Part 17

_Mrs. B._ Just so, my dear. The composition of the two winds, north and east, produces a constant north-east wind; and that of the two winds, south and east, produces a regular south-east wind; these winds extend to about thirty degrees on each side of the equator, the regions further distant from it, experiencing only their respective northerly and southerly winds.

_Caroline._ But, Mrs. B., if the air is constantly flowing from the poles, to the torrid zone, there must be a deficiency of air, in the polar regions?

_Mrs. B._ The light air about the equator, which expands, and rises into the upper regions of the atmosphere, ultimately flows from thence, back to the poles, to restore the equilibrium: if it were not for this resource, the polar, atmospheric regions, would soon be exhausted by the stream of air, which, in the lower strata of the atmosphere, they are constantly sending towards the equator.

_Caroline._ There is then a sort of circulation of air in the atmosphere; the air in the lower strata, flowing from the poles towards the equator, and in the upper strata, flowing back from the equator, towards the poles.

_Mrs. B._ Exactly; I can show you an example of this circulation, on a smaller scale. The air of this room, being more rarefied, than the external air, a wind or current of air is pouring in from the crevices of the windows and doors, to restore the equilibrium; but the light air, with which the room is filled, must find some vent, in order to make way for the heavy air that enters. If you set the door a-jar, and hold a candle near the upper part of it, you will find that the flame will be blown outwards, showing that there is a current of air flowing out from the upper part of the room.--Now place the candle on the floor, close by the door, and you will perceive, by the inclination of the flame, that there is also a current of air, setting into the room.

_Caroline._ It is just so; the upper current is the warm light air, which is driven out to make way for the stream of cold dense air, which enters the room lower down.

_Mrs. B._ Besides the general, or trade-winds, there are others, which are called periodical, because they blow in contrary directions, at particular periods.

_Emily._ I have heard, Mrs. B., that the periodical winds, called, in the torrid zone, the sea and land breezes, blow towards the land, in the day time, and towards the sea, at night: what is the reason of that?

_Mrs. B._ The land reflects into the atmosphere, a much greater quantity of the sun's rays, than the water; therefore, that part of the atmosphere which is over the land, is more heated and rarefied, than that which is over the sea: this occasions the wind to set in upon the land, as we find that it regularly does on the coast of Guinea, and other countries in the torrid zone. There, they have only the sea breeze, but on the islands, they have, in general, both a land and sea breeze, the latter being produced in the way described; whilst at night, during the absence of the sun, the earth cools, and the air is consequently condensed, and flows from the land, towards the sea, occasioning the land breeze.

_Emily._ I have heard much of the violent tempests, occasioned by the breaking up of the monsoons; are not they also regular trade-winds?

_Mrs. B._ They are called periodical trade-winds, as they change their course every half year. This variation is produced by the earth's annual course round the sun; the north pole being inclined towards that luminary one half of the year, the south pole, the other half. During the summer of the northern hemisphere, the countries of Arabia, Persia, India, and China, are much heated, and reflect great quantities of the sun's rays into the atmosphere, by which it becomes extremely rarefied, and the equilibrium consequently destroyed. In order to restore it, the air from the equatorial southern regions, where it is colder, (as well as from the colder northern parts,) must necessarily have a motion towards those parts. The current of air from the equatorial regions, produces the trade-winds for the first six months, in all the seas between the heated continent of Asia, and the equator. The other six months, when it is summer in the southern hemisphere, the ocean and countries towards the southern tropic are most heated, and the air over those parts, more rarefied: then the air about the equator alters its course, and flows exactly in an opposite direction.

_Caroline._ This explanation of the monsoons is very curious; but what does their breaking up mean?

_Mrs. B._ It is the name given by sailors to the shifting of the periodical winds; they do not change their course suddenly, but by degrees, as the sun moves from one hemisphere, to the other: this change is usually attended by storms and hurricanes, very dangerous for shipping; so that those seas are seldom navigated at the season of the equinoxes.

_Emily._ I think I understand the winds in the torrid zone perfectly well; but what is it that occasions the great variety of winds, which occur in the temperate zones? for, according to your theory, there should be only north and south winds, in those climates.

_Mrs. B._ Since so large a portion of the atmosphere, as is over the torrid zone, is in continued agitation, these agitations in an elastic fluid, which yields to the slightest impression, must extend every way, to a great distance; the air, therefore, in all climates, will suffer more or less perturbation, according to the situation of the country, the position of mountains, valleys, and a variety of other causes: hence it is easy to conceive, that almost every climate, must be liable to variable winds; this is particularly the case in high latitudes, where the earth is less powerfully affected by the sun's rays, than near the equator.

_Caroline._ I have observed, that the wind, whichever way it blows, almost always falls about sun-set.

_Mrs. B._ Because the rarefaction of air in the particular spot which produces the wind, diminishes as the sun declines, and consequently the velocity of the wind, abates.

_Emily._ Since the air is a gravitating fluid, is it not affected by the attraction of the moon and the sun, in the same manner as the waters?

_Mrs. B._ Undoubtedly; but the aerial tides are as much greater than those of water, as the density of water exceeds that of air, which, as you may recollect, we found to be about 800 to 1.

_Caroline._ What a prodigious protuberance that must occasion! How much the weight of such a column of air, must raise the mercury in the barometer!

_Emily._ As this enormous tide of air is drawn up and supported, as it were, by the moon, its weight and pressure, I should suppose, would be rather diminished than increased?

_Mrs. B._ The weight of the atmosphere is neither increased nor diminished by the aerial tides. The moon's attraction augments the bulk, as much as it diminishes the weight, of the column of air; these effects, therefore, counterbalancing each other, the aerial tides do not affect the barometer.

_Caroline._ I do not quite understand that.

_Mrs. B._ Let us suppose that the additional bulk of air at high tide, raises the barometer one inch; and on the other hand, that the support which the moon's attraction affords the air, diminishes its weight or pressure, so as to occasion the mercury to fall one inch; under these circumstances the mercury must remain stationary. Thus, you see, that we can never be sensible of aerial tides by the barometer, on account of the equality of pressure of the atmosphere, whatever be its height.

The existence of aerial tides is not, however, hypothetical; it is proved by the effect they produce on the apparent position of the heavenly bodies; but this I cannot explain to you, till you understand the properties of light.

_Emily._ And when shall we learn them?

_Mrs. B._ I shall first explain to you the nature of sound, which is intimately connected with that of air; and I think at our next meeting, we may enter upon the subject of optics.

We have now considered the effects produced by the wide, and extended agitation, of the air; but there is another kind of agitation, of which the air is susceptible--a vibratory trembling motion, which, striking on the drum of the ear, produces _sound_.

_Caroline._ Is not sound produced by solid bodies? The voice of animals, the ringing of bells, the music of instruments, all proceed from solid bodies. I know of no sound but that of the wind, which is produced by the air.

_Mrs. B._ Sound, I assure you, results from a tremulous motion of the air; and the sonorous bodies you enumerate, are merely the instruments by which that peculiar species of motion, is communicated to the air.

_Caroline._ What! when I ring this little bell, is it the air that sounds, and not the bell?

_Mrs. B._ 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.

_Emily._ I can with difficulty conceive that. A person born deaf, it is true, has no idea of sound, because he hears none; yet that does not prevent the real existence of sound, as all those who are not deaf, can testify.

_Mrs. B._ I do not doubt the existence of sound, to all those who possess the sense of hearing; but it exists neither in the sonorous body, nor in the air, but in the mind of the person whose ear is struck, by the vibratory motion of the air, produced by a sonorous body. Sound, therefore, is a sensation, produced in a living body; life, is as necessary to its existence, as it is to that of feeling or seeing.

To convince you that sound does not exist in sonorous bodies, but that air or some other vehicle, is necessary to its production, endeavour to ring the little bell, after I have suspended it under a receiver in the air pump, from which I shall exhaust the air....

_Caroline._ This is indeed very strange: though I agitate it so violently, it produces but little sound.

_Mrs. B._ By exhausting the receiver, I have cut off the communication between the air and the bell; the latter, therefore, cannot impart its motion, to the air.

_Caroline._ Are you sure that it is not the glass, which covers the bell, that prevents our hearing it?

_Mrs. B._ That you may easily ascertain, by letting the air into the receiver, and then ringing the bell.

_Caroline._ Very true; I can hear it now, almost as loud, as if the glass did not cover it; and I can no longer doubt but that air is necessary to the production of sound.

_Mrs. B._ Not absolutely necessary, though by far the most common vehicle of sound. Liquids, as well as air, are capable of conveying the vibratory motion of a sonorous body, to the organ of hearing; as sound can be heard under water. Solid bodies also, convey sound, as I can soon convince you by a very simple experiment. I shall fasten this string by the middle, round the poker; now raise the poker from the ground, by the two ends of the string, and hold one to each of your ears:--I shall now strike the poker, with a key, and you will find that the sound is conveyed to the ear by means of the strings, in a much more perfect manner, than if it had no other vehicle than the air.

_Caroline._ That it is, certainly, for I am almost stunned by the noise. But what is a sonorous body, Mrs. B.? for all bodies are capable of producing some kind of sound, by the motion they communicate to the air.

_Mrs. B._ Those bodies are called sonorous, which produce clear, distinct, regular, and durable sounds, such as a bell, a drum, musical strings, wind instruments, &c. They owe this property to their elasticity; for an elastic body, after having been struck, not only returns to its former situation, but having acquired momentum by its velocity, like the pendulum, it springs out on the opposite side. If I draw the string A B, (fig. 6, plate 14,) which is made fast at both ends, to C, it will not only return to its original position, but proceed onwards, to D.

This is its first vibration; at the end of which, it will retain sufficient velocity to bring it to E, and back again to F, which constitutes its second vibration; the third vibration, will carry it only to G and H, and so on, till the resistance of the air destroys its motion.

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 the sonorous body. They differ also, in the one taking place in a plane, the other, in all directions: the aerial undulations, being spherical.

_Emily._ But if the air moves backwards, as well as forwards, how can its motion extend so as to convey sound to a distance?

_Mrs. B._ The first sphere of undulations, which are produced immediately around 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.

_Caroline._ The vibrations of sound, must extend much further than the circular waves in water, since sound is conveyed to a great distance.

_Mrs. B._ The air is a fluid so much less dense than water, that motion is more easily communicated to it. The report of a cannon produces vibrations of the air, which extend to several miles around.

_Emily._ Distant sound takes some time to reach us, since it is produced at the moment the cannon is fired; and we see the light of the flash, long before we hear the report.

_Mrs. B._ The air is immediately put in motion, by the firing of a cannon; but it requires time for the vibrations to extend to any distant spot. The velocity of sound, is computed to be at the rate of 1142 feet in a second.

_Caroline._ With what astonishing rapidity the vibrations must be communicated! But the velocity of sound varies, I suppose, with that of the air which conveys it. If the wind sets towards us from the cannon, we must hear the report sooner than if it set the other way.

_Mrs. B._ The direction of the wind makes less difference in the velocity of sound, than you would imagine. If the wind sets from us, it bears most of the aerial waves away, and renders the sound fainter; but it is not very considerably longer in reaching the ear, than if the wind blew towards us. This uniform velocity of sound, enables us to determine the distance of the object, from which it proceeds; as that of a vessel at sea, firing a cannon, or that of a thunder cloud. If we do not hear the thunder, till half a minute after we see the lightning, we conclude the cloud to be at the distance of six miles and a half.

_Emily._ Pray, how is the sound of an echo produced?

_Mrs. B._ 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 opposite direction, the angle of reflection being equal to the angle of incidence.

_Caroline._ Oh, then, Emily, I now understand why the echo of my voice behind our house is heard so much plainer by you than it is by me, when we stand at the opposite ends of the gravel walk. My voice, or rather, I should say, the vibrations of air it occasions, fall obliquely on the wall of the house, and are reflected by it, to the opposite end of the gravel walk.

_Emily._ Very true; and we have observed, that when we stand in the middle of the walk, opposite the house, the echo returns to the person who spoke.

_Mrs. B._ Speaking-trumpets, are constructed on the principle, that sound is reflected. The voice, instead of being diffused in the open air, is confined within the trumpet; and the vibrations which would otherwise spread laterally, fall against the sides of the instrument, and are reflected from the different points of incidence, so as to combine with those vibrations which proceed straight forwards. The vibrations are thus forced onwards, in the direction of the trumpet, so as greatly to increase the sound, to a person situated in that direction. Figure 7, plate 14, will give you a clearer idea, of the speaking-trumpet; in this, lines are drawn to represent the manner, in which we may imagine the sound to be reflected. There is a point in front of the trumpet, F, which is denominated its focus, because the sound is there more intense, than at any other spot. The trumpet used by deaf persons, acts on the same principle; although it does not equally increase the sound.

_Emily._ Are the trumpets used as musical instruments, also constructed on this principle?

_Mrs. B._ So far as their form tends to increase the sound, they are; but, as a musical instrument, the trumpet becomes itself the sonorous body, which is made to vibrate by blowing into it, and communicates its vibrations to the air.

I will attempt to give you, in a few words, some notion of the nature of musical sounds, which, as you are fond of music, must be interesting to you.

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, interrupt it in its course, and produce harsh jarring sounds, which are called _discords_.

_Emily._ But each set of these irregular vibrations, if repeated alone, and at equal intervals, would, I suppose, produce a musical tone? It is only their irregular interference, which occasions discord.

_Mrs. B._ Certainly. The quicker a sonorous body vibrates, the more acute, or sharp, is the sound produced; and the slower the vibrations, the more grave will be the note.

_Caroline._ But if I strike any one note of the piano-forte, repeatedly, whether quickly or slowly, it always gives the same tone.

_Mrs. B._ Because the vibrations of the same string, at the same degree of tension, are always of a similar duration. The quickness, or slowness of the vibrations, relate to the single tones, not to the various sounds which they may compose, by succeeding each other. 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 strings, or wires, depends upon their length, their thickness, or weight, and their degree of tension: thus, you find, the low bass notes are produced by long, thick, loose strings; and the high treble notes by short, small, and tight strings.

_Caroline._ Then, the different length, and size, of the strings of musical instruments, serve to vary the duration of the vibrations, and consequently, the acuteness or gravity of the notes?

_Mrs. B._ Yes. 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. Thus, if the vibrations of two strings are performed in equal times, the same tone is produced by both, and they are said to be in unison.

_Emily._ Now, then, I understand why, when I tune my harp, in unison with the piano-forte, I draw the strings tighter, if it is too low, or loosen them, if it is too high a pitch: it is in order to bring them to vibrate, in equal times, with the strings of the piano-forte.

_Mrs. B._ But concord, you know, is not confined to unison; for two different tones, harmonize in a variety of cases. When the vibrations of one string (or other sonorous body) vibrate 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 octave.

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.

_Caroline._ So, then, when I strike the key-note with its fifth, I hear every second vibration of one, and every third of the other, at the same time?

_Mrs. B._ Yes; and the key-note, struck with the fourth, is likewise a concord, because the vibrations, are as three to four. The vibrations of a major third, with the key-note, are as four to five; and those of a minor third, as five to six.

There are other tones, which, though they cannot be struck together without producing discord, if struck successively, give us that succession of pleasing sounds, which is called melody. Harmony, you perceive, arises from the combined effect of two, or more concordant sounds, while melody, is the result of certain simple sounds, which succeed each other. Upon these general principles, the science of music is founded; but, I am not sufficiently acquainted with it, to enter into it any further.

We shall now, therefore, take leave of the subject of sound; and, at our next interview, enter upon that of optics, in which we shall consider the nature of light, vision, and colours.

Questions

1. (Pg. 146) What is wind, and how is it generally produced?

2. (Pg. 146) How do the winds blow, around the place where the air becomes rarefied?

3. (Pg. 146) What effect is likely to be produced where the winds meet?

4. (Pg. 147) In what part of the globe is the air most rarefied, and what is the consequence?

5. (Pg. 147) How do these winds change their direction as they approach the equator?

6. (Pg. 147) How are the trade-winds produced, and how far do they extend?

7. (Pg. 147) How is the equilibrium in the air restored?

8. (Pg. 148) How can contrary currents of air be shown in a room?

9. (Pg. 148) What causes this?

10. (Pg. 148) What is meant by a periodical wind?

11. (Pg. 148) What occasions the land and sea breezes, and where do they prevail?

12. (Pg. 149) What are monsoons?

13. (Pg. 149) How do they change, and what is the cause?

14. (Pg. 149) What is meant by their breaking up, and what effect is in general produced?

15. (Pg. 149) Why is the wind most variable in high latitudes?

16. (Pg. 150) Why is the wind apt to lessen about sunset?

17. (Pg. 150) What effect must the sun and moon produce upon the atmosphere, from their attraction?

18. (Pg. 150) Why do not the aerial tides affect the barometer?

19. (Pg. 151) How is sound produced?