In this apparatus there is no provision made for the unequal pressure which the gas suffers, accordingly as the gasometer is more or less immersed in water. It will be observed that, in this apparatus, the weight of the interior vessel is constantly increasing, in proportion as it fills with gas, and rises out of the water, and consequently, if a constant, uniform, counterpoising weight, equal only to that of the gasometer in the first moment of its rise, be employed, the gas becomes gradually more and more compressed by that part of the weight of the gasometer which is not counterpoised, and if its pressure or quantity be then estimated by the bulk which it occupies, without making allowance for the increasing pressure, a material error must arise, and this, in the large way, would give rise to insurmountable difficulties with regard to the regulation of the size of the flames; which could not be rendered uniform. Suppose the cistern or exterior vessel full of water, and the gasometer partly filled with gas and partly with water, it is evident that the balance-weight may be so adjusted, as to occasion an exact equilibrium, so that the external air shall not tend to enter into the gasometer nor the gas to escape from it; and in this case the water will stand exactly at the same level both within the gasometer and within the outer cistern. On the contrary, if the balance-weights be diminished, the gasometer will then press downwards from its own gravity, and the water will stand lower in the gasometer than it does in the cistern; in this case, the included air or gas will suffer a degree of compression above that experienced by the external air, exactly proportioned to the weight of a column of water, equal to the difference of the external and internal surfaces of the water. To compensate for this increasing weight of the gasometer, and render a scale of equal graduations accurate, some have ingeniously adopted the plan of a spiral pulley to the chain, which has the effect of gradually avoiding the evil, but the best way of accomplishing it will be stated hereafter. With regard to the philosophy or the production of coal-gas, it proves that pit-coal contains solid hidrogen, carbon, and oxigen. When the intensity of the heat has reached a certain degree, a part of the carbon unites with part of the oxigen and produces carbonic acid, which by means of caloric is melted into the gazeous state and forms carbonic acid gas; at the same time, part of the hidrogen of the coal combines with another portion of carbon and caloric, and forms the carburetted hidrogen gas, which varies considerably in its constitution, according to the circumstances under which it is produced; a portion of olifiant gas, carbonic oxid, hidrogen, and sulphuretted hidrogen, is also produced during the process. The quantities of these products vary according to the nature of the coal employed in the process.