in that it enables the driver to operate the steam engine in a continuous range of settings from maximum economy to maximum power. At any setting, the valve gear satisfies the following two conditions: The valve opens to admit steam to the cylinder just before the start of a piston stroke. The pressure of this steam provides the driving force. Soon before the space on one side of the piston starts to contract, the valve starts to release steam from that space to the atmosphere, so as not to impede the movement of the piston. In an economical setting, steam is admitted to the expanding space for only part of the stroke; at a point set by the driver, the intake is cut off. Since the exhaust is also shut, during the rest of the stroke the steam that has entered the cylinder expands in isolation, and so its pressure decreases. Thus, the most energy available from the steam (in the absence of a condenser) is used. The Walschaerts valve gear enables the engine driver to change the cutoff point without changing the points at which intake starts. Economy also requires that the throttle be wide open and that the boiler pressure is at the maximum safe level to maximise thermal efficiency. For economy, a steam engine is used of a size such that the most economical settings yield the right amount of power most of the time, such as when a train is running at steady speed on level track. When greater power is necessary, e.g. when gaining speed when pulling out of a station and when ascending a gradient, the Walschaerts valve gear enables the engine driver to set the cutoff point near the end of the stroke, so that the full pressure of the boiler is exerted on the piston for almost the entire stroke. With such a setting, when the exhaust opens, the steam in the cylinder is near full boiler pressure. The pressure in the steam at that moment serves no useful purpose; it drives a sudden pulse of pressure into the atmosphere and is wasted . This sudden pulse of pressure causes the loud puffing sound that members of the public associate with steam engines, because they mostly encounter engines at stations, where efficiency is sacrificed as trains pull away. A steam engine well adjusted for efficiency makes a soft hissing sound that lasts