What is doubtless a very unique feature of the Pumpherston retort is the mechanical arrangement for withdrawing the spent shale continuously, and thus keeping the whole mass inside the retorts in constant movement. Below each pair of retorts is fixed a hopper made of cast-iron, and fixed to girders supported on the brick piers or columns between the ovens. At the top of each hopper, and immediately underneath the bottom of the retorts, is fixed a cast-iron disc or table, with a space left between its edge and the sides of the hopper. The whole mass of shale in the retort rests upon the table, the space permitting some to pass over the edge. Through the centre of the table a steel spindle projects, on the upper end of which is fixed a curved arm, and this, when rotated, pushes some of the shale off, causing it to fall over the edge of the table into the hopper below. The shaft carrying the curved arm passes through a stuffing-box on the hopper, and has a ratchet and lever fitted to the lower end, actuated by a rod of T-iron which is made to travel horizontally, and is driven by a small electric motor. The motion is comparatively slow, the arm making but one revolution in about 20 minutes, but the action is most satisfactory, the through-put of shale being regulated at will. The ammonia water got from the atmospheric condensers is pumped through a heater, in which it is raised in temperature by the waste water flowing from the still, and passes into the top of the still, which is circular in shape, about 30 feet high, and has a series of cast-iron shelves or trays fixed horizontally every 2 feet or thereabouts from the top to near the bottom. Steam is put into the bottom of the still at a pressure of 40 lb., and passes to the top through a series of conical arrangements on the shelves carrying with it the volatile ammonia, while the water, after traversing the whole area of each tray, passes out into a concrete tank containing a cast-iron worm, which is the heater already referred to, for the ammonia water on its way to the still. During its progress from the top to the bottom of the still, the water is diverted into a chamber containing milk of lime, setting free the fixed ammonia which cannot be got by steaming.