The cylindrical vessel P, of fig. 3, surrounding the orifice of the pipe O, which delivers the tar into the tar cistern, fig. 3, serves to keep this pipe constantly immersed into a portion of tar, so that the contents of the cistern may be drawn off by the cock without admitting air into any part of the apparatus. The tar cistern has a small hole at the top, to allow the air which it encloses to escape, as it becomes filled with tar and ammoniacal liquor. The main condenser L, is placed, as shown in the drawing, higher than the level of the water in the gasometer cistern, to allow a free descent of the distillatory liquids as they pass from this vessel along into the pipes M, N, O, &c. The cistern of the gasometer, as well as the lime machine, and tar cistern, are constructed of cast iron plates, bolted and cemented together with iron cement. The gasometer is made of sheet iron plates rivetted together--E, E, are two iron stays--G, G, are friction wheels. _METHOD of correcting the relative pressure of the Gasometer, so as to cause the gas which it contains to be uniformly of an equal density._[42] [42] For this elegant contrivance we are also indebted to Mr. CLEGG. We have mentioned already that the pressure of the gas in the gasometer should be invariable, for it is obvious that the weight of the gasometer is constantly increasing in proportion as it fills with gas, and rises out of the water--see p. 88, and 167. To render its pressure uniform, we first take the _absolute_ weight of that part of the gasometer which becomes immersed in the water, and knowing the _specific weight_ of the substance of which it is composed, we divide its absolute weight by the specific weight of the substance of which it is composed; and this being done, we make part of the chain, (measured at right angles from the axis of the wheels over which it passes downwards towards the top of the gasometer,) which is equal to the length of that part of the gasometer which becomes immersed in water, equal in weight to the specific gravity of the substance of which the gasometer is composed. For example, let us suppose that the part of the gasometer which becomes immersed in water weighs 861 _lb._ and that it is composed of sheet iron, the specific gravity of which, in round numbers, we will take to be 7. It is then evident, that the part of the chain of the gasometer measured downward from the axis of the wheel over which it passes, and which is equal in length to the height of the gasometer, must be loaded with a weight of, or must itself weigh, 123_lb._ for this would be the weight of the water displaced by the gasometer; or let us suppose the gasometer to be made of sheet copper, the specific weight of which (omitting decimals) is 8; and that the absolute weight of the gasometer is 1792_lbs._ then the chain of the gasometer equal in length to the height of the gasometer, immersed into the water must weigh 224_lb._ for this