The amount of compression or displacement necessary to force the gas in bulk through the traps has been accurately measured, to know what quantity of liquid was required to be thrown into a drain or sewer of any size to force the gas in bulk through the water-trap. The lifting power of the gas on the water by compression was found to be 1/300 part of its bulk. Thus, if a drain perfectly sound, and sealed with a water-seal each end, held 300 feet of gas, 1 cubic foot of water thrown into the drain would force the gas in bulk through the water-seal. It became evident, that if both ends of any drain were sealed with a water-trap or otherwise for testing, the capacity of the drain or leaks of any kind could be determined without excavating. As it was inconvenient to watch the working of the drain through the traps, I constructed an instrument called a detector[1] to observe the working of the atmosphere in the drain. This instrument, or a gas-pressure gauge, when attached to the drain, will denote by the rising of the liquid the amount of compression in the drain. This, when compared with the quantity of water thrown into it, will give the size and capacity of the drain, and will also indicate any siphoning of traps or leaks which exist in any of the sanitary fittings of the house. Footnote 1: This instrument with instructions as to reagents can be obtained from E. Cetti, Meteorological Instrument Maker, 36, Brooke Street, Holborn, price 12_s._ 8_d._ It is cheaper and more convenient than the pressure-gauge, and registers any pressure during the testing of drains. The following table will show the amount of gas in every 100 feet of circular pipe or drain, from 4 to 30 inches in diameter, also the amount of water thrown into a trap to produce the necessary pressure of gas to lift the liquid 1 inch in the detector or pressure gauge: the quantity being as near as possible 3⅓ ozs. of water to 1 cubic foot of gas space. [Illustration: Plate 1.]