits of superheated steam. It was found that all sliding valves, as flat, piston and Corliss valves, were not suitable for high temperatures, as they have to depend upon their surrounding walls for steam tightness and required. careful fitting for varying temperatures and abundant lubrication.

German engineers have, for that reason, devoted their efforts for the last fifty years to the perfection of the balanced and frictionless poppet valve, and have fitted all their engines with this device. On account of the great success obtained with poppet valve engines, this type entirely replaced the Corliss engine in Germany, Austria, and Switzerland, during the last thirty years..

The advantages of the poppet valve are: no rubbing surfaces, hence no lubrication, and its adaptability for high temperature; small weight and practical balance, hence its adaptability for high speed and high steam pressure; no possible wear, consequently the economy maintained for an indefinite period.

There are a number of excellent types of poppet valve engines, built on the European continent and in England. Of these, the Lentz engine has figured most prominently as the best representative of its class. As the Lentz engine is to my knowledge the only modern poppet valve engine built in this country, a short description of its constructional features will no doubt be of interest.

Figure 1 shows the valve gear side of the Lentz engine. A lay shaft is running along side the engine driven from the main shaft by a pair of bevel gears. All the eccentric are keyed on this lay shaft, each separate eccentric operating one valve by means of eccentric strap, rod and lever. In such a way, the different valves are entirely independent from each other and can be set very accurately to the requirements of steam distribution. The governor is placed right between, and is directly connected to the two high pressure steam eccentrics, and changes their position according to the requirements of the load without the help of chains or belts.

FIGURE 1.-VALVE GEAR SIDE OF THE LENTZ ENGINE.

Figure 2 shows a cross section through the high pressure cylinder.

The valve gear itself is shown in larger scale in Figure 3. The principle of the poppet valve is, that by resting with its two faces on the corresponding seats of the cylinder, it prevents the flow of steam from the steam chest into the cylinder. Admission and cut-off are effected by breaking and re-establishing the contact of the seats. The valve is entirely balanced but for the width of the seat, which even in the largest sizes does not exceed one-eighth of an inch, and hence takes very little power to operate. The valve gear consists only of a roller and cam lever. The latter is oscillated by means of the eccentric rod, and is so curved that it opens the valve and closes it in a very rapid manner. Dash pots are not used, as the whole operation

is entirely positive. On account of the noiseless operation of the valve gear, and its great simplicity, it is possible to operate the engine safely at high speeds. A limit has not been reached at even 350 revolutions per minute.

A feature is the absence of any spindle packing as the spindles are simply ground into cast iron bushings, and provided with grooves, thus producing a labyrinth system in which the condensed steam and oil collect and by which the escape of any steam is successfully prevented.

A similar principle is used for the piston rod packing, Figure 4, which is made entirely of cast iron rings, but so arranged as to follow the floating action of the piston rod.

A longitudinal section through the cylinders of a tandem compound engine is given in Figure 5. The low pressure cylinder is placed next to the bed. This arrangement gives the high pressure cylinder a chance to expand freely, without being obstructed by the distance piece and the low pressure cylinder.