The virtue or equivalent eccentric for the Meyer’s expansion valve is defined as an eccentric having such a length and angle of advance that will cause cut-off to take place at the same position, as is caused by the combined effect of main eccentric and expansion eccentric.

The valve gear of a steam engine is a mechanism that controls the inlet and exhaust valves. It also serves as a reversing gear. 

The reversing gear controls the direction of the engine's travel, and adjusts the timing of the slide valves. It also controls when and how long the intake- and exhaust-ports of the cylinders are open. 
A slide valve engine has a constant direction of pressure. It's only reversed by changing the valve position relative to the crank shaft.