advantage gained is in proportion to the space passed over by the moving power.

10. The wheel and axis, fig. 6, consists of a wheel W W on an axis a b, and the advantage gained is in proportion as the circumference of the wheel is greater than that of the axis. The projecting spokes X have the effect of increasing the diameter and circumference of the wheel, and of course of increasing the power.

11. Cranes of all kinds, windlasses, capstans, and those axles turned by means of winches, are all to be referred to the wheel and axis, of which they are so many species.

Fig. 6.

QUESTIONS FOR EXAMINATION.

1. For what are the mechanical powers used?

2. What are the mechanical powers?

3. What is a lever, and how many kinds of levers are there? 4. Explain the different kinds of levers.

5. What instruments are to be referred to the lever of the first kind?

6. What are to be referred to levers of the second kind?

7. What are to be referred to levers of the third kind?

8. To which of the levers is a hammer in the act of drawing a nail to be referred?

9. What is the general maxim with regard to the mechanical powers?

10. Of what does the wheel and axis consist?

11. What machines are to be referred to the wheel and axis?

LESSON THE FIFTH

MECHANICAL POWERS.

1. The third mechanical power is the pulley, which is a small wheel turning on an axis, with a rope passing over it. It is used to change the direction of a weight; which may be raised to any height by a person not moving from his place.

2. Pulleys may be either fixed, as ZZ, or moveable, as X: the single fixed pulley, fig. 7, gives no mechanical advantage, but is only used to change the direction of a weight. The moveable pulley X, fig. 8, to which the weight W is attached, rises and falls with the weight, and the advantage gained by this pulley is as two to one; that is, a power p of 4lb. will balance a weight W, of 81b.: for the power moves through twice as much space as the weight.

3. The advantage gained by pulleys is found by multiplying the number of pulleys in the lower block by 2; that is, if instead of one pulley X, there were 3 or 4, there must, in that case, be 3 or 4 in the upper block, and the power gained would be as 6 or 8.

A

Z

B B

W

Fig. 7.

Fig. 8.

4. The fourth power is the inclined plane, which is made by planks, &c. laid in a sloping direction, on which large and heavy bodies may be more easily lowered or raised, than by a mere lift.

5. The wedge may be considered as two equally inclined planes.

6. The advantage gained by an inclined plane is in proportion as the length of the plane C D, fig. 9, is

greater than the height AC: and the advantage gained by the wedge is in proportion as the length of the two sides SS, SS, see fig. 10, p. 221, is greater than the length of the back.

Explanation. If CB, see fig. 9, p. 221, be three feet, and CA one foot, then a force of one pound will sustain the roller E of three pounds. And if the sides of the wedge, see fig. 10, be each 6 inches, and the back be 2 inches, then two weights, r and s, each 1lb. would sustain 12 lb. placed on the back SS, of the wedge, or suspended by the hook s.

7. Chisels, hatchets, and other sharp instruments, which are sloped down to an edge on one side only, are to be referred to the inclined plane; and those instruments that are sloped down on both sides, act on the principle of the wedge.

8. The principal use of the wedge consists in its being urged by the stroke of a hammer, mallet, &c. and not by mere pressure. By repeated blows the wedge is used to split wood, rocks, &c.

Fig. 11.

9. The last mechanical power is the screw, A B. fig. 11, which is always used with a lever C D, and which gains advantage in proportion as the circumference of the circle made by the lever is greater than the length between two threads of the screw.

10. The screw is used for pressing light bodies close together, as in presses for paper-makers, bookbinders, packers, &c. A common corkscrew in passing through the cork acts upon this principle.

11. When a screw acts in a wheel, it is called an endless screw.

12. In the application of the mechanical powers onefourth, and often one-third, must be allowed to overcome the friction to which in practice they are all liable: that is, if 60 pounds are required to balance any weight, then 75 or 80 pounds will be required to put it

in motion.

13. The capital advantages of the mechanical powers are, that by their means, we can raise and move from

place to place large weights, as blocks of stone; that we can give almost any direction to the moving power, and apply its action at a distance from the body to be moved.

QUESTIONS FOR EXAMINATION.

1. What is the fourth mechanical power?

2. How many kinds of pulleys are there?
3. How do you estimate the advantage gained?
4. For what is the inclined plane used?

5. What is the wedge?

6. How do you estimate the power gained by the inclined plane and wedge?

Give the explanation by means of the figure.

7. What instruments are to be referred to the inclined plane, and what to the wedge?

8. In what does the chief use of the wedge consist?

9. Which is the other mechanical power, and in what way

does it gain advantage?

10. For what is the screw used?

11. What is meant by an endless screw?

12. What is to be allowed for friction in the application of th mechanical powers?

13. What are the capital advantages of the mechanical powers?

LESSON THE SIXTH.

MOVING POWERS.

1. The principal moving powers are the strength of men, horses, and oxen; the force of running water and of wind: the force of steam, and the weight of heavy bodies.

2. In clocks, jacks, &c. a weight is the first mover, which is easily applied; and its action is very uniform, but as the power requires to be frequently renewed, it is used only in slow movements.

3. The spring is a useful moving power, but requires also to be wound up occasionally, and it differs from the weight in this, that the action is never uniform, being strongest when most bent.

4. A pendulum is a heavy body suspended by a

small string, wire, &c. which is moveable on a centre; each swing of a pendulum is called a vibration or oscillation.

5. All the vibrations of the same pendulum, whether great or small, are performed in equal times.

6. The swinging of a pendulum is used for measuring time; one that swings seconds is little more than 39 inches long and one that swings half seconds would be 39.2

4

=

= 9.75, &c., or a little more than 9 inches; and to vibrate once in two seconds it must be 39 x 4 = 156 inches.

7. The longer the pendulum, the slower are its vibrations; therefore, as heat lengthens the wire on which it swings, a clock is liable to lose time in summer, and to gain time in winter, when the cold contracts the wire.

QUESTIONS FOR EXAMINATION.

1. What are the principal moving powers?

2. What is the principle of a weight attached to clocks and

jacks?

3. What are the properties of a spring?

4. What do you mean by a pendulum?

5. How are the vibrations of a pendulum performed?

6. For what is the swinging of a pendulum used?

7. What is the cause of irregularity in clocks?

HYDROSTATICS.

Hydrostatics is the science of weighing fluids: and weighing bodies in fluids."-Bentley.

THE object of hydrostatics is the consideration of the properties and effects of fluids. By the term fluid is understood every body whose parts have so small cohesion, that the least force applied more in one direction than another, will produce a motion among them.

The cause of fluidity is not perfectly known. Some are of opinion that the particles of fluids are spherical, and in con