1815] Illumination by Coal Gas. 19 
then produced, which has the capital disadvantage of affording a 
suffocating odour when the gas is burnt. 
I need scarcely mention that it makes no difference in what 
form the coal is used, and that the very refuse or small coal, which 
passes through the screen at the pit’s mouth, and which finds no 
market, nay, even the sweepings of the pit, which are thrown 
away, may be employed for the production of the gas. 
With regard to the pressure of the gazometer, your Correspondent 
is informed that experience has shown that a pressure of a column 
of water from an half'to one inch is sufficient for regulating the 
proper supply of the gas to the lamps and burners; but this pressure 
must be constant and uniform. It is obvious that the weight of the 
gazometer or vessel which contains the gas is constantly increasing 
in proportion as it fills with gas and rises out of the water or cistern 
in which it is immersed ; and consequently, if a constant or uniform 
balance weight equal only to that of the gazometer in the first 
moment of its immersion be employed, the gas becomes gradually 
more and more compressed by that part of the weight of the 
gazometer which is not counterpoised ; therefore insurmountable 
difficulties would follow, because it would be impossible to regulate 
the size of the flames, &c. To compensate for this increasing 
weight of the gazometer, the chain by which this vessel is sus- 
pended, or at least such a part of it as is equal in length to the 
height of the gazometer (measured at right angles to the axis of the 
wheel over which it passes downwards) must be loaded with a 
weight equal to the quantity of water which the gazometer -dis- 
places ;* and thus the density of the gas will be uniform, or at all 
times the same. : 
The diameter of the pipes which convey the gas is not taken at 
random, as your Correspondent imagines. Their diameters is a 
simple matter of calculation, depending upon the quantity of gas 
which they have to deliver in a given time, and the diameters of the 
branch pipes proceeding from them. 
Further information concerning the general nature of the gas 
light illumination, together with a description of the best machine- 
ries employed in this new branch of civil economy, your Corres- 
pondent will find in a Treatise on Gas Light, illustrated with 
copper plates, which will be published on the 10th of next month, 
by, Sir, Your most obedient humble servant, 
Jompton street, Soho 
¢ aeige, lel.” Freprerick Accum, 
* For this elegant contrivance we are indebted to Mr, Clegg, the engineer of 
the Gas Light Company, 
