Chapter X: Introduction: General Properties of Bodies. Impenetrability. Extension (8)

You may make a similar experiment with this little bladder, which you see is perfectly flaccid, and appears to contain no air: in this state I shall tie up the neck of the bladder, so that whatever air remains within it, may not escape, and then place it under the receiver. Now observe, as I exhaust the receiver, how the bladder distends; this proceeds from the great dilatation of the small quantity of air, which was enclosed within the bladder, when I tied it up; but as soon as I let the air into the receiver, that which the bladder contains, condenses and shrinks into its small compass, within the folds of the bladder.

_Emily._ These experiments are extremely amusing, and they afford clear proofs, both of the weight, and elasticity of the air; but I should like to know, exactly, how much the air weighs.

_Mrs. B._ A column of air reaching to the top of the atmosphere, and whose base is a square inch, weighs about 15 lbs. therefore, every square inch of our bodies, sustains a weight of 15 lbs.: and if you wish to know the weight of the whole of the atmosphere, you must reckon how many square inches there are on the surface of the globe, and multiply them by 15.

_Emily._ But can we not ascertain the weight of a small quantity of air?

_Mrs. B._ With perfect ease. I shall exhaust the air from this little bottle, by means of the air pump: and having emptied the bottle of air, or, in other words, produced a vacuum within it, I secure it by turning this screw adapted to its neck: we may now find the exact weight of this bottle, by putting it into one of the scales of a balance. It weighs, you see, just two ounces; but when I turn the screw, so as to admit the air into the bottle, the scale which contains it, preponderates.

_Caroline._ No doubt the bottle filled with air, is heavier than the bottle void of air; and the additional weight required to bring the scales again to a balance, must be exactly that of the air which the bottle now contains.

_Mrs. B._ That weight, you see, is almost two grains. The dimensions of this bottle, are six cubic inches. Six cubic inches of air, therefore, at the temperature of this room, weighs nearly 2 grains.

_Caroline._ Why do you observe the temperature of the room, in estimating the weight of the air?

_Mrs. B._ Because heat rarefies air, and renders it lighter; therefore the warmer the air is, which you weigh, the lighter it will be.

If you should now be desirous of knowing the specific gravity of this air, we need only fill the same bottle, with water, and thus obtain the weight of an equal quantity of water--which you see is 1515 grs.; now by comparing the weight of water, to that of air, we find it to be in the proportion of about 800 to 1.

As you are acquainted with decimal arithmetic, you will understand what I mean, when I tell you, that water being called 1000, the specific gravity of air, will be 1.2.

I will show you another instance, of the weight of the atmosphere, which I think will please you: you know what a barometer is?

_Caroline._ It is an instrument which indicates the state of the weather, by means of a tube of quicksilver; but how, I cannot exactly say.

_Mrs. B._ It is by showing the weight of the atmosphere, which has great influence on the weather. The barometer, is an instrument extremely simple in its construction. In order that you may understand it, I will show you how it is made. I first fill with mercury, a glass tube A B, (fig. 3, plate 14.) about three feet in length, and open only at one end; then stopping the open end, with my finger, I immerse it in a cup C, containing a little mercury.

_Emily._ Part of the mercury which was in the tube, I observe, runs down into the cup; but why does not the whole of it subside, for it is contrary to the law of the equilibrium of fluids, that the mercury in the tube, should not descend to a level with that in the cup?

_Mrs. B._ The mercury that has fallen from the tube, into the cup, has left a vacant space in the upper part of the tube, to which the air cannot gain access; this space is therefore a perfect vacuum; the mercury in the tube, is relieved from the pressure of the atmosphere, whilst that in the cup, remains exposed to it.

_Caroline._ Oh, now I understand it; the pressure of the air on the mercury in the cup, forces it to rise in the tube, where there is not any air to counteract the external pressure.

_Emily._ Or rather supports the mercury in the tube, and prevents it from falling.

_Mrs. B._ That comes to the same thing; for the power that can support mercury in a vacuum, would also make it ascend, when it met with a vacuum.

Thus you see, that the equilibrium of the mercury is destroyed, only to preserve the general equilibrium of fluids.

_Caroline._ But this simple apparatus is, in appearance, very unlike a barometer.

_Mrs. B._ It is all that is essential to a barometer. The tube and the cup, or a cistern of mercury, are fixed on a board, for the convenience of suspending it; the brass plate on the upper part of the board, is graduated into inches, and tenths of inches, for the purpose of ascertaining the height at which the mercury stands in the tube; and the small moveable metal plate, serves to show that height, with greater accuracy.

_Emily._ And at what height, will the weight of the atmosphere sustain the mercury?

_Mrs. B._ About 28 or 29 inches, as you will see by this barometer; but it depends upon the weight of the atmosphere, which varies much, in different states of the weather. The greater the pressure of the air on the mercury in the cup, the higher it will ascend in the tube. Now can you tell me whether the air is heavier, in wet, or in dry weather?

_Caroline._ Without a moment's reflection, the air must be heaviest in wet weather. It is so depressing, and makes one feel so heavy, while in fine weather, I feel as light as a feather, and as brisk as a bee.

_Mrs. B._ Would it not have been better to have answered with a moment's reflection, Caroline? It would have convinced you, that the air must be heaviest in dry weather; for it is then, that the mercury is found to rise in the tube, and consequently, the mercury in the cup, must be most pressed by the air.

_Caroline._ Why then does the air feel so heavy, in bad weather?

_Mrs. B._ Because it is less salubrious, when impregnated with damp. The lungs, under these circumstances, do not play so freely, nor does the blood circulate so well; thus obstructions are frequently occasioned in the smaller vessels, from which arise colds, asthmas, agues, fevers, &c.

_Emily._ Since the atmosphere diminishes in density, in the upper regions, is not the air more rare, upon a hill, than in a plain; and does the barometer indicate this difference?

_Mrs. B._ Certainly. This instrument, is so exact in its indications, that it is used for the purpose of measuring the height of mountains, and of estimating the elevation of balloons; the mercury descending in the tube, as you ascend to a greater height.

_Emily._ And is no inconvenience experienced, from the thinness of the air, in such elevated situations?

_Mrs. B._ Oh, yes; frequently. It 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 distention, and sometimes causes the bursting of the smaller blood-vessels, in the nose, and ears. Besides in such situations, you are more exposed, both to heat, and cold; for though the atmosphere is itself transparent, its lower regions, abound with vapours, and exhalations, from the earth, which float in it, and act in some degree as a covering, which preserves us equally from the intensity of the sun's rays, and from the severity of the cold.

_Caroline._ Pray, Mrs. B., is not the thermometer constructed on the same principles as the barometer?

_Mrs. B._ Not at all. The rise and fall of the fluid in the thermometer, is occasioned by the expansive power of heat, and the condensation produced by cold: the air has no access to it. An explanation of it would, therefore, be irrelevant to our present subject.

_Emily._ I have been reflecting, that since it is the weight of the atmosphere, which supports the mercury, in the tube of a barometer, it would support a column of any other fluid, in the same manner.

_Mrs. B._ Certainly; but as mercury, is heavier than all other fluids, it will support a higher column, of any other fluid; for two fluids are in equilibrium, when their height varies, inversely as their densities. We find the weight of the atmosphere, is equal to sustaining a column of water, for instance, of no less than 32 feet above its level.

_Caroline._ The weight of the atmosphere, is then, as great as that of a body of water of 32 feet in height.

_Mrs. B._ Precisely; for a column of air, of the height of the atmosphere, is equal to a column of water of about 32 feet, or one of mercury, of from 28 to 29 inches.

The common pump, is dependent on this principle. By the act of pumping, the pressure of the atmosphere is taken off the water, which, in consequence, rises.

The body of a pump, consists of a large tube or pipe, whose lower end is immersed in the water which it is designed to raise. A kind of stopper, called a piston, is fitted to this tube, and is made to slide up and down it, by means of a metallic rod, fastened to the centre of the piston.

_Emily._ Is it not similar to the syringe, or squirt, with which you first draw in, and then force out water?

_Mrs. B._ It is; but you know that we do not wish to force the water out of the pump, at the same end of the pipe, at which we draw it in. The intention of a pump, is to raise water from a spring, or well; the pipe is, therefore, placed perpendicularly over the water, which enters it at the lower extremity, and it issues at a horizontal spout, towards the upper part of the pump; to effect this, there are, besides the piston, two contrivances called valves. The pump, therefore, is rather a more complicated piece of machinery, than the syringe.

_Caroline._ Pray, Mrs. B., is not the leather, which covers the opening, in the lower board of a pair of bellows, a kind of valve?

_Mrs. B._ It is, valves are made in various forms; any contrivance, which allows a fluid to pass in one direction, and prevents its return, is called a valve; that of the bellows, and of the common pump, resemble each other, exactly. You can now, I think, understand the structure of the pump.

Its various parts, are delineated in this figure: (fig. 4. plate 14.) A B is the pipe, or body of the pump, P the piston, V a valve, or little door in the piston, which, opening upwards, admits the water to rise through it, but prevents its returning, and Y, is a similar valve, placed lower down in the body of the pump; H is the handle, which in this model, serves to work the piston.

When the pump is in a state of inaction, the two valves are closed by their own weight; but when, by working the handle of the pump, the piston ascends; it raises a column of air which rested upon it, and produces a vacuum, between the piston, and the lower valve Y; the air beneath this valve, which is immediately over the surface of the water, consequently expands, and forces its way through it; the water, then, relieved from the pressure of the air, ascends into the pump. A few strokes of the handle, totally excludes the air from the body of the pump, and fills it with water, which, having passed through both the valves, runs out at the spout.

_Caroline._ I understand this perfectly. When the piston is elevated, the air, and the water, successively rise in the pump, for the same reason as the mercury, rises in the barometer.

_Emily._ I thought that water was drawn up into a pump, by suction, in the same manner as water may be sucked through a straw.

_Mrs. B._ It is so, into the body of the pump; for the power of suction, is no other than that of producing a vacuum over one part of the liquid, into which vacuum the liquid is forced, by the pressure of the atmosphere, on another part. The action of sucking through a straw, consists in drawing in, and confining the breath, so as to produce a vacuum in the mouth; in consequence of which, the air within the straw, rushes into the mouth, and is followed by the liquid, into which, the lower end of the straw, is immersed. The principle, you see, is the same, and the only difference consists in the mode of producing a vacuum. In suction, the muscular powers answer the purpose of the piston and valve.

_Emily._ Water cannot, then, be raised by a pump, above 32 feet; for the pressure of the atmosphere will not sustain a column of water, above that height.

_Mrs. B._ I beg your pardon. It is true that there must never be so great a distance as 32 feet, from the level of the water in the well, to the valve in the piston, otherwise the water would not rise through that valve; but when once the water has passed that opening, it is no longer the pressure of air on the reservoir, which makes it ascend; it is raised by lifting it up, as you would raise it in a bucket, of which the piston formed the bottom. This common pump is, therefore, called the sucking, or lifting pump, as it is constructed on both these principles. The rod to which the piston is attached, must be made sufficiently long, to allow the piston to be within 32 feet of the surface of the water in the well, however deep it may be. There is another sort of pump, called the forcing pump: it consists of a forcing power, added to the sucking part of the pump. This additional power, is exactly on the principle of the syringe: by raising the piston, you draw the water into the pump, and by causing it to descend, you force the water out.

_Caroline._ But the water must be forced out at the upper part of the pump; and I cannot conceive how that can be done by the descent of the piston.

_Mrs. B._ Figure 5, plate 14, will explain the difficulty. The large pipe, A B, represents the sucking part of the pump, which differs from the lifting pump, only in its piston P, being unfurnished with a valve, in consequence of which the water cannot rise above it. When, therefore, the piston descends, it shuts the valve Y, and forces the water (which has no other vent) into the pipe D: this is likewise furnished with a valve V, which, opening upwards, admits the water to pass, but prevents its return.

The water, is thus first raised in the pump, and then forced into the pipe, by the alternate ascending, and descending motion of the piston, after a few strokes of the handle to fill the pipe, from whence the water issues at the spout.

_Emily._ Does not the air pump, which you used in the experiments, on pneumatics, operate upon the same principles as the sucking pump?

_Mrs. B._ Exactly. The air pump which I used (plate 1, fig. 2,) has two hollow, brass cylinders, called barrels, which are made perfectly true. In each of those barrels, there is a piston; these are worked up, and down, by the same handle; the pistons, are furnished with valves, opening upwards, like those of the common pump: there are valves also, placed at the lower part of each barrel, which open upwards; there are therefore two pumps, united to produce the same effect: two tubes, connect these barrels with the plate, upon which I placed the receivers, which were to be exhausted.

_Emily._ I now understand how the air pump acts; the receiver contains air, which is exhausted, just as it is by the common pump, before the water begins to rise.

_Mrs. B._ Having explained the mechanical properties of air, I think it is now time to conclude our lesson. When next we meet, I shall give you some account of wind, and of sound, which will terminate our observations on elastic fluids.

_Caroline._ And I shall run into the garden, to have the pleasure of pumping, now that I understand the construction of a pump.

_Mrs. B._ And, to-morrow, I hope you will be able to tell me, whether it is a forcing, or a common lifting pump.

Questions

1. (Pg. 136) Into what two kinds are fluids divided?

2. (Pg. 136) There are different kinds of elastic fluids, in what properties are they alike, and in what do they differ?

3. (Pg. 136) In what particular do elastic, differ from non-elastic, fluids?

4. (Pg. 136) What is meant by the elasticity of air?

5. (Pg. 137) What is said respecting the weight of the atmosphere?

6. (Pg. 137) Why do we not feel the pressure of the air?

7. (Pg. 137) What would be the effect of relieving us from atmospheric pressure?

8. (Pg. 138) How may the weight of the air be shown by the aid of the air pump, and a piece of bladder?

9. (Pg. 138) How is this explained?

10. (Pg. 138) How may its elasticity be exhibited, by an apple, and by a bladder?

11. (Pg. 139) What is the absolute weight of a given column of atmospheric air, and how could its whole pressure upon the earth be ascertained?

12. (Pg. 139) How can the weight of a small bulk of air be found?

13. (Pg. 140) In ascertaining the weight of air, we take account of its temperature--Why?

14. (Pg. 140) How could you ascertain the specific gravity of air, and what would it be?

15. (Pg. 140) What are the essential parts of a barometer, as represented plate 14, fig. 3?

16. (Pg. 141) What sustains the mercury in the tube?

17. (Pg. 141) Of what use are the divisions in the upper part of the instrument?

18. (Pg. 141) To what height will the mercury rise, and what occasions this height to vary?

19. (Pg. 141) When is the mercury highest, in wet, or in dry weather?

20. (Pg. 141) What occasions the sensation of oppression, in damp weather?

21. (Pg. 142) Why will the barometer indicate the height of mountains, or of balloons?

22. (Pg. 142) Is any inconvenience experienced by persons ascending to great heights, and from what cause?

23. (Pg. 142) What occasions the rise and fall of the mercury, in a thermometer?

24. (Pg. 142) To what height will the pressure of the atmosphere raise a column of water?

25. (Pg. 142) What governs the difference between the height of the mercury, and of the water?

26. (Pg. 143) How does the common pump, raise water from a well?

27. (Pg. 143) What is meant by a piston?

28. (Pg. 143) Describe the construction, and use, of a valve.

29. (Pg. 143) What are the parts of the pump, as represented, fig. 4, plate 14.?

30. (Pg. 144) How do these parts act, in raising the water?

31. (Pg. 144) In what does that which is commonly called suction, consist?

32. (Pg. 144) How must the piston be situated in the pump?

33. (Pg. 144) What other kind of pump is described?

34. (Pg. 145) How is the forcing pump constructed, as shown in plate 14, fig. 5?

35. (Pg. 145) Describe the construction and operation of the air pump, (fig. 2, plate 1.)

CONVERSATION XIII.

ON WIND AND SOUND.

OF WIND IN GENERAL. OF THE TRADE-WIND. OF THE PERIODICAL TRADE-WINDS. OF THE AERIAL TIDES. OF SOUNDS IN GENERAL. OF SONOROUS BODIES. OF MUSICAL SOUNDS. OF CONCORD OR HARMONY, AND MELODY.

MRS. B.

Well, Caroline, have you ascertained what kind of pump you have in your garden?

_Caroline._ I think it must be merely a lifting pump, because no more force is required to raise the handle than is necessary to lift its weight; and as in a forcing pump, by raising the handle, you force the water into the smaller pipe, the resistance the water offers, must require an exertion of strength, to overcome it.

_Mrs. B._ I make no doubt you are right; for lifting pumps, being simple in their construction, are by far the most common.

I have promised to-day to give you some account of the nature of wind. Wind is nothing more than the motion of a stream, or current of air, generally produced by a partial change of temperature in the atmosphere; for when any one part is more heated than the rest, that part is rarefied, the air in consequence rises, and the equilibrium is destroyed. When this happens, there necessarily follows a motion of the surrounding air towards that part, in order to restore it; this spot, therefore, receives winds from every quarter. Those who live to the north of it, experience a north wind; those to the south, a south wind:--do you comprehend this?

_Caroline._ Perfectly. But what sort of weather must those people have, who live on the spot, where these winds meet and interfere?

_Mrs. B._ They have most commonly turbulent and boisterous weather, whirlwinds, hurricanes, rain, lightning, thunder, &c. This stormy weather occurs most frequently in the torrid zone, where the heat is greatest: the air being more rarefied there, than in any other part of the globe, is lighter, and consequently, ascends; whilst the air from the north and south, is continually flowing in, to restore the equilibrium.

_Caroline._ This motion of the air, would produce a regular and constant north wind, to the inhabitants of the northern hemisphere; and a south wind, to those of the southern hemisphere, and continual storms at the equator, where these two adverse winds would meet.

_Mrs. B._ These winds do not meet, for they each change their direction before they reach the equator. The sun, in moving over the equatorial regions from east to west, rarefies the air as it passes, and causes the denser eastern air to flow westwards, in order to restore the equilibrium, thus producing a regular east wind, about the equator.

_Caroline._ The air from the west, then, constantly goes to meet the sun, and repair the disturbance which his beams have produced in the equilibrium of the atmosphere. But I wonder how you will reconcile these various winds, Mrs. B.; you first led me to suppose there was a constant struggle between opposite winds at the equator, producing storm and tempest; but now I hear of one regular invariable wind, which must naturally be attended by calm weather.

_Emily._ I think I comprehend it: do not these winds from the north and south, combine with the easterly wind about the equator, and form, what are called, the trade-winds?

_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?

20. (Pg. 151) Does sound exist in the sonorous body, if not, what is it?

21. (Pg. 151) By what experiment might we prove that air is the principal vehicle of sound?

22. (Pg. 152) What other bodies convey sound, and how can it be shown that they do so?

23. (Pg. 152) What is meant by a sonorous body?

24. (Pg. 152) To what do they owe this property?

25. (Pg. 152) How is this explained by fig. 6, plate 14?

26. (Pg. 152) How is it illustrated by a stone thrown into water, and how far does this illustration apply?

27. (Pg. 153) How are the vibrations propagated?

28. (Pg. 153) How can we prove that sound, does not travel as rapidly as light?

29. (Pg. 153) At what rate is sound said to travel?

30. (Pg. 153) Is the velocity much influenced by the direction of the wind?

31. (Pg. 153) How will sound enable us to judge of the distance of objects?

32. (Pg. 154) How are echoes produced?

33. (Pg. 154) What is the operation and effect of the speaking-trumpet (fig. 7, plate 14)?

34. (Pg. 155) How is a musical tone produced?

35. (Pg. 155) What occasions discords?

36. (Pg. 155) Upon what does the acuteness or gravity of a sound depend?

37. (Pg. 155) Does the force, with which a string is struck, affect the rapidity of its vibrations?

38. (Pg. 155) How are the strings made to produce the high and low notes?

39. (Pg. 155) What is meant by harmony, or concord, and how is it produced?

40. (Pg. 156) When are strings said to be in unison?

41. (Pg. 156) How are octaves produced?

42. (Pg. 156) How are fifths produced?

43. (Pg. 156) How major and minor thirds?

44. (Pg. 156) What is meant by melody, and in what particular does it differ from harmony?

CONVERSATION XIV.

ON OPTICS.

OF LUMINOUS, TRANSPARENT, AND OPAQUE BODIES. OF THE RADIATION OF LIGHT. OF SHADOWS. OF THE REFLECTION OF LIGHT. OPAQUE BODIES SEEN ONLY BY REFLECTED LIGHT. VISION EXPLAINED. CAMERA OBSCURA. IMAGE OF OBJECTS ON THE RETINA.

CAROLINE.

I long to begin our lesson to-day, Mrs. B., for I expect that it will be very entertaining.

_Mrs. B._ _Optics is that branch of philosophy, which treats of the nature and properties of light._ It is certainly one of the most interesting branches of Natural Philosophy, but not one of the easiest to understand; I must, therefore, beg that you will give me your undivided attention.

I shall first inquire, whether you comprehend the meaning of a _luminous body_, an _opaque body_, and a _transparent body_.

_Caroline._ A luminous body is one that shines; an opaque....

_Mrs. B._ Do not proceed to the second, until we have agreed upon the definition of the first. All bodies that shine, are not luminous; for a luminous body is one that shines by its own light; as the sun, the fire, a candle, &c.

_Emily._ Polished metal then, when it shines with so much brilliancy, is not a luminous body?

_Mrs. B._ No, for it would be dark, if it did not receive light from a luminous body; it belongs, therefore, to the class of dark, as well as of opaque bodies, which comprehends all such as are neither luminous, nor will admit the light to pass through them.

_Emily._ And transparent bodies, are those which admit the light to pass through them, such as glass and water.

_Mrs. B._ You are right. Transparent, or pellucid bodies, are frequently called mediums, because they allow the rays of light to pass through them; and the rays which pass through, are said to be transmitted by them.

Light, when emanated from the sun, or any other luminous body, is projected forward in straight lines, in every possible direction; so that the luminous body, is not only the general centre, from whence all the rays proceed; but every point of it, may be considered as a centre, which radiates light in every direction. (Fig. 1, plate 15.)

_Emily._ But do not the rays which are projected in different directions, and cross each other, interfere, and impede each other's course?