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.