I shall take a concrete example in explanation of the method of balance. Taking the rate of growth per second of a plant to be 1/50,000 inch or 0·5 µ, per second (equal to the wave length of sodium light), the tip of the plant will be maintained at the same point in space if we succeeded in making the plant-holder subside exactly at the same rate. The growth-elongation of the plant will then be exactly balanced by a compensating movement downwards. The state of exact balance is indicated when the recording lever of the Crescograph traces a horizontal line on the moving plate. Overbalance or underbalance will deflect the record below or above the horizontal line. [Illustration: FIG. 93.--Arrangement for compensation of growth-movement by equal subsidence of plant-holder; S, adjusting screw for regulation of speed of rotation; G, governor; W, heavy weight; P, plant-holder.] COMPENSATING MOVEMENT. For securing exact balance the holder of the plant P, in the given example, will have to subside at a rate of 1/50,000 inch per second. This is accomplished by a system of reducing worm and pinion, also of clock wheels (Fig. 93). The clock at first used for this purpose was worked by the usual balance wheel. Though this secured an _average_ balance yet as each tick of the clock consisted of sudden movement and stoppage, it caused minute variation in the rate of subsidence; this became magnified by the Crescograph and appeared as a series of oscillations about a mean position of equilibrium. This particular defect was obviated by the substitution of a fan governor for the balance wheel. But the speed of rotation slows down with the unwinding of the main spring, and the balance obtained at the beginning was found to be insufficient later on. The difficulty was finally overcome by the use of a heavy weight W, in the place of coiled spring. The complete apparatus is seen in figure 94. [Illustration: FIG. 94.--Photographic reproduction of the Balanced Crescograph. L, L', magnifying compound lever. R, recording plate. P, plant. C, clock work for oscillation of the plate and lateral movement. G, governor. M, circular growth-scale. V, plant-chamber.]