[Illustration: Plate 2.] Experience in working the detector will not allow me to do this. Although for months not the slightest particle of sewer gas has entered the building, until one evening about eight o’clock a sickly smell is observed in the kitchen, and this is attributed to a change in the weather, heavy rain having fallen during the day, and no notice was taken of it before retiring to rest. In the morning the house is unbearable. The inspector of nuisances is sent for, who cannot detect anything wrong in the drains. The surveyor to the Local Board visits the house with the same result, and it is not until the middle of the day before the nuisance has abated, its cause still a mystery to the sanitary officials and to the owner of the house, who is a medical man of great experience in sanitary matters, and a sanitary writer. Now what really did occur was this. The sewer at the back nearly filled with water and soil caused by the heavy rains, and when this was rising, about 2 cubic feet of gas was forced from the drain B through the grating at the bottom of the soil-pipe. The junction where the drain at A joined the sewer was made as usual about two-thirds the height of the sewer, consequently the drain from A to B filled some 20 feet during the storm. When the storm abated, the water leaving the drain at A sucked the trap at the bottom of the soil-pipe 3. The seal being gone, the gas from the sewer at once came through the trap, the current being estimated at from 80 to 200 feet per minute; so that the quantity of gas given off at the trap, which is 2 feet from the door, would be about 2000 cubic feet from the time the trap was sucked until it was filled again by the flushing of the closet. This is by no means an exceptional occurrence, two similar cases occurred in the suburbs last year. In one case the owner of the house was seriously ill for several days, and was for some weeks obliged to neglect his business and seek a change of air. In the other case the daughter was taken ill with a zymotic disease which nearly cost her her life, and it was months before she regained her strength. The easiest method of preventing this siphoning of the traps is to fix a small mica valve at the most convenient part of the drain between A and B, fixing it above the ground. You can also prevent it as well as the gas coming near the house by putting in a trap at A and having an open grating between A and B. This would not prevent the 2000 cubic feet of gas before referred to from escaping from the drains, but would cause it to be discharged some distance from the house. You would also have about 23 cubic feet of gas in the drain always mixing with the atmosphere of the garden at this point when the traps are full and tight. Plate 2 shows the plan and drains of a hospital which I tested by this system in 1880. As it was an old building the testing was somewhat different to that described in Plate 1. [Illustration: Plate 3.]