The eleventh part is appropriated to the description of another apparatus, called the governor, also of recent invention, and now in use at numerous establishments. The design of this machine is, to regulate the pressure of the gas, before it enters into the mains, the importance of which must be sufficiently manifest. I have also pointed out the application of this apparatus for regulating the magnitude of the flames of gas burners and lamps. The twelfth part treats on gas mains and branch pipes, I have here stated the rules and practical proceedings necessary to be observed, for applying and distributing gas pipes to the greatest advantage. The most efficient method of introducing the gas to the interior of houses, forms the subject of part thirteen. All the necessary instructions are here given to workmen, for adapting the gas pipes, and insuring success at the least cost, under every variety of circumstances. The fourteenth part gives an account of the illuminating power of coal gas--the quantity of gas consumed in a given time, by different kinds of gas burners and lamps, the relative cost of gas, tallow, and oil lights of different intensities, and the most improved method employed for ventilating apartments lighted by gas. In the fifteenth and sixteenth parts, I have added an account of the manufacture of carburetted hydrogen gas, from coal tar, vegetable tar, and oil, with such other observations as may enable the reader to form a proper estimate of the comparative advantage of manufacturing gas from oil, or tar, under certain circumstances. I have here also given an account of the manufacture of carbonate of ammonia, as now practised, from the ammoniacal liquor obtained in the Gas Light process, and of the manufacture of other saleable products obtainable from coal, namely; pitch, coal tar, and oil. In conclusion I have to observe that my object throughout has been to make the work a compendium of all the best information which the practice of the art down to the present moment has been able to afford, embodying a great number of data, with which I have been obligingly favoured by gentlemen, the most practically versant in the art, and for which I beg they will individually accept this public expression of my thanks, and obligations, as well as the results which my own labours in this department, neither few, nor inconsiderable have furnished. To supply the reader with a work of practical utility in a most valuable, and growing branch of national economy has been my object; and I need scarcely add, that the suffrages of the public to the zeal and industry at least with which I have endeavoured to obtain that object, will be a source of infinite satisfaction. FREDRICK ACCUM. _LONDON, 1819._ CONTENTS. PART I. PAGE