64 CONSTRUCTION OF RETORTS. 



E is the " hydraulic main," running the entire length of the retort-house, 

 over the benches, in a perfectly horizontal direction, and sufficiently high up 

 to allow of head-room, and to be removed from the flame issuing from the 

 retorts while charging. They are sometimes turned the reverse way to that 

 shown in the Plate, and made to rest upon the brickwork of the benches ; but 

 this is inconvenient when the brickwork has to be taken down or repaired. 



This main is three-quarters of an inch in thickness, and cast in convenient 

 lengths, contrived to reach over two benches ; in this case they would be 

 equal to 13 feet 6 inches. The joints are made with iron cement. Its use is 

 to cut off the communication between the retorts, when one or more benches 

 are charging or open. Being half full of tar, the gas evolved from the retorts 

 in action remains in the upper part, and the ends of the dip-pipes immersed 

 under the surface are effectually sealed. The pressure of the gas on the sur- 

 face of the tar will force some up into the dip-pipes connected with the open 

 retorts, the height to which such tar is forced being equal to that pressure. 



The diameter of the hydraulic main must be sufficient to form a reservoir 

 capable of supplying the quantity of tar contained in the open dip- pipes with- 

 out suffering it to fall below their immersed ends, and thus open a communi- 

 cation between the open and working retorts. There are other methods of 

 connecting the retorts with the hydraulic main, as shown in Figs. 12 and 13. 



In Fig. 12, A is the main, cast square at the bottom, to allow of more tar 

 being contained therein, and a greater facility in making the bottom joint of 

 the stand-pipe. 



B is the stand-pipe from the retort, and C the sealing-pipe covering the 

 stand-pipe and dipping into the tar contained in the main : the annular space 

 between the two pipes must be more than equal to the area of the stand-pipe. 

 D is a bonnet, to be removed when the pipes require clearing. This arrange- 

 ment has a neat appearance, and is ceconomical ; but its great disadvantage is 

 the difficulty of cleaning the hydraulic main between the stand-pipes. 



Fig. 13. is a contrivance for placing the hydraulic main under the firing 

 floor. This can only be used in a few instances where the retorts are set 

 singly or in couples, and where a coke-cellar is built. It is very simple, how- 

 ever, and in the cases just quoted may be adopted with advantage. A is the 

 main ; B is the dip-pipe, connected to the retort by an elbow-piece jointed 

 on to the mouth by a flanch or a socket, as usual ; C is the bonnet, by remov- 

 ing which both dip and elbow may be cleared. 



