Abstract: Transportable booms having lift platforms are now well established in the art. In the lift platform where the lift controls are located, there are means to operate an actuator and four bar linkage system in such manner that the lifting torque is substantially constant, and the arcuate or angular movement of the upper boom about its articulate connection on the lower boom is substantially constant, being at all times proportioned to the linear travel of the fluid motor actuator. In this way, the central purpose of the invention is achieved which is to obtain a controllable actuating system.The four bar linkage system achieves optimum results when the driving link a is related to the follower link b driven link c and base link d in the proportion a; b; c; d = 1.52; 1.28; 1.00; 0.76 and the fixed pivot of the driving link is offset 0.27 units from the fixed pivot for the driven link.