But as comparisons seldom go on all four, the mill and the human machine differ in one respect: the miller, when he takes up his flashes, lays them it may be on the bank, goes whistling into his mill, and thinks no more till his grist is ground, for the water will work on forever unless he shuts it out again : but the valves used by the mind fall back again of them selves when the mind withdraws her activity.
The mind in this case works like the miller of an overshot mill, he has shoots lying over every one of his wheels, stopped by flash-boards at their upper ends, against which the water lies bearing always ready to drive the wheels whenever it can find a passage : so tbe miller by drawing a little board, which any child might pull up with a finger, turns the stream upon this wheel or that as he pleases, and twirls round a massive stone which he could not stir with both his arms.  But as comparisons seldom go on all four, the mill and the human machine differ in one respect: the miller, when he takes up his flashes, lays them it may be on the bank, goes whistling into his mill, and thinks no more till his grist is ground, for the water will work on forever unless he shuts it out again : but the valves used by the mind fall back again of them selves when the mind withdraws her activity.  Therefore if you would point with your hand at some object for any time, you must continue to exert yourself all the while : for the moment the mind forbears her volition, the valves close, the stream ceases to flow upon the brachial muscles, and the arm no longer supported falls to your side.