222 THE SCIENTIFIC PAPERS OF 



ascends into the upper part of the producer with a slight outward 

 pressure. It is necessary to maintain this pressure through the 

 whole length of the gas flue, in order to ensure a free supply of 

 gas to the furnaces, and to prevent its deterioration in the flue, 

 through the indraught of air at crevices in the brickwork. The 

 slight loss of gas by leakage, which results from a pressure in the 

 flue, is of no moment, as it ceases entirely in the course of a day 

 or two, when the crevices become closed by tar and soot. 



Where the furnace stands so much higher than the gas producer, 

 that the flue may be made to rise considerably, the required 

 plenum of pressure is at once obtained ; but more frequently the 

 furnaces and gas producers are placed nearly on the same level, 

 and some special arrangement is necessary to maintain the pressure 

 in the flue. The most simple contrivance for this purpose is the 

 " elevated cooling tube." The hot gas is carried up by a brick 

 stack to a height of eight or ten feet above the top of the gas 

 producer, and is led through a horizontal sheet-iron cooling tube, 

 J, of not less than 60 square feet of surface per gas producer, from 

 which it passes down either directly to the furnace, or into an 

 underground brick flue. 



The gas rising from the producer at a temperature of about 

 1100 F., is cooled as it passes along the overhead tube, and the 

 descending column is consequently denser and heavier than the 

 ascending column of the same length, and continually over-balances 

 it. The system forms, in fact, a syphon in which the two limbs 

 are of equal length, but the one is filled with a heavier fluid than 

 the other. The height of cooling tube required to produce as 

 great a pressure in the flue as would be obtained by placing the 

 gas producers, say ten feet deeper in the ground, may be readily 

 calculated. The temperature of the gas as it rises from the pro- 

 ducers has been taken as 1,100 F., and we may assume that it is 

 cooled in the overhead tube to 100 F., an extent of cooling very 

 easily attained. The calculated specific gravity, referred to 

 hydrogen, of the gas of which I have quoted the analysis, being 

 13*14, we obtain the following data : 



LB. 



Weight of the gas per cube foot at 1,100 F. = -022 



100 F. = -061 



Weight of atmospheric air per cube foot at 60 F. = -076 



