“=Cushioning=” is determined by causing the discharge of a considerable mass of water (as from a bath-tub or water-closet) into the main pipe, at some distance above the suspected fixture. If there is a regurgitation in the fixture under observation, there is certainly an obstruction in the pipe at some point below the fixture. If the obstruction is caused by something lodged in the pipe, the back-flow in the fixture will be a steady one, because there will be some leakage through the obstruction, while if it is due to compressed air, which is elastic, the regurgitation will be by spurts, and the water in the trap may even be thrown up to a considerable height. “=Siphoning=” is also determined by filling the pipe with water in the manner just explained. If the trap of the fixture under observation is emptied by siphoning, a sucking or gurgling noise will be heard in it, and a flame held over the outlet of the fixture will be drawn downward by the inward current of air. =One trap to several pipes= (when it is suspected but not certainly known because the trap is out of sight) may be detected by the fact that there will always be a current of air in one direction or the other through such pipes, and more or less odor will escape. If the back of the hand is wet, it becomes very sensitive to such a current, and by holding alternately the palm and the back of the hand, or even of the forefinger (wet), over an opening, the existence and direction of such a current can be easily determined. If the odors are from such a source, it can then be made evident by covering all the outlets but one with wet paper, so as to prevent the air-currents, when the odors will temporarily disappear. If a trap is accessible, it is easy to determine whether its water-seal is preserved or not by tapping it from top to bottom with some metallic substance, when the difference in sound will indicate the exact level of the water inside. Those parts of the plumbing which are not accessible to inspection must be examined by introducing into the pipes some strong-smelling substance, which will indicate defects by its escape through them into the house. Oil of peppermint is often used for this purpose. _The Peppermint-Test._ For testing the pipes of an ordinary dwelling, one ounce of the =oil= of peppermint is sufficient. For a large building more may be needed. The drug costs from twenty-five to seventy-five cents an ounce, according to its quality and the place where it is bought.