MUSICAL BAROMETER

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If

tions of two strings, for instance, are performed in equal times, the same tone is produced by both, and they are said to be in unison. But concord is not confined to unison; for two different tones harmonize in a variety of cases. one string or sonorous body vibrates in double the time of another, the second vibration of the latter will strike upon the ear at the same instant as the first vibration of the former; and this is the concord of an eighth or octaye. If the vibrations of two strings are as two to three, the second vibration of the first corresponds with the third vibration of the latter, producing the harmony called a fifth. There are other tones which, though they cannot be struck together without producing discord, yet if struck successively, give us the pleasure which is called melody.

A sort of musical barometer has been invented in Switzerland, called the weather harp, which possesses the singuiar property of indicating changes of the weather by musical tones. In the year 1787, one was constructed in the following manner. Thirteen pieces of iron wire, each three hundred and twenty feet long, were extended across a garden, in a direction parallel to the meridian. They were placed about two inches apart; the largest were two lines in diameter, the smallest only one, and the others one and a half; they were on the side of the house, and made an angle of twenty or thirty degrees with the horizon; they were stretched and kept tight by wheels made for that purpose. Every time the weather changes, these.wires make so much noise that it is impossible to continue concerts in the parlour, and the sound resembles that of a tea-urn when boiling, and sometimes that of a distant bell, or an organ.

QUESTIONS.-1. When do the vibrations of a sonorous body produce the same musical tone? 2. How are discords produced? 3. On what does the sharpness or acuteness of a musical sound depend? 4. On what does the duration of the vibrations of strings or chords depend? 5. How is harmony or concord produced? 6. How is an octave concord produced? 7. The harmony called a fifth? 8. Describe the musical barometer or weather harp. [NOTE. In the opinion of a celebrated chemist, this is an electro-magnetical phenomenon.] 9. Illustrate the vibrations of a musical string by figures 17, 18, and 19.

Lu'minous, shining by its own light.

Transparent, admitting rays of light to pass through.
Opaque', stopping the rays of light.

Zenith, a point in the heavens directly over our heads, the pole of
the horizon. Na'dir is a point diametrically opposite to the
zenith, constituting the other pole of the horizon.

OPTICS is the science which treats of light, and of the instruments by which it is applied to useful purposes. It is one of the most interesting branches of natural philosophy, but not one of the easiest to understand; it will be necessary, therefore, that you give to it the whole of your attention.

Light, when emanated from the sun, or any other luminous body, is projected forwards in straight lines in every possible direction; so that the luminous body is not only the centre from whence all the rays proceed, but every point of it may be considered as a centre which radiates light in every direction. The particles of light are so extremely minute, that although they are projected in different direc tions, and cross each other, yet they are never known to interfere, and impede each other's course. It is still a disputed point, however, whether light be a substance composed of particles like other bodies. In some respects it is obedient to the laws which govern bodies; in others, it appears to be independent of them: thus, though its course is guided by the laws of motion, it does not seem to be influenced by the laws of gravity. It has never been discovered to have weight, though a variety of interesting experiments have been made with a view of ascertaining that point. Some suppose that the rays of light, instead of being particles, consist of the undulations of an elastic medium, which fills all space, and which produces the sensation of light to the eye, just as the vibrations of the air produce the sensation of sound to the ear. Most of the phenomena may be accounted for by either hypothesis, but that of their being particles applies more happily to some of the facts respecting the modifications of light by refraction and reflection.

When rays of light encounter an opaque body, part of them are absorbed, and part are reflected, and rebound just

REFLECTION OF LIGHT.

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as an elastic ball which is struck against a wall. A ray of light striking perpendicularly upon a plane mirror, is reflected back in the same direction; but those rays which strike it obliquely, are reflected back in an opposite direction, but with the same obliquity; the angle of reflection, therefore, is exactly equal to the angle of incidence. If you stand directly before a looking-glass, you see your image reflected back to you. If you stand a little to the side, you cannot see yourself; but a person who stands just as far on the other side of it, can see your image in the glass, and you can see his. If you place a candle a little to one side, you must go as far on the other to see its image in the glass. This is the same rule which takes place in the collision of elastic bodies against any surface. If you strike an ivory ball or common marble perpendicularly against the wainscot, it returns to you; but if you make it strike sideways, it goes off at the same angle with which it came to the wainscot. So it is with rays of light; the incident ray, or the ray which falls upon a surface, makes an angle with a perpendicular line, drawn from the point where it strikes, equal to that which the reflected ray makes with it.

With respect to a looking-glass, it is the silvering on the glass which causes the reflection, otherwise the rays would pass through it without being stopped, and if they were not stopped they could not be reflected. No glass, however, is so transparent but it reflects some rays: if you put your hand near a window, you clearly see its image on the other side, and the nearer the hand is to the glass, the more evident is the image. Whatever suffers the rays of light to pass through it is called a medium, and the more transparent the body, the more perfect is the medium. But rays of light do not pass through a transparent medium, (unless they fall perpendicularly upon it) in precisely the same direction in which they were moving before they entered it. They are bent out of their former course, and this is called refraction. When they pass out of a rarer into a denser medium, as from air into water or glass, they are always refracted towards a perpendicular to the surface, and the refraction is, more or less, in proportion as the rays fall, more or less, obliquely on the refracting surface. But when they pass from a denser into a rarer medium, as from glass or water into air, they move in a direction farther from the

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REFRACTION OF LIGHT.

perpendicular. If you put a piece of money into an empty basin, and stand at such a distance that it may not be visible; then let another person pour water into the basin, and the money will be seen; for the rays of light, in passing from a denser into a rarer medium, are bent from the perpendicular, and thus are directed to your eye. The following, therefore, may be established as a sort of axiom in optics we see every thing in the direction of that line in which the rays approach us last. If you place a candle before a looking-glass, and stand before it, the image of the candle appears behind it; but if another looking-glass be so placed as to receive the reflected rays of the candle, and you stand before this second glass, the candle will appear behind that; because the mind imagines every object to be in the direction from which the rays come to the eye last. Hence, when the rays of light coming from the celestial bodies, arrive at our atmosphere, they are bent downward; and those bodies appear, when in the horizon, higher than they are. The effect of this refraction is about six minutes of time, but the higher they rise, the less are the rays refracted; and when they are in the zenith, they suffer no refraction. The sun is visible about three minutes before he rises, and about the same time after he sets; making in the course of a year about a day and a half. Twilight is occasioned partly by refraction, but chiefly by reflection of the sun's rays by the atmosphere, and it lasts till the sun is eighteen degrees below the horizon. Were there no atmosphere to reflect and refract the sun's rays, only that part of the heavens would be luminous in which the sun is placed; and if we could live without air, and should turn our backs to the sun, the whole heavens would appear as dark as in the night. In this case also, a sudden transition from the brightest sunshine to dark night would immediately take place upon the setting of the sun..

QUESTIONS.-1. What is said of optics? 2. In what manner is light projected from luminous bodies? 3. What is still a disputed point, and what is said of it? 4. How are rays of light reflected? 5. How is it shown that the angle of reflection is equal to the angle of incidence? 6. What is meant by the refraction of rays of light? 7. How are they refracted in passing from a rarer into a denser medium? 8. From a denser into a rarer? 9. What is the example for illustration?" 10. What may be established as a sort of axiom in optics? 11. Give 3 illustration. 12. What is the effect of rays of light, coming from

celestial bodies, being refracted by the atmosphere? 13. What occa sions twilight 14. How would the heavens appear if there were no atmosphere? 15. Illustrate the reflection of light by fig. 29. Engr. III. 16. Refraction of light by fig. 29. [NOTE. Fig. 31. is a vessel with a flower in water at the bottom, seen by the eye in the direction of the rays which enter it. This experiment, and many others, may be easily performed.]

LESSON 32.

Different kinds of Lenses.

Diverge', rays of light coming from a point, and continually separating as they proceed, are said to diverge; the point is called the radiant point.

Converge', rays which tend to a common point are said to converge.

A Beam of light is a body of parallel rays; a Pencil of rays is a body of diverging or converging rays.

Cam'era obscu'ra, a chamber darkened; an optical machine used

in a darkened chamber.

If

A LENS is a glass ground into such a form as to collect or disperse the rays of light which pass through it. They are of different shapes, from which they take their names. rays proceed from a radiant point distant as far as the sun, their divergency is so trifling that they may be considered as parallel. When parallel rays fall on a piece of glass having a double convex surface, that ray only, which falls in the direction of the axis of the lens, is perpendicular to the surface, the other rays falling obliquely, are refracted towards the axis, and they will meet beyond the lens at a point called its focus. The distance of the focus from the centre of the lens depends both upon the form of the lens, and upon the refractive power of the substance of which it is made; in a glass lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere of which the. surface of the lens forms a portion; it is at the distance, therefore, of half the diameter of the sphere. The property of a lens which has a double concave surface is to disperse the rays of light. Instead of converging towards the ray, which falls on the axis of the lens, they will be attracted towards its thick edges, both on entering and quitting it, and will, therefore, be made to diverge. Lenses which have