It is the most necessary substance for our life; it is the vehicle of all odors and smells; it is the medium of all sounds, and brings to our ear and so to our mind an immense knowledge of the outside world; it is the cause of the beauty of the blue firmament or sky, of the aurora and twilight; it is the great nurse of the whole vegetable kingdom by clouds, rain, and dew.

What is an Aneroid Barometer?

It is a barometer in the construction of which no quicksilver or other liquid is used. It consists of a metal box, exhausted of air, the top of which is of thin metal, so elastic that it readily yields to alterations in the pressure of the atmosphere. When the pressure increases, the top is pressed inwards; when, on the contrary, it decreases, the elasticity of the lid, aided by a spring, tends to move it in the opposite direction. These motions are transmitted by delicate levers to an index which moves on a scale. This barometer has the advantage of being portable.

What is the Ear-trumpet?

A trumpet-like instrument used to aid deaf persons in hearing. Its form is conical, and the larger end is of a bell shape; the small end is placed in the ear, and the person talks in the large end. It acts by concentrating the voice on the listener"s ear.

What is a Stethoscope?

An instrument used by physicians for ascertaining the action of the lungs, judging by the sound of their motion whether they are healthy or not.

Describe the Audiphone.

It is a fan-shaped instrument to help deaf people, and is made of flexible carbonized rubber. Fine silk cords attached to the upper edge bend it over, and are fastened by a wedge in a handle. The top edge of this fan rests upon the upper teeth, and the sound waves strike its surface; the vibrations are thus conveyed by the teeth and the bones of the face to the acoustic nerve in the ear.

Describe the Telephone.

It is an instrument by which conversation may be carried on at a distance, and is composed of three parts--a thin disk of soft metal, a small coil or bobbin of silk-covered copper wire, and a small bar magnet about four inches long. The bobbin is placed on one pole of the magnet, so that the wire is as it were steeped in the magnetic s.p.a.ce round the pole. The metal disk is placed face close to the pole and bobbin, so that when it vibrates in front of the pole a series of wave currents will be set up in the coil of wire on the bobbin. The whole is encased in wood, and a mouth-piece is provided for speaking against the disk. The coil of wire on the bobbin is of course connected by its two ends into the circuit of a telegraph line.

Who invented the Telephone?

It was invented, almost simultaneously, by Alex. Graham Bell, a native of Scotland, and Professor of Vocal Physiology in the Boston University, and Elisha Gray, of Chicago.

What is a Phonograph?

It is an instrument for recording the vibrations of sounds, and consists of a revolving cylinder covered with tin-foil. To this cylinder is attached a mouth-piece, fitted with a thin plate or disk, on the outer side of which, next to the cylinder, is a needle or point. The cylinder runs on a screw, so that the whole length of it, from end to end, may pa.s.s under the point. On speaking into the mouth-piece the voice causes the disk to vibrate, and the point to trace marks corresponding to these vibrations on the tin-foil. By turning the cylinder so that the point again pa.s.ses into the marks in the tin-foil, the sounds that entered at the mouth-piece can be reproduced at any time.

By whom was the phonograph invented?

By Thomas A. Edison, who was born in Ohio in 1847. Mr. Edison is the inventor of many improvements in telegraphy, which have been adopted into general use, and are to him the source of a large income. To him, also, we are indebted for the megaphone, microphone, tasimeter, an improvement in the telephone, a system of electric lighting, and many other inventions.

What is a Microphone?

This instrument is a variety of telephone by means of which faint sounds can be heard at a very great distance. It consists of a small battery for generating a weak current of electricity, a telephone for the receiving instrument, and a speaking or transmitting instrument.

The last is a small rod of gas carbon with the ends set loosely in blocks of the same material. The blocks are attached to an upright support, glued into a wooden base board. This instrument is connected with the battery and the telephone. So wonderfully sensitive is it, that the ticking of a watch, the walking of a fly across a board, or the brush of a camel"s-hair pencil can be heard even though it be hundreds of miles distant.

Will you describe the Megaphone?

It is a subst.i.tute for the ear and speaking trumpet. It consists of three paper funnels placed side by side. The two larger ones are about 6 feet 8 inches long and 27-1/2 inches in diameter, and are each provided with a flexible tube, the ends of which are held to the ear.

The centre funnel, which is used as a speaking-trumpet, does not differ materially from an ordinary trumpet, except that it is larger and has a larger bell mouth. Two persons, each provided with a megaphone, can, without other apparatus, carry on a conversation at a distance of one and a half or two miles.

What is the Tasimeter?

It is an instrument, sensitive to the smallest degree of heat, and is mostly used in astronomy. Attached to a telescope it will show the heat coming from the stars.

What is a Bathometer?

This ingenious instrument, the invention of Prof. Siemens of London, enables those on board of ships to read from an index the depths of the ocean beneath them. It consists of a highly sensitive steel spring to which a heavy piece of metal is attached. The changes in weight to which the latter is subject in consequence of the variations of attractive force (the deeper the ocean the smaller the latter, and vice versa) are registered on a scale by the indicator that is in connection with the steel spring.

What is an Anemometer?

An instrument for measuring the velocity and force of the wind, and by which storms, at a distance, can be predicted.

What is a Chronometer?

A time-piece of delicate and exact construction, chiefly employed by astronomers and navigators. It differs only from an ordinary watch in its delicate springs, in not being so much influenced by heat and cold, and consequently in its accuracy in giving the time.

CHAPTER XX.

LIGHT, LIME LIGHT, MAGNESIUM LIGHT, ELECTRIC LIGHT, RAINBOW, PRISM, SPECTRUM, COLORS, PHOTOGRAPHY, CAMERA OBSCURA, STEREOSCOPE, KALEIDOSCOPE.

Do you know something about the nature of Light?

Light is a mere form of vibration like sound, and like sound it requires some source to set this vibration going, and some medium to carry this vibration as air carries sound.

Is not the air this medium?

No, it is supposed that there is an elastic fluid called "ether" which pervades all s.p.a.ce and matter, and if the molecules of a body are in motion they have the power of setting this ether in motion. The movement thus produced will appear either as heat or light according to its velocity.

What sources of light do you know?

We are told that the princ.i.p.al source of light on earth is the sun, either directly with its own beams or indirectly by supplying us with combustibles to produce light; for oil, gas, candles, and most of the substances used for producing light and heat when burning are but sending forth in another form the rays of the sun which were stored up in nature"s economy.

Another source of light is the result of chemical action, such as the lime, magnesium, and electric light. A third source of light is phosph.o.r.escence, as we see it in the glow-worm and fireflies.