As the quantity of air introduced at every stroke is constant, the work developed at every stroke is varied by regulating the temperature of the gas that fills the cylinder. When the temperature falls, the pressure, and consequently the work developed, diminishes. This result is obtained by varying the respective quantities of air that pass through the fire box and around the piston. In measure as less air passes through the fire box, the quantity that passes around the piston augments by just so much, and the pressure diminishes. A valve, _n'_, in the conduit that runs to the fire box is controlled by the regulator, L', in the interior of the column. When the work to be transmitted diminishes, the regulator closes the valve more or less, and the work developed diminishes. The coke is put by shovelfuls into a hopper, I. Four buckets mounted upon the periphery of a wheel, I', traverse the coke, and, taking up a piece of it, let it fall upon the cover, J, of the slide valve, _j_, whence it falls into the cavity of the latter when it is uncovered, and from thence into the conduit, _c'_, of the box, _j'_, when the cavity of the valve is opposite the conduit. From the conduit, _c'_, the coke falls upon the grate. A small sight hole covered with glass, in the cover, J, permits the grate to be seen when the cavity of the valve is opposite _c'_. As in gas engines, a current of water is made to flow around the cylinder, C', in order to keep the sides from getting too hot. In order to set the engine in motion, we begin by opening the bottom, C, of the cylinder, C', to clean the grate. This done, we close C and introduce lighted charcoal through the conduit, _c'_ (the valve being open). The valve is put in place, two or three revolutions are given to the fly wheel, and the motor starts. The feeding is afterward done with coke. The parts that transmit motion operate under conditions analogous to those under which the same parts of a steam engine do. The air pump sucks and forces nothing but cold air, and nothing but cold air passes through the distributing slide valve. The pump and valve are therefore rendered very durable. The piston and cylinder, at the points where friction exists, are at a temperature of 60 or 80 degrees. These surfaces are protected against hot gas charged with dust. The hot gas, which escapes from the cylinder through a valve, has previously been cooled by contact with the sides of the cylinder and by expansion. The eduction valve just mentioned works about like that of a steam engine, and it is only necessary to polish it now and then in order to keep it in good condition.--_Annales Industrielles._ * * * * * YOUR FUTURE PROBLEMS.[1] [Footnote 1: An address to the graduating class, Stevens Institute, Hoboken, N.J., 1887.] By CHARLES E. EMERY.