and are occasioned by the reservoir being imperfectly supplied.
Independently of the situation, this is always the case, when the duct,
or ducts, which convey the water into the reservoir, are smaller than
those which carry it off.
_Caroline._ If it runs out, faster than it runs in, it will of course
sometimes be empty. Do not rivers also, derive their source from
springs?
_Mrs. B._ Yes, they generally take their source in mountainous
countries, where springs are most abundant.
_Caroline._ I understood you that springs were more rare, in elevated
situations.
_Mrs. B._ You do not consider that mountainous countries, abound equally
with high, and low situations. Reservoirs of water, which are formed in
the bosoms of mountains, generally find a vent, either on their
declivity, or in the valley beneath; while subterraneous reservoirs,
formed in a plain, can seldom find a passage to the surface of the
earth, but remain concealed, unless discovered by digging a well. When a
spring once issues at the surface of the earth, it continues its course
externally, seeking always a lower ground, for it can no longer rise.
_Emily._ Then what is the consequence, if the spring, or, as I should
now rather call it, the rivulet, runs into a situation, which is
surrounded by higher ground?
_Mrs. B._ Its course is stopped; the water accumulates, and it forms a
pool, pond, or lake, according to the dimensions of the body of water.
The lake of Geneva, in all probability, owes its origin to the Rhone,
which passes through it: if, when the river first entered the valley,
which now forms the bed of the Lake, it found itself surrounded by
higher grounds, its waters would there accumulate, till they rose to a
level with that part of the valley, where the Rhone now continues its
course beyond the Lake, and from whence it flows through valleys,
occasionally forming other small lakes, till it reaches the sea.
_Emily._ And are not fountains, of the nature of springs?
_Mrs. B._ Exactly. A fountain is conducted perpendicularly upwards, by
the spout or adjutage A, through which it flows; and it will rise nearly
as high as the reservoir B, from whence it proceeds. (Plate 14. fig. 2.)
_Caroline._ Why not quite as high?
_Mrs. B._ Because it meets with resistance from the air, in its ascent;
and its motion is impeded by friction against the spout, where it rushes
out.
_Emily._ But if the tube through which the water rises be smooth, can
there be any friction? especially with a fluid, whose particles yield to
the slightest impression.
_Mrs. B._ Friction, (as we observed in a former lesson,) may be
diminished by polishing, but can never be entirely destroyed; and though
fluids, are less susceptible of friction, than solid bodies, they are
still affected by it. Another reason why a fountain will not rise so
high as its reservoir, is, that as all the water which spouts up, has to
descend again, it in doing so, presses, or strikes against the under
parts, and forces them sideways, spreading the column into a head, and
rendering it both wider, and shorter, than it otherwise would be.
At our next meeting, we shall examine the mechanical properties of the
air, which being an elastic fluid, differs in many respects, from
liquids.
Questions
1. (Pg. 129) Why do not the frequent rains, fill the earth with water?
2. (Pg. 129) Why will vapour rise? to what height will it ascend, and
what will it form?
3. (Pg. 129) How may drops of rain be formed?
4. (Pg. 130) What becomes of the water after it has fallen to the earth?
5. (Pg. 130) What is the difference between rain water, and that from
springs?
6. (Pg. 130) Why is rain more pure than spring water?
7. (Pg. 130) Why is spring water more agreeable to the palate?
8. (Pg. 131) What causes the water to collect and form springs?
9. (Pg. 131) Why cannot water penetrate through clay?
10. (Pg. 131) What is represented by fig. 9, plate 13?
11. (Pg. 132) How can you account for its rising upwards, as represented
at C?
12. (Pg. 132) In conveying water by means of pipes, how must the
reservoir be situated?
13. (Pg. 132) What is the instrument called, which is represented in
plate 14, fig. 1,--and how does it operate?
14. (Pg. 133) Why are wells rarely well supplied with water, in elevated
situations?
15. (Pg. 133) When water is found in elevated situations, whence is it
supplied?
16. (Pg. 133) Wells and springs, at some periods well supplied, fail at
others; how is this accounted for?
17. (Pg. 134) Some springs flow abundantly in dry weather, which
occasionally fail in wet weather, how may this be explained?
18. (Pg. 134) What is meant by intermitting springs?
19. (Pg. 134) Whence do rivers, in general, derive their water?
20. (Pg. 134) Why do springs abound more in mountainous, than in level
countries?
21. (Pg. 135) How are lakes formed?
22. (Pg. 135) What causes water to rise in fountains, and how is this
explained by figure 2, plate 14?
23. (Pg. 135) Why will not the fountain rise to the height of the water
in the reservoir?
CONVERSATION XII.
ON THE MECHANICAL PROPERTIES OF AIR.
OF THE SPRING OR ELASTICITY OF THE AIR. OF THE WEIGHT OF THE AIR.
EXPERIMENTS WITH THE AIR PUMP. OF THE BAROMETER. MODE OF WEIGHING AIR.
SPECIFIC GRAVITY OF AIR. OF PUMPS. DESCRIPTION OF THE SUCKING PUMP.
DESCRIPTION OF THE FORCING PUMP.
MRS. B.
At our last meeting we examined the properties of fluids in general, and
more particularly of such as are called non-elastic fluids, or liquids.
There is another class of fluids, distinguished by the name of aeriform,
or elastic fluids, the principal of which is the air we breathe, which
surrounds the earth, and is called the atmosphere.
_Emily._ There are then other kinds of air, besides the atmosphere?
_Mrs. B._ Yes; a great variety; but they differ only in their chemical,
and not in their mechanical properties; and as it is the latter we are
to examine, we shall not at present inquire into their composition, but
confine our attention to the mechanical properties of elastic fluids in
general.
_Caroline._ And from whence arises this difference, between elastic, and
non-elastic fluids?
_Mrs. B._ There is no attraction of cohesion, between the particles of
elastic fluids; so that the expansive power of heat, has no adversary to
contend with, but gravity; any increase of temperature, therefore,
expands elastic fluids considerably, and a diminution, proportionally
condenses them.
The most essential point, in which air, differs from other fluids is in
its spring or elasticity; that is to say, its power of increasing, or
diminishing in bulk, accordingly as it is more, or less, compressed: a
power of which I have informed you, liquids are almost wholly deprived.
_Emily._ I think I understand the elasticity of the air very well from
what you formerly said of it; but what perplexes me is, its having
gravity; if it is heavy, and we are surrounded by it, why do we not feel
its weight?
_Caroline._ It must be impossible to be sensible of the weight of such
infinitely small particles, as those of which the air is composed:
particles which are too small to be seen, must be too light to be felt.
_Mrs. B._ You are mistaken, my dear; the air is much heavier than you
imagine; it is true, that the particles which compose it, are small; but
then, reflect on their quantity: the atmosphere extends in height, a
great number of miles from the earth, and its gravity is such, that a
man of middling stature, is computed (when the air is heaviest) to
sustain the weight of about 14 tons.
_Caroline._ Is it possible! I should have thought such a weight would
have crushed any one to atoms.
_Mrs. B._ That would, indeed, be the case, if it were not for the
equality of the pressure, on every part of the body; but when thus
diffused, we can bear even a much greater weight, without any
considerable inconvenience. In bathing we support the weight and
pressure of the water, in addition to that of the atmosphere; but
because this pressure is equally distributed over the body, we are
scarcely sensible of it; whilst if your shoulders, your head, or any
particular part of your frame, were loaded with the additional weight of
a hundred pounds, you would soon sink under the fatigue. Besides this,
our bodies contain air, the spring of which, counterbalances the weight
of the external air, and renders us insensible of its pressure.
_Caroline._ But if it were possible to relieve me from the weight of the
atmosphere, should I not feel more light and agile?
_Mrs. B._ On the contrary, the air within you, meeting with no external
pressure to restrain its elasticity, would distend your body, and at
length bursting some of the parts which confined it, put a period to
your existence.
_Caroline._ This weight of the atmosphere, then, which I was so
apprehensive would crush me, is, in reality, essential to my
preservation.
_Emily._ I once saw a person cupped, and was told that the swelling of
the part under the cup, was produced by taking away from that part, the
pressure of the atmosphere; but I could not understand how this pressure
produced such an effect.
_Mrs. B._ The air pump affords us the means of making a great variety of
interesting experiments, on the weight, and pressure of the air: some
of them you have already seen. Do you not recollect, that in a vacuum
produced within the air pump, substances of various weights, fell to the
bottom in the same time; why does not this happen in the atmosphere?
_Caroline._ I remember you told us it was owing to the resistance which
light bodies meet with, from the air, during their fall.
_Mrs. B._ Or, in other words, to the support which they received from
the air, and which prolonged the time of their fall. Now, if the air
were destitute of weight, how could it support other bodies, or retard
their fall?
I shall now show you some other experiments, which illustrate, in a
striking manner, both the weight, and elasticity of air. I shall tie a
piece of bladder over this glass receiver, which, you will observe, is
open at the top as well as below.
_Caroline._ Why do you wet the bladder first?
_Mrs. B._ It expands by wetting, and contracts in drying; it is also
more soft and pliable when wet, so that I can make it fit better, and
when dry, it will be tighter. We must hold it to the fire in order to
dry it; but not too near, lest it should burst by sudden contraction.
Let us now fix it on the air pump, and exhaust the air from underneath
it--you will not be alarmed if you hear a noise?
_Emily._ It was as loud as the report of a gun, and the bladder is
burst! Pray explain how the air is concerned in this experiment.
_Mrs. B._ It is the effect of the weight of the atmosphere, on the upper
surface of the bladder, when I had taken away the air from the under
surface, so that there was no longer any reaction to counterbalance the
pressure of the atmosphere, on the receiver. You observed how the
bladder was pressed inwards, by the weight of the external air, in
proportion as I exhausted the receiver: and before a complete vacuum was
formed, the bladder, unable to sustain the violence of the pressure,
burst with the explosion you have just heard.
I shall now show you an experiment, which proves the expansion of the
air, contained within a body, when it is relieved from the pressure of
the external air. You would not imagine that there was any air contained
within this shrivelled apple, by its appearance; but take notice of it
when placed within a receiver, from which I shall exhaust the air.
_Caroline._ How strange! it grows quite plump, and looks like a
fresh-gathered apple.
_Mrs. B._ But as soon as I let the air again into the receiver, the
apple, you see, returns to its shrivelled state. When I took away the
pressure of the atmosphere, the air within the apple, expanded, and
swelled it out; but the instant the atmospheric air was restored, the
expansion of the internal air, was checked and repressed, and the apple
shrunk to its former dimensions.
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.