74 THE POPULAR SCIENCE MONTHLY. 



anxieties, the often permanent darkening of life, the straitened means 

 of subsistence, the very frequent destitution and pauperism, which 

 attend or follow such suffering, death statistics, to which alone I can 

 refer, testify only in sample or by suggestion." 



The means by which infection is likely to be conveyed to house- 

 holds are far too numerous to be dealt with in a single lecture, and I 

 have thought it best to select for consideration three or four of what 

 I feel to be among the more important, and to deal with these in 

 detail. 



In a report on an epidemic of enteric fever at Croydon, in 1875, Dr. 

 Buchanan, F. R. S., makes use of the following words : " The air of 

 the sewers is, as it were, ' laid on ' to houses." That significant ex- 

 pression " laid on " comes in aptly, as giving prominence to the special 

 features of one of the channels for conveying infection to households, 

 to which I propose drawing your attention. From the inside of every 

 ordinary dwelling-house there pass certain waste-pipes intended to 

 convey liquid refuse, first to the house-drains without, and thence to 

 the public sewers. It is the custom to regard these conduits as pass- 

 ing from house to sewer, but this evening I would ask you to compare 

 them with the pipes for the supply of coal-gas, and to view them 

 rather as passing from the sewer as a center to the periphery within 

 our dwelling-houses. In our comparison the public sewer may be re- 

 garded as corresponding with the gasometer ; the house-drain and the 

 waste-pipes as representing the service-pipes for gas ; and the so-called 

 " trap " indoors as taking the place of the metal tap found in connec- 

 tion with each gas-bracket. Sewer-air, even in its normal state, is a 

 grave source of danger to health ; but when the sewers receive in 

 their course along the streets the infectious refuse discharged from 

 houses where specific disease prevails, then the sewer-air harmful 

 hitherto is changed into an intense poison. 



How is it usually sought to debar this poisonous agent from dwell- 

 ings ? The sole means adopted, in nine cases out of ten, consists in 

 placing at some point of the pipe which connects the interior of the 

 house with the interior of the sewer a small body of water which is 

 known as a " trap," and which is designed to act as a barrier to the 

 passage of all sewer-air. The contrivance most commonly resorted to 

 is the so-called bell-trap, an apparatus in which the rim of a bell- 

 shaped cup is suspended in a small body of water contained within a 

 circular depression. This form of trap is of all the least efficient ; it 

 is not only one in which the water-lock constituting the trapping may 

 at any moment be entirely removed at the will of the individual, but 

 at its best it provides between the house and the sewer a layer of 

 water only about one half or three quarters of an inch in depth. Even 

 the most efficient of all traps, the so-called " siphon-bend," is not much 

 better. Dr. Andrew Fergus maintains that trapping has but little 

 effect in keeping sewer-air out of houses, as the entrance of the con- 



