I general ways in which hydraulic control of servo circuits may be considered are: 1. Source control. 2. Line control. That is, (1) the direction and amount of fluid flow may be controlled at the source at which the hydraulic power is generated—for example, by use of a servo pump; or (2) a constant pressure source may be established and a metering valve placed in the main power line between this source and the load—for example, electro-hydraulic servo metering valves.