1. In a transfer mechanism: a pantograph comprising four arms a first and second arms of which each have pivotal connection at one end thereof to a third arm at spaced points and at the other end thereof to the fourth arm at spaced points, said pivotal connections being formed about substantially parallel axes, said pantograph having a pivotal connection about a pantograph axis parallel with said axes at one of said first and second arms; means for pivoting said pantograph about said pantograph axis by lineal displacement of a point in one of said third and fourth arms; and counterbalance means for applying uniform opposition to pivoting of said pantograph about said pantograph axis, said counterbalance means comprising a hydraulic cylinder and piston, one fixed to the pantograph and movable relative to the other in response to the difference between pressures acting to expand and compress the space within the cylinder.

9. A hydraulic counterweight for connection to a movable load comprising in combination: a hydraulic cylinder and piston, one fixed to the load and the other to a point with respect to which the load is movable, said piston and cylinder being movable relative to one another in response to differences between expansive cylinder pressure and compressive pressure applied thereto; a hydraulic pressure line for connection between a pressure source and a drain, said line including a check valve and a relief valve in series in that order beTween its pressure and drain ends; said cylinder having fluid communication with said line at a point intermediate said valves, said fluid line including a pilot pressure open valve at a point between said cylinder and the pressure end of said line; means for supplying pilot pressure to said pressure-opened valve only when said fluid line is pressurized in selected degree; and a pressure-sensing line for connection between the pressure source and drain and including a flow-resistance element and relief valve upstream from the flow-resistance element, said means for supplying pilot pressure comprising a fluid connection to said pilot pressure operated valve from a point in said pressure-sensing line intermediate the relief valve and flow-resistance element.