[Illustration:  FIG. 108—­A screw as a simple machine.]

The ridge encircling the screw is called the thread, and the distance between two successive threads is called the pitch.  It is easy to see that the closer the threads and the smaller the pitch, the greater the advantage of the screw, and hence the less force needed in overcoming resistance.  A corkscrew is a familiar illustration of the screw.

160.  Pulleys.  The pulley, another of the machines, is merely a grooved wheel around which a cord passes.  It is sometimes more convenient to move a load in one direction rather than in another, and the pulley in its simplest form enables us to do this.  In order to raise a flag to the top of a mast, it is not necessary to climb the mast, and so pull up the flag; the same result is accomplished much more easily by attaching the flag to a movable string, somewhat as in Figure 109, and pulling from below.  As the string is pulled down, the flag rises and ultimately reaches the desired position.

If we employ a stationary pulley, as in Figure 109, we do not change the force, because the force required to balance the load is as large as the load itself.  The only advantage is that a force in one direction may be used to produce motion in another direction.  Such a pulley is known as a fixed pulley.

[Illustration:  FIG. 109.—­By means of a pulley, a force in one direction produces motion in the opposite direction.]

161.  Movable Pulleys.  By the use of a movable pulley, we are able to support a weight by a force equal to only one half the load.  In Figure 109, the downward pull of the weight and the downward pull of the hand are equal; in Figure 110, the spring balance supports only one half the entire load, the remaining half being borne by the hook to which the string is attached.  The weight is divided equally between the two parts of the string which passes around the pulley, so that each strand bears only one half of the burden.

We have seen in our study of the lever and the inclined plane that an increase in force is always accompanied by a decrease in distance, and in the case of the pulley we naturally look for a similar result.  If you raise the balance (Fig. 110) 12 feet, you will find that the weight rises only 6 feet; if you raise the balance 24 inches, you will find that the weight rises 12 inches.  You must exercise a force of 100 pounds over 12 feet of space in order to raise a weight of 200 pounds a distance of 6 feet.  When we raise 100 pounds through 12 feet or 200 pounds through 6 feet the total work done is the same; but the pulley enables those who cannot furnish a force of 200 pounds for the space of 6 feet to accomplish the task by furnishing 100 pounds for the space of 12 feet.

[Illustration:  FIG. 110.—­A movable pulley lightens labor.]