336 Method of producing Heat, Light, 8c. from Pit-Coal. 
should be changed every two or three days: by these 
means the gas will be deprived of all ts smell, at least as 
far as washing will effect it, and the apparatus will be 
clean. 
The stop-cock at O is for the use of a master, if he 
wishes to lock up the gas in the reservoir, to prevent his 
workman, &c. wasting it in his absence; as also if any 
pipe should leak, or a cock be out of order, in any part of 
the premises, by turning this cock all the gas is kept im the 
reservoir while the pipe is repaired, or any other alteration 
made; it also extinguishes all the lights when turned, if 
any are left burning by careless workmen, nor can they be 
lighted until it is opened again. 
The whole of this apparatus is simple, and not liable to 
be put out of order in such a way, but that any person may 
put it to rights again. All the art required to make the gas 
is to take off the cover of the pot, and without removing 
the pot to take out the coke, and fill it with. fresh coal, 
wedge it down by putting an iron wedge between the bewels 
or ears and the clbow of the vessel, and if required, plaster 
a little clay or loam round the cover, to keep it air-ught; a 
fire is then made under it, and the whole is done. The 
boy or man who does it, must now and then look at the 
fire and keep it up, until the pot is hot, and the gas is made. 
Now in works where lights are wanted almost always, I 
would recommend two fire-places, and two pots, so that 
when one pot is burned out, the other pot may be ready to 
act; for this purpose the purifier must be provided with 
two of the water-joints B, one communicating with each 
pot, and the elbow pipe of each pot must have a siop-cock, 
as V: now when one pot is burning, the cock in the other 
pipe must be stopped, that the gas may not find its way out 
of the purifier; and when all the gas is extracted from that 
pot, the cock C, leading from it, must be stopped, and the 
pot left to cool; while a fire 13 put under the other pot, its 
cock is opened, and a supply of gas from it is passed into 
the reservoir: by these means one of the pots is constantly 
supplying the reservoir with gas, and the lights are always 
kept burning; one purifier is all that is necessary; the cock 
V must be shut when either of the covers is taken up to 
fill the pot again with coal: when the elbow-pipe is lifted 
out of the water-joint, as the cover is attached to it, a plug 
must be provided to fit into the water-joint pipe the mo- 
ment the elbow is removed from it, or the gas wil] rush out 
of the pipe at the water-joint; but a better way would he, 
to Jengthen the pipe of the water-joints B, and place a large 
coc 
