steam-balance

from The Century Dictionary.

• noun Any device or apparatus in which the pressure of steam is used to oppose and balance a weight or spring or other force.
• noun A steam-accumulator in which steam, acting on a piston, balances a water-pressure upon a plunger or second piston acting in the opposite sense.
• noun A steam-cylinder, in a vertical engine, with a piston to balance the weight of the piston-rod and connecting-rod, so that the up and down strokes may be of equal power.
• noun A steam-cylinder with a piston to balance or reduce the pressure by which a slide-valve is held to its seat by the working pressure on its back.
• noun The weighted lever of a boiler safety-valve which tends to hold the valve on its seat against the steam-pressure underneath it.