Why do sudden turns in a pipe check the flow of water through it? Because every turn in the pipe increases the friction made by the water. Why does a wagon-wheel turn around when the wagon moves? Because the friction between the wheel and the earth, prevents the wheel from sliding along. Why can a steam-engine draw a train of heavy cars? Because the friction, between the wheels and the rails, causes the wheels to turn around instead of sliding; and thus the engine moves forward drawing the cars with it. Why is sand sometimes put on the rails? Sand is put on the rails to increase the friction between the wheels and the rails, so that the engine may start a heavier train. Why do we use sleighs when there is snow on the ground? Because sleighs slip easily on the snow. Why do sleighs slip easily on the snow? Because there is but little friction between the sleigh and the snow. Why do wagons run more heavily when the ground is soft? Because their wheels then sink farther into the earth, and thus the friction is increased. Why do wagons run more heavily when the ground is covered with snow? Because their wheels sink into the snow, and thus the friction is increased. Why will a person slip down upon the ice more easily than upon the earth? Because there is less friction between his feet and the ice than there is between his feet and the earth. Why do we put ashes on our walks when they are covered with ice? We put ashes on our walks to increase the friction between the ice and our feet, so that we may not slip down when walking on them. Why is a lock or a brake used on a wagon? It is used to check the motion of the wagon. How does a lock check the motion of the wagon? The lock is pressed against the wheels, so that enough friction is produced to check the motion of the wagon. Why does it require more force to draw a rusty plough through the ground, than to draw one that is bright and smooth? Because the rust increases the friction between the plough and the ground, and this makes more force necessary in order to move it. Why cannot we hold a live eel in our hand, as easily as we can a live fish? Because the skin of the eel is so smooth, that there is much less friction between it and our hand, than there is between the fish and our hand. Dry sand or dry ashes sprinkled on the eel will increase the friction so that it can be held. [Illustration: Explorer.] [Illustration: Decoration.] CHAPTER X. Mechanical Powers. Lever. [Illustration: Illustrated Capital W] What is a lever? A _Lever_ is a rod or bar of some kind used for raising weights. Of what are levers made? Levers are made of any solid substance, such as wood and iron. When a spoon is balanced on a cup, as in Fig. 40, what may the spoon be called? The spoon may then be called a lever. Upon what does the spoon rest?