CONSTRUCTION OF RETORTS. 



63 



from the retort ; it is three or four inches in diameter at the top, increasing to 

 five inches at the bottom, to prevent the tar which adheres to the lower parts 

 from obstructing the flow of the gas. The lowest joint is made with a socket, 

 instead of a flanch, to allow for some expansion without injury. 



B' is a " bridge-piece," connecting the stand- and dip-pipes. 



C is the " dip-pipe," passing through the upper metal of the hydraulic 

 main, upon which it is jointed, and having its lower extremity, which is three 

 inches in diameter, immersed four or five inches into the tar contained therein. 

 The holes in the hydraulic main, through which the dip-pipes pass, are gene- 

 rally drilled and chipped out while the apparatus is in process of erection ; 

 because they are at unequal distances from one another, and to have them 

 tixed " out of square" would be an eye-sore. The height of the dip-pipes 

 from the surface of the tar, measured from the lower bend of the bridge- 

 piece, ought to be sufficient to contain the perpendicular head of tar forced up 

 into them by the pressure of the gas from the working retorts*. This would 

 probably in no case exceed three feet : I have made it five feet in the engra- 

 ving, which is perhaps unnecessary. 



DD are the bonnets, to be removed when the pipes require clearing, jointed 

 by putty and pasteboard, as just described ; they are represented in Fig. 11. 



Fig. 11. 





* The dip-pipes now spoken of belong to the retorts " thrown off." 



