Liquids are only slightly compressible, nevertheless nice experiments show that even they can be somewhat reduced in bulk by pressure.

Fig 4.

Metals are compressible, as is shown in the process of stamping coins, metals, and the like.

'Dilatability.

12. DILATABILITY is the property that a body possesses of assuming a greater bulk under certain circumstances.

In the experiment upon air, explained in the last article,

Are liquids compressible? Are metals compressible? How shown? (12.) What is Dilatability?

if the piston be raised after the air has been compressed, it will expand and fill the tube. Almost all bodies expand on being heated. It is on this principle that thermometers are constructed. In cooling, bodies contract.

A familiar example of dilatability and contractibility is shown in the process of fitting the tire upon a carriage wheel. The tire is made a little smaller than the wheel, but on being heated it expands so as to embrace it; on cooling it contracts again and draws the parts of the wheel tightly together.

The same property of metals has been used for producing great pressures, and even for restoring inclined walls to an erect position.

Elasticity.

13. ELASTICITY is the property which bodies possess of recovering their original shape and size after having been either compressed or extended.

Bodies differ in their degree of elasticity, yet all are more or less elastic. India-rubber, ivory, and whalebone are examples of highly elastic bodies. Putty and clay are examples of those which are only slightly elastic.

If air be compressed, its elasticity tends to restore it to its original bulk; this property has been utilized in making air-beds, air-cushions, and even in forming car-springs. If a spring of steel be bent, its elasticity tends to unbend it; this principle is employed in giving motion to watches, clocks, and the like. If a body be twisted, its elasticity tends to untwist it, as is observed in the tendency of yarn and thread to untwist; this principle, under the name of torsion, is used to measure the deflective force of magnetism. If a body be stretched, its elasticity tends to reduce it to its original length, as is shown by stretching a piece of india-rubber, and then allowing it to contract.

We see that the elasticity of a body may be brought into play by four different methods: by pressure, by flexure or bending, by torsion

Example. Application in putting tire upon a wheel. Example of restoring walls. (13.) What is Elasticity? Give examples of highly and slightly elastic bodies. Give examples of the applications of elasticity. How may elasticity be brought into play? Examples.

or twisting, and by tension or stretching. In whatever way it may be developed, it is the result of molecular displacement. Thus, when air is compressed, the repulsions between the molecules tend to expand it. Again, when a spring is bent, the particles on the outside are drawn asunder, whilst those on the inside are pressed to. gether; the attractions of the former and the repulsions of the latter tend to restore the spring to its original shape.

The most elastic bodies are gases; after them come tempered steel, whalebone, india-rubber, ivory, glass, &c.

Fig. 5 illustrates the method of showing that ivory is elastic, and at the same time that the cause of its. elasticity is molecular displacement. It consists of a polished plate of marble, over which is spread a thin layer of oil. If a ball of ivory be let fall upon it from different heights, it will at each time rebound, leaving a circular impression on the plate, which is the larger as the ball falls from a greater height. This experiment shows that the ball is flattened each time by the fall, that the flattening increases as the height increases,

and that the action of the compressed molecules causes it to rebound. The property of elasticity is utilized in the arts in a great variety of ways. When a cork is forced into the mouth of a bottle, its elasticity causes it to expand and fill the neck so as to render it both water and air-tight. It is the elasticity of air which causes indiarubber balls, filled with air, to rebound when thrown upon hard substances. It is the elasticity of steel which renders it of use in

What bodies are most elastic? How is it shown that ivory is elastic? Explain the experiment. Explain some of the applications of elasticity. Corking bottles Springs.

springs for moving machinery, as well as for easing the motion of carriages over rough roads. It is the elasticity of cords that renders them applicable to musical instruments. It is the elasticity of air that renders it a fit vehicle for transmitting sound. It is the elasticity of the etherial medium pervading space which renders it capable of transmitting light.

14. MECHANICS is that branch of Physics which treats of the laws of rest and motion. It also treats of the action of forces upon bodies.

Rest and Motion.

15. A body is at REST when it retains its position in space. It is in MOTION when it continually changes its position in space.

A body is at rest with respect to surrounding bodies, when it retains the same relative position with respect to them, and it is in motion with respect to surrounding objects when it continually changes its relative position with respect to them. These states of rest and motion are called Relative Rest and Relative Motion, to distinguish them from Absolute Rest and Absolute Motion.

When a body remains fixed on the deck of a moving vessel or boat, it is at rest with respect to the parts of the vessel, although it par takes with them in the common motion of the vessel. When a man walks about the deck of a vessel, he is in motion with respect to the parts of the vessel, but he may be at rest with respect to objects on shore; this will be the case when he travels as fast as the vessel sails, but in an opposite direction. In consequence of the earth's motion around its axis and about the sun, together with the motion

Stringed instruments. Transmission of light. (14.) What is Mechanics? (15.) When is a body at rest? When in motion? Explain relative and absolute rest and motion. Illustrate by examples.

of the whole solar system through space, it is not likely that any part of our system is in a state of absolute rest for any appreciable length of time.

Different kinds of Motion.

16. MOTION may be rectilinear or curvilinear; it is rectilinear when the path of the moving body is a straight line, and it is curvilinear when this path is a curved line. The motion of a train of cars along a straight track is an example of rectilinear motion; the motion of the same train in passing round a curve is an example of curvilinear motion.

Uniform Motion-Velocity.

17. UNIFORM MOTION is that in which a body passes over equal spaces in equal times. Thus, every point on the surface of the earth is, by its revolution, carried around the axis with a uniform motion.

In this kind of motion the space passed over in one second of time is called the velocity. Thus, if a train of cars travel uniformly at the rate of 20 miles per hour, its velocity is 293 feet. Instead of taking a second as the unit of time, we might adopt a minute, or an hour. In the same case as before we might say, that the velocity of the train is one third of a mile per minute, or twenty miles per hour.

18.

Varied Motion-Accelerated and Retarded Motion.

VARIED MOTION is that in which a body passes over unequal spaces in equal times. If the spaces passed over in equal times go on increasing, the motion is accelerated; such is the motion of a train of cars when starting, or that of a body falling towards the surface of the earth. If the spaces passed over go on decreasing, the motion is

(16.) What is Rectilinear Motion? Curvilinear Motion? Examples. (17.) What is Uniform Motion? Example. What is meant by velocity? Example. (18.) What is Varied Motion? When is it accelerated and when retard ed?