Then when the left-hand slots of the outer sleeve open upon, or come into register with the left-hand slots of the inner sleeve, a passage into the cylinder is opened for the new gas to enter. When a charge of gas has been drawn into the cylinder, one sleeve rises while the other falls, so that the openings are separated and the passage is tightly closed. The compression stroke then begins with the piston rising to the top. At this juncture the igniting spark explodes the compressed gas and the downward or power stroke takes place. During the upward compression stroke and the downward impulse stroke the slots have been closed, allowing no opportunity for the gas to escape. When the explosion has taken place and the piston has been driven to the bottom of its stroke, the right-hand openings in the inner sleeve and those of the outer sleeve come together, providing a passage for the exhausted gases to escape with the fourth or exhaust stroke. Thus it is plain that the motor is of the four-cycle type and it should not be confounded with two-cycle motors. As the expert explained the motion he showed how it was carried out on an engine from which the casing had been partly removed. The careful mechanical adjustment of the eccentric shaft, which operated the connecting rods that pull the sleeves of the cylinder up and down so that the openings for the entrance of the fresh gas and the expulsion of the exploded fumes come together at just the proper second, was what took the boy's eye. In connection with this the scientist handed the boy a magazine to read. It was a copy of the _Motor Age_ in which an expert said editorially: "Those who pin their faith to the slide-valve motor do so for many reasons, chief of which is that with this motor there is a definite opening and closing of the intake and exhaust parts, no matter at what motor speeds the car be operating. Two years ago one of the leading American engineers experimented with poppet valves and discovered that frequently at the high speeds the exhaust valves did not shut, there not being sufficient time owing to the inability of the valve spring to close the valve in the interval before a cam returned to open it again. With such a condition it is certain that the most powerful mixture was not obtained. With the sleeve valve such failure of operation cannot be, because no matter how fast the motor is operating there is a definite opening and closing for both intake and exhaust valve. "It is a well-known fact that with poppet valves the tension of the springs on the exhaust side varies after five or six weeks' use, and consequently the accuracy of opening and closing is interfered with. Carbon gets on the valve seatings and prevents proper closing of the valve, with the result that the compression is interfered with and the face of the valve injured. These troubles are, as far as can be learned, obviated in the sleeve valve."