exhauster out of action the liquid in the hydraulic main rises up the dip pipe to a considerable height, and through this accumulated pressure the gas has to force its way. The consequence is, that it takes an easier course, i.e., through the cracks and joints of the clay retorts: clay retorts, although being practically gas tight under a few tenths of an inch of pressure, are not so under the pressure which results when the exhauster is out of action; the consequence being that there is great leakage, as just described. The same result does not take place when iron retorts are employed, as the metal of which such retorts are made is more homogeneous than fire-clay: it is not so necessary, therefore, in the case of iron retorts to employ an exhauster, as there is not the same amount of leakage. In addition to the loss of gas by leakage, there is another reason for the employment of the exhauster, viz., when gas is subjected to pressure in the retorts, some of the richest constituents of the gas are deposited on the walls of the retorts, the deposit being technically known as carbon or "scurf:" this carbon takes away from the retort space, and consequently must be periodically removed, which means retorts standing idle for a time. It also means an increased consumption of fuel, owing to the carbon being a non-conductor of heat: further, the removal of the scurf, as ordinarily practised, viz., by chisel bars, is very destructive to the retorts themselves.

Exhausters are divided into two classes, rotary and reciprocating. The exhausters of Jones, Laidlaw, and Beale are on the rotary principle, while Methven's, Anderson's, and Dempster's are reciprocating. The two lastmentioned exhausters are similar in principle, the apparatus essentially consisting of what may be called a gaspump, having a strong resemblance to an ordinary steam cylinder, with its accompanying piston and piston-rods, slide-valve, ports, etc.

Fig. 17 shows a small type of pump exhauster, as manufactured by Messrs. R. and J. Dempster. It is made on the same principle as an ordinary pump, the piston-rod of the steam cylinder being coupled direct to the gas cylinder by means of a cast-steel bow.

The majority of exhausters met with in gas-works, however, are of the rotary class, those which work on the principle originally devised by Mr. Beale being the most numerous.

Beale's exhauster, as manufactured by Messrs. Gwynne and Co., is shown in figs. 18, 19. The apparatus consists of an outer cylinder bored out, provided with inlet and outlet pipes, and having a roll or drum working eccentrically inside it, the diameter of the drum being two-thirds that of the outer case. The axis of the drum is placed at such a level in relation to the outer case that the drum, when revolving, is in contact with the bottom of the outer cylinder, but does not rest upon it.

The drum revolves upon a shaft which passes through

stuffing-boxes fixed on the end plates of the outer cylinder, where it rests on two gun-metal bearings, one at either end of the outer cylinder. The drum has slots cut in it on each

FIG. 18.

side, and through these slots freely slide two diaphragms or blades, which, being planed and faced, slide over one another, and thus form one double diaphragm, which varies in width according to the relative position of the two single diaphragms to

each other. The ends of

these slides have steel-cased pins which work in wroughtiron segments turning in grooves sunk in the end covers of the outer cylinder, so that when the drum revolves out of the centre of the cylinder, the diaphragm being attached to the

FIG. 19.

segments, the slides are drawn in and out, and so draw in the gas on one side of the cylinder and discharge it on the other by continuously sweeping the walls of the outer cylinder, and as the segments travel in a circular course,

concentric to the outer cylinder, the outer edges of the diaphragms are always maintained in the same relative position to the inner walls of the cylinder by the aid of the sliding arrangement previously alluded to. The slides are kept in close contact with the inner walls of the cylinder by means of a T-shaped strip having a flat spring at the back, the web of the T and the spring being recessed into a groove in the edge of the slide. The drum is driven either direct from a steam-engine, which is coupled on to the same shaft, or by means of belting from a separate source of power. Each revo

lution of the drum and slides draws in and discharges double the contents of the cylinder, minus the contents of the drum and slides.

Fig. 20 shows Beale's exhauster, as manufactured by Messrs. B. Donkin and Sons. It will be seen that in this form of exhauster, a rotating block is fixed in

FIG. 20.

the centre of the exhauster, which is carried on a horizonal fixed pin, and having two guides on the slide which engage with the rotating block, and guide the slide independently of the casing.

The chief improvement claimed for this latter form of exhauster is that the slide is guided from the centre of the exhauster, where the velocity is very low, instead of from the periphery, where the velocity is high; the difference being as 15 to 1 for the same number of revolutions.

A later development in exhausters by the same firm is utilizing the space occupied by the internal cylinder, as shown in fig. 21.

G

Waller's Three-Blade Exhauster.-It is claimed for this form of exhauster that with three blades it will deliver one-third more than the Beale's exhauster with the same size cylinder, and at the same speed, the contents of the cylinder being discharged three times in each revolution, against twice in the latter; the oscillation is thereby reduced in proportion (and still further when four blades are used). The blades working on a central pin are radial with the cylinder, and work smoothly round its circumference without ring or segments, instead of, as formerly, being eccentric

exhauster, necessitating the use of a heavy fly-wheel to counteract the effect; for the same reason, this exhauster can be run at a higher speed with steadiness and without increased friction. The exhauster is made with or without combined engine.

The cylinder is made as usual with two branches, and either can be inlet or outlet. The covers are recessed for ends of the roller, and have a boss for a central stud or, in large sizes, for a central pin going through both covers. This stud, or pin, is of steel, and hollow to admit of oil being injected to lubricate the joints of the blades. The roller is open at one end and solid at the