the mercury in the cup must be most pressed by the air. It is true that in damp weather the air feels heaviest, but it is on account of its being less salubrious. The lungs under

these circumstances do not play so freely, nor does the blood circulate so well; and thus obstructions are frequently occa sioned in the smaller vessels, from which arise colds, asthmas, and fevers. The thinness of the air in elevated situations is sometimes oppressive from being insufficient for respiration; and the expansion which takes place in the more dense air contained within the body is often painful. It occasions distension, and sometimes causes the bursting of smaller blood-vessels.

In wet

The barometer has been used for the purpose of measuring the heights of mountains and towers, and of estimating the elevation of balloons. The weight of one hundred and three feet of air is equal to that of one tenth of an inch of mercury. If a barometer, therefore, be carried to any great eminence, the mercury will descend one tenth of an inch for every one hundred and three feet that the barometer ascends. When the surface of the mercury is convex, or stands higher in the middle than at the sides, it is a sign the mercury is then in a rising state; but if the surface be concave, or hollow in the middle, it is then sinking. In very hot weather, the falling of the mercury indicates thunder. In winter, the rising indicates frost, and in frosty weather if the mercury falls three or four divisions, there will be a thaw. But in a continued frost, if the mercury rises, it will snow. weather, when the mercury rises much and high, continues for two or three days before the bad weather is entirely over, then a continuance of fair weather may be expected. In fair weather, when the mercury falls low, and thus continues for two or three days before the rain comes, then much wet weather may be expected and probably high winds. The unsettled motion of the mercury denotes unsettled weather. The words engraved on the scale are not so much to be attended to, as the rising and falling of the mercury. It always sinks lowest of all for great winds, though not accompanied with rain; but it falls more for wind and rain together than for either of them alone. Barometers are frequently made of a tube with a curved neck and bulb, being more commodious than the basin and tube. To make these tolerably exact, however, the circular area

and so

of the bulb should be at least thirty or forty times larger than that of the tube; so that the mass of mercury may be as little affected as possible whilst it rises and falls; for the height of the column is taken from the surface of the mercury in the bulb to its height in the tube.

QUESTIONS.-1. What is the construction of the barometer? 2. Upon what does the height of the mercury depend? 3. Why is the air heaviest in dry weather? 4. Why does it feel heaviest in damp weather? 5. How may the height of a mountain be ascertained by the barometer? 6. What is indicated by the convexity and concavity of the mercury? 7. Upon what other construction are barometers made than that first described?

LESSON 29.

Sound.

Humidity, moisture. The degrees of moisture in the air are measured by an instrument called a Hygrometer, of which there are various kinds; whatever contracts or expands by the moisture or dryness of the atmosphere is capable of being formed into one.

SOUND arises from a tremulous or vibrating motion in elastic bodies, which is caused by a stroke or collision, and is earried to the ear through the medium of the air. The production of sound therefore depends upon three circumstances, a sonorous body to give the impression, a medium to convey it, and the ear to receive it. Sonorous bodies, however, are merely the instruments by which a peculiar species of motion is communicated to the air. It is true that when you ring a bell, both the bell and the air are concerned in the production of sound; but sound, strictly speaking, is a perception excited in the mind by the motion of the air on the nerves of the ear; the air, therefore, as well as the sonorous bodies which put it in motion, is only the cause of sound, the immediate effect is produced by the sense of hearing: for without this sense, there would be no sound. The vibrating air strikes the ear, and causes in the mind the perception of sound.

If you endeavour to ring a small bell, after you have suspended it under the receiver in an air-pump, from which the air has been exhausted, no sound will be produced. By

62

VELOCITY OF SOUND."

exhausting the receiver, you cut off the communication between the air and bell; and the latter, therefore, cannot impart its motion to the air. It has been ascertained that liquids as well as air are capable of conveying the vibratory motion of a sonorous body to the organ of hearing; for sound can be heard under water. Dr. Franklin imagined, that with his ear under water, he heard the collision of stones in that medium, at the distance of a mile.

The vibration of a sonorous body gives a tremulous motion to the air around it, very similar to the motion communicated to smooth water when a stone is thrown into it. This first produces a small circular wave around the spot in which the stone falls; the wave spreads, and gradually communicates its motion to the adjacent waters, producing similar waves to a considerable extent. The same kind of waves are produced in the air by the motion of a sonorous body, but with this difference, that as air is an elastic fluid, the motion does not consist of regularly extending waves, but of vibrations, and are composed of a motion forwards and backwards, similar to those of a sonorous body. They differ also in the one taking place in a plane, the other in all directions: the aerial undulations being spherical. The first sphere of undulations which are produced immediately round the sonorous body, by pressing against the contiguous air, condenses it. The condensed air, though impelled forward by the pressure, reacts on the first set of undulations, driving them back again. The second set of undulations which have been put in motion, in their turn communicate their motion, and are themselves driven back by reaction. Thus there is a succession of waves in the air, corresponding with the succession of waves in the water.

The air is a fluid so much less dense than water, that motion is more easily communicated to it. The firing of a cannon produces vibrations of the air which extend to several miles around. Distant sound, however, takes some time to reach us, and we see the light of the flash long before we hear the report. The velocity of sound is commonly computed at the rate of eleven hundred and forty-two feet in a second. Its velocity varies according to the temperature, density, and humidity of the atmosphere. It is influenced also by the force and direction of the wind. The velocity of sound has been applied to the measurement of

distances. If a ship at sea in distress fires a gun, the light of which is seen on shore twenty seconds before the report is heard, it is therefore known to be at the distance of twenty times eleven hundred and forty-two feet, or a little more than four miles and one third. By counting the number of seconds elapsed between the flash of lightning and the clap of thunder, you may ascertain how far distant you are from the cloud.

When the aerial vibrations meet with an obstacle, having a hard and regular surface, such as a wall or rock, they are reflected back to the ear, and produce the same sound a second time; but the sound will then appear to proceed from the object by which it is reflected. If the vibrations fall perpendicularly on the obstacle, they are reflected back in the same line; if obliquely, the sound returns obliquely in the same direction. This reflected sound is called an echo. At Rosneath, near Glasgow, there is an echo that repeats a tune, played with a trumpet, three times, completely and distinctly. At Brussels there is an echo that answers fifteen times; and in Italy, near Milan, the sound of a pistol is returned fifty-six times. Speaking trumpets, and those made to assist the hearing of deaf persons, depend on the reflection of sound from the sides of the trumpet, and also by its being confined and prevented from spreading in every direction.

QUESTIONS.-1. From what does sound arise? 2. Upon what three circumstances does the production of sound depend? 3. What is sound, strictly speaking? 4. How can it be shown that air is necessary to the production of sound? 5. Why cannot a bell be heard in an exhausted receiver? 6. What are conductors of sounds besides the atmosphere? (Ans. water, wood, flannel.) Tie a piece of iron or any metal to the middle of a strip of flannel, 2 or 3 ft. long. Press the ends of the flannel in your ears, and if the metal be struck against iron, you will hear a sound like that of a heavy church bell. 7. How is the tremulous motion of the air as produced by a sonorous body illustrated? 8. What is said of the velocity of sound? 9. Ship at sea? 10. Distance of lightning? 11. How is the sound of an echo produced? 12. Describe the speaking trumpet, fig. 20.

NOTE. The science which treats of sound in general is called acoustics.

64

MUSICAL SOUNDS

LESSON 30.

Nature of Musical Sounds.

Ten'sion, act of stretching, state of being stretched.
Gravity, in music, the modification of any sound, by which it be
comes deep or low in respect of some other sound.
Con'cert, many performers playing the same tune.

Line, a small French measure, containing the 12th part of an inch
geometricians conceive the line subdivided into six points.

If a sonorous body be struck in such a manner, that its vibrations are all performed in regular times, the vibrations. of the air will correspond with them; and striking in the same regular manner on the drum of the ear, will produce the same uniform sensation on the auditory nerve and excite the same uniform idea in the mind; or, in other words, we shall hear one musical tone. But if the vibrations of the sonorous body are irregular, there will necessarily follow a confusion of aerial vibrations; for a second vibration may commence before the first is finished, meet it half way on its return, intercept it in its course, and produce harsh jarring sounds which are called discords. But each set of these irregular vibrations, if repeated at equal intervals, would produce a musical tone. It is only their irregular succession which makes them interfere, and occasions discord.

The quicker a sonorous body vibrates, the more acute, or sharp is the sound produced; and the vibrations of the same string, at the same degree of tension, are always of a similar duration. Striking the note in quick succession, produces a more frequent repetition of the tone, but does not increase the velocity of the vibrations of the string. The duration of the vibrations of the strings or chords depends upon their length, their thickness, or weight, and their degree of tension. The different length and size of the strings of musical instruments, therefore, serve to vary the duration of the vibrations, and consequently the acuteness or gravity of the

notes.

Among the variety of tones, there are some which, sounded together, please the ear, producing what we call harmony or concord. This arises from the agreement of the vibrations of the two sonorous bodies; so that some of the vibrations of each strike upon the ear at the same time. If the vibra