In front of houses Nos. 4 and 5 (which by the way are the oldest retort houses inside these works) is situated the boiler and engine house. There are three boilers 28 feet by 6 in diameter. In the engine house four of Beal's exhausters occupy prominent positions, they are used to exhaust or suck the gas from the retorts and afterwards force it through the vessels for purification; two of these driven by engines of 20 horse power work 150,000 cubic feet per hour each. Two driven by engines of 12 horse power work 100,000 per hour each. Attached to the inlet of each exhauster is one of Wright's exhauster governors, it is made on the principle of pressure or suction elevating or depressing a light cylinder working in a water-lute of sufficient depth. When an exhaust is maintained on the water gauge, counter balance weights equal to the exhaust on the area of the cylinder are applied, and the oscillations, as the suction increases or diminishes, regulate to a nicety the exhaust. The whole of the machinery in this department is in excellent order and will bear the minutest inspection. Over the engine house, which is reached outside by a corkscrew or spiral iron staircase, is a workshop fitted up with machinery; it contains a horizontal engine of eight horse power, which drives two lathes, one bolt screwing machine, two drilling machines, and a saw bench. Against the wall of the engine house is one of Tangye's Special Pumps for raising water from the dock to supply the whole of the works with water for cooling purposes. Outside the engine house an apparatus called a jet exhauster has recently been erected composed of a series of vertical iron tubes, a steam boiler, a generator, and jet. A vacuum is created by a blast of steam, thereby compelling the gas to rapidly leave the retorts and at the same time the ammonia is supposed to be entirely removed by means of water which percolates through shavings with which the tubes or pipes are filled. On the south side of the works, in addition to the coolers, there are thirteen purifiers and fifteen plots or courts including the foreman's lobby. Each purifier is of cast-iron, it is oblong in form, the cover is wrought iron riveted together in sheets, and the seal is made by means of a water-lute round the edge of the purifier. The purifying material, which is sometimes lime but principally oxide of iron, is carefully spread out on trays and these are disposed in tiers or sets in such a manner as to leave a clear open space between each succeeding layer to allow the gas to diffuse itself thoroughly throughout the mass. Lime when once fouled cannot profitably be renewed for gas purifying purposes, but the oxide of iron can be further utilized by spreading out the oxide in an open court when the oxygen of the atmosphere precipitates the sulphur and the oxide is again fit for use.