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slot, to serve as a guide in raising and lowering the slide which carries the lens. On the hole in the shelf rests a large

watch-glass, or shallow dish, about 4 inches (10.1 centimetres) in diameter. A plano-convex lens may be used in its place. On each side of the shelf are two upright wooden arms, and on screws, which go through them, is swung a looking-glass, 7 inches (17.8 centimetres) long and 4 inches (10.1 centimetres) wide.

EXPERIMENT 2.—Place this lantern before the heliostat, so that the full beam of light will be reflected from the mirror upward through the glass bowl and the watchglass. Fill each of these with clear water, and then place the swinging mirror at an angle of 45°. Hang up a large screen of white cotton cloth, or sheet, in front of the lantern, and from 15 to 40 feet (4.5 to 12.2 metres) distant. On this screen will appear a circle of light projected from the lantern. Get a piece of smoked glass, and trace upon it some letters, and then lay it on the water-lens. The image of the letters will appear on the screen, in white on a black ground. If they are not distinct, loosen the nut at the back of the box, and move the wooden slide up or down till the right focus is obtained.

This water-lantern may now be used for all the work performed with ordinary magic-lanterns. Place a sheet of clear glass over the large lens, to keep the dust out of the water, and then you can lay common lantern-slides on this as in a magic-lantern.

1 Dr. R. M. Ferguson first used a condensing lens made of a glass shade filled with water. See Quarterly Journal of Science, April, 1872. Subsequently, Professor Henry Morton made a watch-glass filled with water, or other liquid, serve for the projecting lens of the lantern.

2

CHAPTER II.

ON THE ORDER OF THE EXPERIMENTS IN THIS

BOOK.

In Chapter I. are explained the construction and use of the heliostat and water-lantern. In Chapter IV. we begin by experimenting on the three ways in which a body may vibrate. We show that it may swing to and fro like a pendulum ; that it may vibrate by shortening and lengthening ; and that it may vibrate by twisting and untwisting itself. Then we study the nature of vibratory motions, and find that they are like the motion of a swinging pendulum ; and the motion of the pendulum we discover is exactly like the apparent motion of a ball looked at in the direction of the plane of a circle, in which it revolves with a uniform velocity.

We then, in Chapter V., experiment on those vibrations whose frequency is so great that they cause sound; and show, in this and the next chapter, that whenever we perceive a sound some solid, liquid, or gaseous body is in a state of rapid vibration, and that these vibrations go from the vibrating body to the ear through a solid, liquid, or gas-air being generally the medium which transmits the vibrations. These vibrations, acting on the ear, make the auditory-nerve fibrils tremble, and thus is caused the sensation of sound.

In Chapter VIII. are experiments which show how these vibrations are transmitted through solids, liquids, and gases, to a distance from the source of the sound. The knowledge of how the sonorous vibrations travel through the air leads to experiments in which we make two sonorous vibrations meet, and, by their mutual action, or interference, cause rest in the air and silence to the ear. This silence may be continuous, or it may be of short duration alternating with sound, and in this case we have “beats.”

Chapter IX. gives Professor Rood's very striking experiment showing the reflection of sound. In Experiment No. 73, of Chapter VIII., I show how we may readily obtain reflection of sound from a gas-flame.

In Chapter X. we give experiments with a siren made of card-board, and with it show that the pitch of sounds rises with the frequency of the vibrations causing them. With the same siren, in connection with a resonant tube tuned to a tuning-fork, we determine the number of vibrations the fork makes in a second. With the same tube and fork we then measure the velocity of sound in air. With the same siren, in Chapter XI., the experimenter finds that the notes of the gamut are given by a series of vibrations whose numbers per second bear to one another certain fixed numerical relations.

In Chapter XII. we experiment with a cheap sonometer, and find the law which connects the length of a string with the frequency of its vibrations; then, with this law in our possession, we make the sonometer give all the notes of the gamut and the sounds of the harmonic series.

In Chapters XIII., XIV., and XV., are described experiments showing the cause of the varying intensities of

sounds, experiments on the sympathetic vibrations of bodies, and on the change made in the pitch of a sounding body by moving it.

The cause of the different quality of sounds is explained in Chapter XVI., and then follow, in Chapter XVII., experiments on the analysis of compound sounds, and on the formation of compound sounds by sounding together the simple sounds which compose them. In this chapter is also found an experiment in which is reproduced the motion of a molecule of air when it is acted on, at the same time, by the vibrations giving the first six harmonics of a compound sound ; also, directions for making a very simple form of König's vibrating flame, and a cheap revolving mirror in which to view the flame.

Chapter XVIII. contains experiments on the voice in talking and singing. After explaining how we speak, I give experiments on the resonance of the oral cavity, and then show how a toy trumpet can be made to speak, and a talking machine made out of the trumpet and an orange. This chapter concludes with accounts of the talking machine of Faber, of Vienna, and of the recently invented talking and singing machine of Mr. Edison, which is indeed the acoustic marvel of the century.

Chapter XIX. concludes the book, and gives a short explanation of the causes of harmony and discord.

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