If water is poured into a U-shaped tube, it will stand just as high in one side of the U as in the other, will it not? When a house is supplied with water from a spring on the side of a hill, we have a big irregular U-tube like this dotted line. As even the garret of this house is lower than the spring, the water will have _force_ when it comes from the pipes, that is, it could yet go higher because it has not run as far up in the U on the house side, as it is on the spring side. Sometimes, as we approach a town, we see a water tower on a hill, or a tall iron stand-pipe. They are one side of a U in a water system. Water is pumped into the tower or the stand-pipe, then it runs into the houses of the town through many pipes which are the other side of the U. There is a library of books one may read about this U performance--its relation to other laws, its limitations and the thousand uses to which it has been put. But all there is to the simple, extraordinary fact, can be seen in a bent glass tube which you can hold in your hand. The side of the U which comes into a town is no longer one pipe but many water mains in streets and multitudes of little pipes in each house. These last are part of the house plumbing. A plan of the house with the position of all the pipes indicated should be one of the housekeeper's possessions. She may not be able to do much about disordered plumbing--in fact, she had better not try to do much; it is not a safe direction for amateur effort--but such a plan is of use to workmen who come to do jobs in the house, and it may keep some zealous husband or brother from driving a nail into a gas pipe in an effort to hang a picture. Water is frequently got out of the house by giving it a good start and then letting it run down according to its nature. Waste pipes are as far as possible perpendicular, and the start is given the water by the weight of a basin or a tubful, or by the sudden emptying of the tank of the closet. That principle of the U, however, is used also in the disposal of waste water. It is the principle on which many traps are constructed. Traps are contrivances for closing the connection between a house and the public sewer. If you have an imagination, or if you will read Victor Hugo's description of the Paris sewers in "Les Misérables" as a help to imagination, it will not be necessary to explain why this connection should be closed. [Illustration]