Where is this principle used to benefit mankind? It is used in supplying the inhabitants of cities with water. What must be built in order to supply a city with water? A reservoir must be built. What is a reservoir? It is a large basin made so as to hold water. How high is the water in the basin? It is usually as high as the tops of the houses in the city. How is the water raised into the basin? It is raised into the basin by means of pumps. How is the water conveyed from the basin to the houses in the city? By means of pipes laid under the surface of the earth, and extending to all parts of the city. How high will the water rise in these pipes? Nearly as high as it is in the basin. If an opening be made in a pipe, how high will the water be forced up? Nearly as high as the water in the basin. What is the water flowing from such an opening called? It is called a _fountain_. What makes the water shoot up in a fountain? The pressure of the water in the basin forces it to shoot up in the fountain. To what does water always tend? Water always tends to fall to a level. What instrument is constructed on this principle? The _water-level_. What is a water-level? It is a glass tube so nearly filled with water that it contains only a bubble of air. This tube is fastened in a piece of wood, as shown in Fig. 22. [Illustration: Fig. 22.] How can we tell when the instrument is level? By the bubble of air, which will always be at the centre of the tube when it is level. What other liquid may be used to fill the tube? Alcohol may be used to fill the tube, and it is then called a _spirit-level_. Of what use is the spirit-level? It is used for levelling the walls and wood-work of houses, bridges, and other structures. Springs and Streams. When we dig into the earth, can we always find water? Yes; we can always find water. What reason may be given for believing this? Because thousands of wells have been dug in different parts of the country, and water is obtained from them. Are these wells all of the same depth? No; they vary in depth from four or five feet to eighty or ninety feet. What makes the water _hard_ in some wells? When the stream of water passes through iron ore, before reaching the well, it absorbs some of the iron, which makes it hard. How else may it become hard? By passing through other mineral substances, such as limestone and sulphur, it will become hard. Why is the water in some wells much warmer than in others? In wells where the water is warm, the streams flowing into them are near the surface of the earth, and are heated by the sun; but in the wells of cold water, the streams are much deeper in the earth. When these streams flow out at the surface of the earth, what are they called? They are called _springs_. Where are springs found? They are mostly found on hill-sides, or in valleys; but they are sometimes found on the top of the hills.