How is the friction in a sewing machine, or a carriage, reduced as much as possible? You know that we use oil or grease; we do the same in a railroad car, an automobile, or a wheelbarrow. The oil and grease we use permit the two surfaces that touch each other to pass with less sticking or rubbing; therefore we get more useful work out of the machine.

What becomes of the energy lost in friction? It is used not only in rubbing off the two surfaces that pass each other, but is partly changed into heat (see § 234). Have you ever heard of a "hot box" on a train? What causes it? Why does a drill used in making a hole in a board become hot? So far we have thought of friction only as it is a disadvantage, and causes us to get less work from a machine than we ought to. Is friction ever necessary? Suppose the rope passing over the wheel of a pulley did not stick a little to the wheel, in order to turn it? Could you roll a barrel up an inclined plane, if the barrel did not stick somewhat to the plane? Is it easy for you to walk upon a highly polished floor? Why not?

282. Exercises. 1. How many simple machines can you discover in an egg beater? A door knob? A pair of cutting pliers? A window shade?

2. What simple machines do you make use of when you open a weighted window, close a door against the wind, draw a nail with a claw hammer, crank an automobile engine, cut meat with your knife, saw a board, take off the cover of a Mason jar, turn a key, or turn a grindstone?

3. Why are casters put under tables or other heavy furniture? Why are glides used instead in some cases?

4. Examine several of the can-openers used for "tin" cans, and

find one that is a lever of the first class; also one of the second class. Describe each. Which is the more efficient?

5. What kind of lever is a full suitcase when you are trying to close it? The pump handle shown in Fig. 86, Ą, § 135?

6. With a pair of shears try to cut a match stick. Where must you place the stick in order to cut it most easily? Tell why.

7. Place a book before you, and put upon the cover an iron weight or a heavy stone. Lift the cover. What forms the fulcrum? The power? The weight? What class of lever is it? Now put the weight near the free edge of the cover, grasp the cover near the hinge, and lift it. What class of lever is the cover now? In which case is it easier to lift the cover?

8. Examine a screw eye, and try to force it into wood by means of your fingers. Then try to do so with the help of a long nail which is put through the "eye." Of what advantage is it to use the nail? Why is it easier to force a screw into wood by the use of a carpenter's brace than with an ordinary screw driver?

9. How many different kinds of wrenches can you find in a hardware store? For what is each used? How many kinds of saws can you find?

10. How many different kinds of machines could you use in cracking a nut? What simple machine is there in a bread-mixer?

Summary.

Man lightens his work by means of machines.

The advantage we obtain from machines is that we can use a small effort through a great distance to exert a great force through a small distance, or we can make the reverse exchange, if we wish; but we never create energy, and we never get something for nothing.

There are six simple machines.

Levers may be of the first, second, or third class. In the first class the fulcrum is between the power and the weight; in the second class the weight is between the other two; in the third the power is applied between the other two.

In a compound pulley we pull a good deal of rope over the pulley wheels in order to lift a great weight a short distance.

The winch is a form of the wheel and axle.

By means of it we use

a small force upon the handle, or crank, to lift a great weight attached to the axle.

The inclined plane is a machine in which we move a heavy object a long distance up an incline in order to lift it a short vertical distance. A wedge is a double inclined plane.

A screw is a spiral inclined plane.

Friction is the resistance one body meets when moving in contact with another. The lost motion due to friction is changed largely into heat. Oils and greases (lubricants) are used to make friction as small as possible. Rolling friction is less than sliding friction.

CHAPTER XXX

MAN'S GREAT MACHINES

283. How Does a Sailboat Use the Wind? - - Have you ever watched a sailboat (Fig. 214) making a landing?

(Courtesy of A. M. Lythgoe, Metropolitan Museum of Art, New York City.) FIG. 214. Boats on the Nile.

Did you notice that no matter from what direction the wind was blowing, so long as there was a wind at all, the pilot could make the boat come to the dock? How is

this done? We can easily understand sailing when the wind is blowing behind the boat. The sail is then only a large surface that receives the pressure of the wind, and as the sail is pushed through the air, it draws the boat through the water. But how can a boat sail in other di

FIG. 215. Traveling-boat, with sail set. Found by the Metropolitan Museum Egyptian Expedition, in March, 1920, in the tomb of the Prince Mehenkwetre, at Thebes, XIth dynasty (about 2000 B.C.). This boat is now in the Cairo Museum.

rections? To give the answer we must know that a sailboat has not only a sail, but a keel. The sail makes use of the pressure of the air, but the keel uses the resistance, or inertia (see § 101), of the water to help direct the boat in its motion.

Put a wide board, edge down, into the water and try pushing it sidewise; you cannot easily do it. In the