The record of the sanitary improvements in any period would be a comparatively barren one if no reference was made to the great and difficult question of sewerage and its attendant evils. Fortunately for humanity, and especially for that portion of it which lives in cities, the period which we are reviewing has been one fruitful in results in this important branch of sanitary science. Not only have great discoveries and important practical improvements been made, but the whole field of previous knowledge and experience on the subject has been thoroughly worked up and codified. So numerous are the works that have appeared, so accurate and systematic are the laws that have been laid down, so precise is our knowledge of its leading principles, that this subject of sewerage has almost grown into the dignity of a science.

TRAPPINGS.

It will be useful to take up a few of the principles of this new science (if we may so term it), to illustrate the kind of progress that has been made. The great difficulty which has to be met in every system of sewerage (Col. Waring's Memphis system perhaps excepted) is the generation of sewer gas and its escape into houses. To meet this difficulty, the following axioms have been formulated and accepted by the great mass of the authorities on the subject:

1st. The sewer should be ventilated by grated man-holes opening into the streets.

2d. Every house-drain connecting with the sewer should be trapped without the house, and the trap ventilated.

3d. The soil pipe connecting with the drain should be ventilated by extending the pipe above the house, and several feet above any window, and attaching to its top a proper ventilation cap.

4th. Every waste pipe connecting with the soil pipe should be trapped as near as possible to its basin, and the trap should be correctly ventilated either into a separate ventilating pipe or into ventilated soil pipes when there are no other basins connecting with it above.

These four principles about cover the whole subject of house drainage. If the traps are correctly situated and made, and properly ventilated; if the pipes are of ample size and are duly connected, and are of proper material, and these principles are applied, there need be no reasonable fear of sewer gas escaping into a house.

I will name two or three modern or improved traps, viz., the Cudell, the Du Bois and the Pitt traps, by way of illustrating these points.

The recent literature of the subject of drainage has been full of examples of the stupid blunders of plumbers, and their mistakes, although death to their victims, serve a useful purpose when exposed as they are by the different sanitary writers of the day, to call the attention of the public in an emphatic manner to the subject of sanitary science, and to illustrate in a negative way the proper principles of drainage.

One example of defective drainage is shown in Cut No. 4, which represents the effect of the settling of new made ground in the Back Bay district in Boston. The houses in this district are built on piles. The cut represents the walls of a house pressing down a pile, and snapping off by the pressure the drain pipe leading from the house. The break causes all the neighboring soil to be saturated with sewage.