which in some measure supports the gold, and by so doing, diminishes its
weight. If the body immersed in water, was of the same weight as that
fluid, it would be wholly supported by it, just as the water which it
displaces, was supported, previous to its making way for the solid body.
If the body is heavier than the water, it cannot be wholly supported by
it; but the water will offer some resistance to its descent.
_Caroline._ And the resistance which water offers to the descent of
heavy bodies immersed in it, (since it proceeds from the upward pressure
of the particles of the fluid,) must in all cases, I suppose, be the
same?
_Mrs. B._ Yes: the resistance of the fluid, is proportioned to the bulk,
and not to the weight, of the body immersed in it; all bodies of the
same size, therefore, lose the same quantity of their weight in water.
Can you form any idea what this loss will be?
_Emily._ I should think it would be equal to the weight of the water
displaced; for, since that portion of the water was supported before the
immersion of the solid body, an equal weight of the solid body, will be
supported.
_Mrs. B._ You are perfectly right; a body weighed in water, loses just
as much of its weight, as is equal to that of the water it displaces; so
that if you were to put the water displaced, into the scale to which the
body is suspended, it would restore the balance.
You must observe, that when you weigh a body in water, in order to
ascertain its specific gravity, you must not sink the dish of the
balance in the water; but either suspend the body to a hook at the
bottom of the dish, or else take off the dish, and suspend to the arm of
the balance a weight to counterbalance the other dish, and to this
attach the solid to be weighed, (fig. 7.) Now suppose that a cubic inch
of gold, weighed 19 ounces out of water, and lost one ounce of its
weight by being weighed in water, what would be its specific gravity?
_Caroline._ The cubic inch of water it displaced, must weigh that one
ounce; and as a cubic inch of gold, weighs 19 ounces, gold is 19 times,
as heavy as water.
_Emily._ I recollect having seen a table of the comparative weights of
bodies, in which gold appeared to me to be estimated at 19 thousand
times, the weight of water.
_Mrs. B._ You misunderstood the meaning of the table. In the estimation
you allude to, the weight of water was reckoned at 1000. You must
observe, that the weight of a substance when not compared to that of any
other, is perfectly arbitrary; and when water is adopted as a standard,
we may denominate its weight by any number we please; but then the
weight of all bodies tried by this standard, must be signified by
proportional numbers.
_Caroline._ We may call the weight of water, for example, one, and then
that of gold, would be nineteen; or if we choose to call the weight of
water 1000, that of gold would be 19,000. In short, specific gravity,
means how many times more a body weighs, than an equal bulk of water.
_Mrs. B._ It is rather the weight of a body compared with a portion of
water equal to it in bulk; for the specific gravity of many substances,
is less than that of water.
_Caroline._ Then you cannot ascertain the specific gravity of such
substances, in the same manner as that of gold; for a body that is
lighter than water, will float on its surface, without displacing any of
it.
_Mrs. B._ If a body were absolutely without weight, it is true that it
would not displace a drop of water, but the bodies we are treating of,
have all some weight, however small; and will, therefore, displace some
quantity. If the body be lighter than water, it will not sink to a level
with its surface, and therefore it will not displace so much water as is
equal to its bulk; but only so much, as is equal to its weight. A ship,
you must have observed, sinks to some depth in water, and the heavier it
is laden, the deeper it sinks, as it always displaces a quantity of
water, equal to its own weight.
_Caroline._ But you said just now, that in the immersion of gold, the
bulk, and not the weight of body, was to be considered.
_Mrs. B._ That is the case with all substances which are heavier than
water; but since those which are lighter, do not displace so much as
their own bulk, the quantity they displace is not a test of their
specific gravity.
In order to obtain the specific gravity of a body which is lighter than
water, you must attach to it a heavy one, whose specific gravity is
known, and immerse them together; the specific gravity of the lighter
body, may then be easily calculated from observing the loss of weight it
produces, in the heavy body.
_Emily._ But are there not some bodies which have exactly the same
specific gravity as water?
_Mrs. B._ Undoubtedly; and such bodies will remain at rest in whatever
situation they are placed in water. Here is a piece of wood which I have
procured, because it is of a kind which is precisely the weight of an
equal bulk of water; in whatever part of this vessel of water you place
it, you will find that it will remain stationary.
_Caroline._ I shall first put it at the bottom; from thence, of course,
it cannot rise, because it is not lighter than water. Now I shall place
it in the middle of the vessel; it neither rises nor sinks, because it
is neither lighter nor heavier than the water. Now I will lay it on the
surface of the water; but there it sinks a little--what is the reason of
that, Mrs. B.?
_Mrs. B._ Since it is not lighter than the water, it cannot float upon
its surface; since it is not heavier than water, it cannot sink below
its surface: it will sink therefore, only till the upper surface of both
bodies are on a level, so that the piece of wood is just covered with
water. If you poured a few drops of water into the vessel, (so gently as
not to give them momentum) they would mix with the water at the surface,
and not sink lower.
_Caroline._ I now understand the reason, why, in drawing up a bucket of
water out of a well, the bucket feels so much heavier when it rises
above the surface of the water in the well; for whilst you raise it in
the water, the water within the bucket being of the same specific
gravity as the water on the outside, will be wholly supported by the
upward pressure of the water beneath the bucket, and consequently very
little force will be required to raise it; but as soon as the bucket
rises to the surface of the well, you immediately perceive the increase
of weight.
_Emily._ And how do you ascertain the specific gravity of fluids?
_Mrs. B._ By means of an hydrometer; this instrument is made of various
materials, and in different forms, one of which I will show you. It
consists of a thin brass ball A, (fig. 8, plate 13.) with a graduated
tube B, and the specific gravity of the liquid, is estimated by the
depth to which the instrument sinks in it, or by the weight required to
sink it to a given depth. There is a small bucket C, suspended at the
lower end, and also a little dish on the graduated tube; into either of
these, small weights may be put, until the instrument sinks in the
fluid, to a mark on the tube B; the amount of weight necessary for this,
will enable you to discover the specific gravity of the fluid.
I must now take leave of you; but there remain yet many observations to
be made on fluids: we shall, therefore, resume this subject at our next
interview.
Questions
1. (Pg. 118) What are the two divisions of the science which treats of
the mechanical properties of liquids?
2. (Pg. 118) Of what do hydrostatics and hydraulics treat?
3. (Pg. 118) What is a fluid defined to be?
4. (Pg. 118) From what is fluidity supposed to arise?
5. (Pg. 118) Into what two classes are fluids divided?
6. (Pg. 119) What is said of the incompressibility of liquids, and what
experiment is related?
7. (Pg. 119) Ought this experiment to be considered as conclusive?
8. (Pg. 119) Why do fluids appear to gravitate more freely than solids?
9. (Pg. 120) When is a fluid said to be in equilibrium?
10. (Pg. 120) What is there in the nature of a fluid, which causes it to
seek this level?
11. (Pg. 120) What circumstances occasion oil to float upon water?
12. (Pg. 120) What is the nature and use of the instrument represented
in fig. 1, plate 13?
13. (Pg. 120) What difference is there in the gravitation of solid
masses, and of fluids?
14. (Pg. 121) What results as regards the pressure of fluids?
15. (Pg. 121) How is this illustrated by fig. 2, 3, plate 13?
16. (Pg. 121) From what does the lateral pressure proceed? and to what
is it proportioned, as exemplified in fig. 5, plate 13?
17. (Pg. 122) Has the extent of the surface of a fluid, any effect upon
its pressure downwards?
18. (Pg. 122) What will be the difference between the pressure upon the
bottom, and upon one side of a cubical vessel?
19. (Pg. 122) What occasions the upward pressure, and how is it
explained by fig. 4, plate 13?
20. (Pg. 123) How could the equilibrium of fluids be exemplified by
pouring water in at the spout of a tea-pot?
21. (Pg. 123) How by the apparatus represented at fig. 6, plate 13?
22. (Pg. 123) What is meant by the specific gravity of a body?
23. (Pg. 123) What do we in common mean by calling a body heavy, or
light?
24. (Pg. 124) Why would not the metals answer to compare other bodies
with?
25. (Pg. 124) What must be supposed equal in estimating the specific
gravity of a body?
26. (Pg. 124) What has been adopted as a standard for comparison?
27. (Pg. 125) What is the first step in ascertaining the specific
gravity of a solid?
28. (Pg. 125) What quantity of water will the solid displace?
29. (Pg. 125) Why will a solid weigh less in water than in air, and to
what will the loss of weight be equal?
30. (Pg. 126) What is the arrangement represented by fig. 7, plate 13?
31. (Pg. 126) What is stated of gold as an example?
32. (Pg. 126) In comparing a body with water, this is sometimes called
1000, what must be observed?
33. (Pg. 126) What quantity of water is displaced, by a body floating
upon its surface?
34. (Pg. 127) How can you find the specific gravity of a solid which is
lighter than water?
35. (Pg. 127) What is observed of a body whose specific gravity is the
same as that of water?
36. (Pg. 127) What is the reason that in drawing a bucket of water from
a well, its weight is not perceived until it rises above the surface?
37. (Pg. 128) Describe the instrument represented by fig. 8, plate 13,
and also how, and for what it is used?
CONVERSATION XI.
OF SPRINGS, FOUNTAINS, &c.
OF THE ASCENT OF VAPOUR AND THE FORMATION OF CLOUDS. OF THE FORMATION
AND FALL OF RAIN, &c. OF THE FORMATION OF SPRINGS. OF RIVERS AND LAKES.
OF FOUNTAINS.
CAROLINE.
There is a question I am very desirous of asking you, respecting fluids,
Mrs. B., which has often perplexed me. What is the reason that the great
quantity of rain which falls upon the earth and sinks into it, does not,
in the course of time, injure its solidity? The sun and the wind, I
know, dry the surface, but they have no effect on the interior parts,
where there must be a prodigious accumulation of moisture.
_Mrs. B._ Do you not know, that, in the course of time, all the water
which sinks into the ground, rises out of it again? It is the same
water which successively forms seas, rivers, springs, clouds, rain, and
sometimes hail, snow and ice. If you will take the trouble of following
it through these various changes, you will understand why the earth is
not yet drowned, by the quantity of water which has fallen upon it,
since its creation; and you will even be convinced, that it does not
contain a single drop more water now, than it did at that period.
Let us consider how the clouds were originally formed. When the first
rays of the sun warmed the surface of the earth, the heat, by separating
the particles of water, rendered them lighter than the air. This, you
know, is the case with steam or vapour. What then ensues?
_Caroline._ When lighter than the air, it will naturally rise; and now I
recollect your telling us in a preceding lesson, that the heat of the
sun transformed the particles of water into vapour; in consequence of
which, it ascended into the atmosphere, where it formed clouds.
_Mrs. B._ We have then already followed water through two of its
transformations; from water it becomes vapour, and from vapour clouds.
_Emily._ But since this watery vapour is lighter than the air, why does
it not continue to rise; and why does it unite again, to form clouds?
_Mrs. B._ Because the atmosphere diminishes in density, as it is more
distant from the earth. The vapour, therefore, which the sun causes to
exhale, not only from seas, rivers, and lakes, but likewise from the
moisture on the land, rises till it reaches a region of air of its own
specific gravity; and there, you know, it will remain stationary. By the
frequent accession of fresh vapour, it gradually accumulates, so as to
form those large bodies of vapour, which we call clouds: and the
particles, at length uniting, become too heavy for the air to support,
and fall to the ground.
_Caroline._ They do fall to the ground, certainly, when it rains; but,
according to your theory, I should have imagined, that when the clouds
became too heavy, for the region of air in which they were situated, to
support them, they would descend, till they reached a stratum of air of
their own weight, and not fall to the earth; for as clouds are formed of
vapour, they cannot be so heavy as the lowest regions of the atmosphere,
otherwise the vapour would not have risen.
_Mrs. B._ If you examine the manner in which the clouds descend, it will
obviate this objection. In falling, several of the watery particles
come within the sphere of each other's attraction, and unite in the form
of a drop of water. The vapour thus transformed into a shower, is
heavier than any part of the atmosphere, and consequently descends to
the earth.
_Caroline._ How wonderfully curious!
_Mrs. B._ It is impossible to consider any part of nature attentively,
without being struck with admiration at the wisdom it displays; and I
hope you will never contemplate these wonders, without feeling your
heart glow with admiration and gratitude, towards their bounteous
Author. Observe, that if the waters were never drawn out of the earth,
all vegetation would be destroyed by the excess of moisture; if, on the
other hand, the plants were not nourished and refreshed by occasional
showers, the drought would be equally fatal to them. If the clouds
constantly remained in a state of vapour, they might, as you remarked,
descend into a heavier stratum of the atmosphere, but could never fall
to the ground; or were the power of attraction more than sufficient to
convert the vapour into drops, it would transform the cloud into a mass
of water, which, instead of nourishing, would destroy the produce of the
earth.
Water then ascends in the form of vapour, and descends in that of rain,
snow, or hail, all of which ultimately become water. Some of this falls
into the various bodies of water on the surface of the globe, the
remainder upon the land. Of the latter, part reascends in the form of
vapour, part is absorbed by the roots of vegetables, and part descends
into the earth, where it forms springs.
_Emily._ Is there then no difference between rain water, and spring
water?
_Mrs. B._ They are originally the same; but that portion of rain water
which goes to supply springs, dissolves a number of foreign particles,
which it meets with in its passage through the various soils it
traverses.
_Caroline._ Yet spring water is more pleasant to the taste, appears more
transparent, and, I should have supposed, would have been more pure than
rain water.
_Mrs. B._ No; excepting distilled water, rain water is the most pure we
can obtain; it is its purity which renders it insipid; whilst the
various salts and different ingredients, dissolved in spring water, give
it a species of flavour, which habit renders agreeable; these salts do
not, in any degree, affect its transparency; and the filtration it
undergoes, through gravel and sand, cleanses it from all foreign
matter, which it has not the power of dissolving.
_Emily._ How is it that the rain water does not continue to descend by
its gravity, instead of collecting together, and forming springs?
_Mrs. B._ When rain falls on the surface of the earth, it continues
making its way downwards through the pores and crevices in the ground.
When several drops meet in their subterraneous passage, they unite and
form a little rivulet; this, in its progress, meets with other rivulets
of a similar description, and they pursue their course together within
the earth, till they are stopped by some substance, such as rock, or
clay, which they cannot penetrate.
_Caroline._ But you say that there is some reason to believe that water
can penetrate even the pores of gold, and it cannot meet with a
substance more dense?
_Mrs. B._ But if water penetrate the pores of gold, it is only when
under a strong compressive force, as in the Florentine experiment; now
in its passage towards the centre of the earth, it is acted upon by no
other power than gravity, which is not sufficient to make it force its
way, even through a stratum of clay. This species of earth, though not
remarkably dense, being of great tenacity, will not admit the particles
of water to pass. When water encounters any substance of this nature,
therefore, its progress is stopped, and it is diffused through the
porous earth, and sometimes the pressure of the accumulating waters,
forms a bed, or reservoir. This will be more clearly explained by fig.
9, plate 13, which represents a section, of the interior of a hill or
mountain. A, is a body of water, such as I have described, which, when
filled up as high as B, (by the continual accession of water it receives
from the ducts or rivulets _a_, _a_, _a_, _a_,) finds a passage out of
the cavity, and, impelled by gravity, it runs on, till it makes its way
out of the ground at the side of the hill, and there forms a spring, C.
_Caroline._ Gravity impels downwards towards the centre of the earth;
and the spring in this figure runs in an horizontal direction.
_Mrs. B._ Not entirely. There is some declivity from the reservoir, to
the spot where the water issues out of the ground; and gravity, you
know, will bring bodies down an inclined plane, as well as in a
perpendicular direction.
_Caroline._ But though the spring may descend, on first issuing, it must
afterwards rise to reach the surface of the earth; and that is in direct
opposition to gravity.
_Mrs. B._ A spring can never rise above the level of the reservoir
whence it issues; it must, therefore, find a passage to some part of the
surface of the earth, that is lower, or nearer the centre, than the
reservoir. It is true that, in this figure, the spring rises in its
passage from B to C; but this, I think, with a little reflection, you
will be able to account for.
_Emily._ Oh, yes; it is owing to the pressure of fluids upwards; and the
water rises in the duct, upon the same principle as it rises in the
spout of a tea-pot; that is to say, in order to preserve an equilibrium
with the water in the reservoir. Now I think I understand the nature of
springs: the water will flow through a duct, whether ascending or
descending, provided it never rises higher than the reservoir.
_Mrs. B._ Water may thus be conveyed to every part of a town, and to the
upper part of the houses, if it is originally brought from a height,
superior to any to which it is conveyed. Have you never observed, when
the pavements of the streets have been mending, the pipes which serve as
ducts for the conveyance of the water through the town?
_Emily._ Yes, frequently; and I have remarked that when any of these
pipes have been opened, the water rushes upwards from them, with great
velocity; which, I suppose, proceeds from the pressure of the water in
the reservoir, which forces it out.
_Caroline._ I recollect having once seen a very curious glass, called
Tantalus's cup; it consists of a goblet, containing a small figure of a
man, and whatever quantity of water you pour into the goblet, it never
rises higher than the breast of the figure. Do you know how that is
contrived?
_Mrs. B._ It is by means of a syphon, or bent tube, which is concealed
in the body of the figure. This tube rises through one of the legs, as
high as the breast, and there turning, descends through the other leg,
and from thence through the foot of the goblet, where the water runs
out. (fig. 1, plate 14.) When you pour water into the glass A, it must
rise in the syphon B, in proportion as it rises in the glass; and when
the glass is filled to a level with the upper part of the syphon, the
water will run out through the other leg of the figure, and will
continue running out, as fast as you pour it in; therefore the glass can
never fill any higher.
_Emily._ I think the new well that has been made at our country-house,
must be of that nature. We had a great scarcity of water, and my father
has been at considerable expense to dig a well; after penetrating to a
great depth, before water could be found, a spring was at length
discovered, but the water rose only a few feet above the bottom of the
well; and sometimes it is quite dry.
[Illustration: PLATE XIV.]
_Mrs. B._ This has, however, no analogy to Tantalus's cup; but is owing
to the very elevated situation of your country-house.
_Emily._ I believe I guess the reason. There cannot be a reservoir of
water near the summit of a hill; as in such a situation, there will not
be a sufficient number of rivulets formed, to supply one; and without a
reservoir, there can be no spring. In such situations, therefore, it is
necessary to dig very deep, in order to meet with a spring; and when we
give it vent, it can rise only as high as the reservoir from whence it
flows, which will be but little, as the reservoir must be situated at
some considerable depth below the summit of the hill.
_Caroline._ Your explanation appears very clear and satisfactory; but I
can contradict it from experience. At the very top of a hill, near our
country-house, there is a large pond, and, according to your theory, it
would be impossible there should be springs in such a situation to
supply it with water. Then you know that I have crossed the Alps, and I
can assure you, that there is a fine lake on the summit of Mount Cenis,
the highest mountain we passed over.
_Mrs. B._ Were there a lake on the summit of Mount Blanc, which is the
highest of the Alps, it would indeed be wonderful. But that on Mount
Cenis, is not at all contradictory to our theory of springs; for this
mountain is surrounded by others, much more elevated, and the springs
which feed the lake must descend from reservoirs of water, formed in
those mountains. This must also be the case with the pond on the top of
the hill; there is doubtless some more considerable hill in the
neighbourhood, which supplies it with water.
_Emily._ I comprehend perfectly, why the water in our well never rises
high: but I do not understand why it should occasionally be dry.
_Mrs. B._ Because the reservoir from which it flows, being in an
elevated situation, is but scantily supplied with water; after a long
drought, therefore, it may be drained, and the spring dry, till the
reservoir be replenished by fresh rains. It is not uncommon to see
springs flow with great violence in wet seasons, which at other times,
are perfectly dry.
_Caroline._ But there is a spring in our grounds, which more frequently
flows in dry, than in wet weather; how is that to be accounted for?
_Mrs. B._ The spring, probably, comes from a reservoir at a great
distance, and situated very deep in the ground: it is, therefore, some
length of time before the rain reaches the reservoir; and another
considerable portion must elapse, whilst the water is making its way,
from the reservoir, to the surface of the earth; so that the dry weather
may probably have succeeded the rains, before the spring begins to flow;
and the reservoir may be exhausted, by the time the wet weather sets in
again.
_Caroline._ I doubt not but this is the case, as the spring is in a very
low situation, therefore, the reservoir may be at a great distance from
it.