136 On the Fetid Cas of Drains. 



mortafity.' I do not krow of any experiments which have been ftriftly Inftltuted to (hew 

 that the Taws of the elafticity and confequent expanfiou of the gafes under variations of 

 preflure are different, and in what dire£lion and quantity. But it is evident, in the prefent 

 cafe, that when the fuperincumbent preflure of the air above us is diminifticd, the gas in 

 the drains expands more than the common air near the furface of the earth : fo that the 

 former is made to afcend into the apartments of houfes and the ftreets. The aftion'of 

 the fires in dwelling houfes caufing an afcending current up the chimneys, there muft of 

 courfe be a fupply at the doors, windows, and other openings : but when thefe openings 

 are moll: effeftiially fhut, as is the cafe at night, they will lefs readily fupnly the air, and 

 a larger portion will be prefled in from the drains. It is certain, neverthelefs, that this 

 preflure, as far as it regards the openings into a dwelling houfe, is in all cafes very fmall. 

 Indeed, there is no probability that it fhould equal the prefliire of half an inch of water. 

 The remedy for this ferious and very difgufl:ing inconvenience is, therefore, very eafy and 

 Cinple, and likewife very well-known, though much lefs fo than could be wifhed when 

 we refleCl. on the number of houfes which are tlius incommoded. I do not, therefore, 

 think that the want of novelty ought to be confidered as a fufficient reafon why 1 (hould 

 hefitate to defcribe for the ufe of my prefent corrcfpondent, and others in the like want 

 of information, the very common apparatus which bricklayers call a {link trap. 



Let us fuppofe a drain, or fubterraneous gutter, to communicate" from beneath the 

 pavdment of a kitchen to the principal drain in the flreet, and that there is a grate, or 

 a fet of holes, in the pavement for the purpofe of fuffering wafte water to flow oflT. 

 Thefe fame holes will admit the noxious vapours of the drain into the houfe. But 

 if any contrivance could be made that the drain fhould fuffer water to run out, and 

 at the fame time prevent air from returning back, we fhould evidently be in pofloflion of a 

 remedy for the fubjedl: of complaint. A variety of methods prefent themfelves for the 

 accomplifliment of this. The nioft common is diftinguifhed by the name given above. 

 It confifts in inclofing the drain with a flat ftone, which ferves as a wall, to prevent 

 all communication as to the upper part of the cavity : but the water is fuffered 

 to pafs o9^ beneath -its lower edge by finking the floor of the drain in that par- 

 ticular part. The deprefled part or cavity, therefore, forms a fmall well or pool, in 

 which water is conftantly lodged ; and the edge of the ftone being defignedly low enough to 

 remain conftantly immerfed, the gas cannot return, unlefs its preflure be fuflicient to fink 

 that furface of the water which is furthcft from the houfe to a depth at leaft equal to 

 that depreflion ; — an event which never happens. 



Though this contrivance is attended with a charge which is very little when counter- 

 balanced againft the evils it removes, yet as it requires the pavement to be taken up, and a 

 few other difpofitions to be made, there are fufficient motives for preferring ftill Ampler 

 methods if ofi'ered. One of thefe confifts in making the cavity or depreffion of the floor 

 immediately beneath the opening in the pavement, and fixing a piece of wooden or other 

 pipe to pafs from the furface of the ground into the ftagnant water. It may eafily be 

 underftood that a perfeft clofure muft be made round the fides of the pipe before the grate 



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