gas to pass into the pipe _t_, whilst it also serves to keep the gasometer steady when nearly out of the outer cistern. When the operation commences, the gasometer is sunk down nearly to a level with the surface of the water in the outer cistern, and is consequently filled with water; but as the gas enters, it rises up to receive it. It is to be noted, that the balance-weights _q_ _q_, should not be quite so heavy as the gasometer, in order that some pressure may be exerted, to force the gas out of the burners with a proper jet. The gas which issues from the retort enters the purifier as stated already, and ascends the pipe _o_, into the vessel, _p_, from which it displaces the water, and passes out at the small holes, as before described, rising through the water into the gasometer, and raising it up: the gas then passes away to the burners, _u_ _u_. In this manner the process proceeds until the whole of the volatile products of the coal in the retort is evaporated. The use of the gasometer is, to equalize the emission of the gas which comes from the retort more quickly at some time than others. When this happens, the interior vessel rises up to receive it, and when the stream from the retort diminishes, the weight of the gasometer expels its contents. When the process is finished, the retort is suffered to cool, and its ground stopper is then removed to replenish it with coal. The residue found in the retort is coke. _v_ _v_ are cocks to let off any liquid that may collect in the pipe _o_ or _t_; for if the smallest portion of liquid were to obstruct the free passage of the gas to the burners, the consequence would be, that the lights would not burn steadily--they would, as it is called, _dance_, or become extinguished. _x_ is the main stop-cock which communicates with the burners--these, of course, may be placed as convenience may require. _z_ _z_ are two projecting parts in the top of the gasometer; they are intended to receive the hood _p_, and the upper extremity of the pipe _t_, so as to allow the gasometer to be wholly immersed into the cistern. The wheels or pullies of the gasometer have a groove to allow the links of the chain to pass freely.