LAYING OUT THE DESIGN OF WORKS. 51 



to accumulate at bends, etc., and often the troubles that 

 are supposed to be unavoidable are simply due to the fact 

 that the whole plant is in a foul and dirty state. All the 

 outlets from the seal traps should converge into one common 

 cesspool, in preference to being carried direct to the tar 

 well. If gas should by any possibility force a seal, the fact 

 at once becomes evident. But small quantities of gas may 

 get into the tar well for some time before they are dis- 

 covered, and there is not only loss of gas but risk of serious 

 explosion. 



Very important improvements have lately been made in 

 the way of simplifying and cheapening the arrangements 

 incidental to dry purification. In place of the set of four 

 small boxes with water lute seals for the covers, contained 

 in a two-storey building, a centre valve, and tortuous 

 series of pipes to each box, one can now have dry-faced 

 covers, and valves resting snugly on the top of the purifiers, 

 with no connecting pipes to speak of. The old arrangement 

 was so costly that the size of the vessels was cut down to 

 the lowest point, and they were so small as to necessitate 

 frequent changing, perhaps several times a week in the 

 winter, which means waste of gas, material and labour, to 

 say nothing of the risk of sending out bad gas. The modern 

 arrangement consists of boxes let into the ground so far as 

 is consistent with a proper drainage from the bottom to the 

 tar cesspool, and with the exception of small pipes for this 

 purpose there is nothing else below ground, the gas pipes 

 being all on the top. A light roof is all that is required, a 

 building being quite unnecessary, and in some respects 

 undesirable. The upstairs arrangement was supposed to 

 economize ground, by leaving a floor space underneath the 

 vessels for preparing lime or exposing oxide, but this space 

 was too circumscribed to be of any great use. And there is 



