Chapter IX: Introduction: General Properties of Bodies. Impenetrability. Extension (7)

_Mrs. B._ Smooth, if you please, but not flat; for the definition of the equilibrium of a fluid is, that every part of the surface is equally distant from the point to which they gravitate, that is to say, from the centre of the earth; hence the surface of all fluids must be spherical, not flat, since they will partake of the spherical form of the globe. This is very evident in large bodies of water, such as the ocean, but the sphericity of small bodies of water, is so trifling, that their surfaces appear flat.

This level, or equilibrium of fluids, is the natural result of their particles gravitating independently of each other; for when any particle of a fluid, accidentally finds itself elevated above the rest, it is attracted down to the level of the surface of the fluid, and the readiness with which fluids yield to the slightest impression, will enable the particle by its weight, to penetrate the surface of the fluid, and mix with it.

_Caroline._ But I have seen a drop of oil, float on the surface of water, without mixing with it.

_Mrs. B._ They do not mix, because their particles repel each other, and the oil rises to the surface, because oil is a lighter liquid than water. If you were to pour water over it, the oil would still rise, being forced up by the superior gravity of the water. Here is an instrument called a spirit-level, (fig. 1, plate 13.) which is constructed upon the principle of the equilibrium of fluids. It consists of a short tube A B, closed at both ends, and containing a little water, or more commonly some spirits: it is so nearly filled, as to leave only a small bubble of air; when the tube is perfectly horizontal, this bubble will occupy the middle of it, but when not perfectly horizontal, the water runs to the lower, and the bubble of air or spirit rises to the upper end; by this instrument, the level of any situation, to which we apply it, may be ascertained.

From the strong cohesion of their particles, you may therefore consider solid bodies as gravitating in masses, while every particle of a fluid may be considered as separate, and gravitating independently of each other. Hence the resistance of a fluid, is considerably less, than that of a solid body; for the resistance of the particles, acting separately, is more easily overcome.

_Emily._ A body of water, in falling, does certainly less injury than a solid body of the same weight.

_Mrs. B._ The particles of fluids, acting thus independently, press against each other in every direction, not only downwards, but upwards, and laterally or sideways; and in consequence of this equality of pressure, every particle remains at rest, in the fluid. If you agitate the fluid, you disturb this equality of pressure, and the fluid will not rest, till its equilibrium is restored.

_Caroline._ The pressure downwards is very natural; it is the effect of gravity; one particle, weighing upon another, presses on it; but the pressure sideways, and particularly the pressure upwards, I cannot understand.

_Mrs. B._ If there were no lateral pressure, water would not run out of an opening on the side of a vessel. If you fill a vessel with sand, it will not continue to run out of such an opening, because there is scarcely any lateral pressure among its particles.

_Emily._ When water runs out of the side of a vessel, is it not owing to the weight of the water, above the opening?

_Mrs. B._ If the particles of fluids were arranged in regular columns, thus, (fig. 2.) there would be no lateral pressure, for when one particle is perpendicularly above the other, it can only press downwards; but as it must continually happen, that a particle presses between two particles beneath, (fig. 3.) these last, must suffer a lateral pressure.

_Emily._ The same as when a wedge is driven into a piece of wood, and separates the parts, laterally.

_Mrs. B._ Yes. The lateral pressure proceeds, therefore, entirely from the pressure downwards, or the weight of the liquid above; and consequently, the lower the orifice is made in the vessel, the greater will be the velocity of the water rushing out of it. Here is a vessel of water (fig. 5.), with three stop cocks at different heights; we shall open them, and you will see with what different degrees of velocity, the water issues from them. Do you understand this, Caroline?

_Caroline._ Oh yes. The water from the upper spout, receiving but a slight pressure, on account of its vicinity to the surface, flows but gently; the second cock, having a greater weight above it, the water is forced out with greater velocity, whilst the lowest cock, being near the bottom of the vessel, receives the pressure of almost the whole body of water, and rushes out with the greatest impetuosity.

_Mrs. B._ Very well; and you must observe, that as the lateral pressure, is entirely owing to the pressure downwards, it is not affected by the horizontal dimensions of the vessel, which contains the water, but merely by its depth; for as every particle acts independently of the rest, it is only the column of particles immediately above the orifice, that can weigh upon, and press out the water.

_Emily._ The breadth and width of the vessel then, can be of no consequence in this respect. The lateral pressure on one side, in a cubical vessel, is, I suppose, not so great as the pressure downwards upon the bottom.

_Mrs. B._ No; in a cubical vessel, the pressure downwards will be double the lateral pressure on one side; for every particle at the bottom of the vessel is pressed upon, by a column of the whole depth of the fluid, whilst the lateral pressure diminishes from the bottom upwards to the surface, where the particles have no pressure.

_Caroline._ And from whence proceeds the pressure of fluids upwards? that seems to me the most unaccountable, as it is in direct opposition to gravity.

_Mrs. B._ And yet it is in consequence of their pressure downwards. When, for example, you pour water into a tea-pot, the water rises in the spout, to a level with the water in the pot. The particles of water at the bottom of the pot, are pressed upon by the particles above them; to this pressure they will yield, if there is any mode of making way for the superior particles, and as they cannot descend, they will change their direction, and rise in the spout.

Suppose the tea-pot to be filled with columns of particles of water, similar to that described in fig. 4., the particle 1, at the bottom, will be pressed laterally by the particle 2, and by this pressure be forced into the spout, where, meeting with the particle 3, it presses it upwards, and this pressure will be continued from 3 to 4, from 4 to 5, and so on, till the water in the spout, has risen to a level with that in the pot.

_Emily._ If it were not for this pressure upwards, forcing the water to rise in the spout, the equilibrium of the fluid would be destroyed.

_Caroline._ True; but then a tea-pot is wide and large, and the weight of so great a body of water as the pot will contain, may easily force up and support so small a quantity, as will fill the spout. But would the same effect be produced, if the spout and the pot, were of equal dimensions?

_Mrs. B._ Undoubtedly it would. You may even reverse the experiment, by pouring water into the spout, and you will find that the water will rise in the pot, to a level with that in the spout; for the pressure of the small quantity of water in the spout, will force up and support, the larger quantity in the pot. In the pressure upwards, as well as that laterally, you see that the force of pressure, depends entirely on the height, and is quite independent of the horizontal dimensions of the fluid.

As a tea-pot is not transparent, let us try the experiment by filling this large glass goblet, by means of this narrow tube, (fig. 6.)

_Caroline._ Look, Emily, as Mrs. B. fills it, how the water rises in the goblet, to maintain an equilibrium with that in the tube.

Now, Mrs. B., will you let me fill the tube, by pouring water into the goblet?

_Mrs. B._ That is impossible. However, you may try the experiment, and I doubt not that you will be able to account for its failure.

_Caroline._ It is very singular, that if so small a column of water as is contained in the tube, can force up and support the whole contents of the goblet; that the weight of all the water in the goblet, should not be able to force up the small quantity required to fill the tube:--oh, I see now the reason, the water in the goblet, cannot force that in the tube above its level, and as the end of the tube, is considerably higher than the goblet, it can never be filled by pouring water into the goblet.

_Mrs. B._ And if you continue to pour water into the goblet when it is full, the water will run over, instead of rising above its level in the tube.

I shall now explain to you the meaning of the _specific gravity_ of bodies.

_Caroline._ What! is there another species of gravity, with which we are not yet acquainted?

_Mrs. B._ No: the specific gravity of a body, means simply its weight, compared with that of another body, of the same size. When we say, that substances, such as lead, and stones, are heavy, and that others, such as paper and feathers, are light, we speak comparatively; that is to say, that the first are heavy, and the latter light, in comparison with the generality of substances in nature. Would you call wood, and chalk, light or heavy bodies?

_Caroline._ Some kinds of wood are heavy, certainly, as oak and mahogany; others are light, as cedar and poplar.

_Emily._ I think I should call wood in general, a heavy body; for cedar and poplar, are light, only in comparison to wood of a heavier description. I am at a loss to determine whether chalk should be ranked as a heavy, or a light body; I should be inclined to say the former, if it was not that it is lighter than most other minerals. I perceive that we have but vague notions of light and heavy. I wish there was some standard of comparison, to which we could refer the weight of all other bodies.

_Mrs. B._ The necessity of such a standard, has been so much felt, that a body has been fixed upon for this purpose. What substance do you think would be best calculated to answer this end?

_Caroline._ It must be one generally known, and easily obtained; lead or iron, for instance.

_Mrs. B._ The metals, would not answer the purpose well, for several reasons; they are not always equally compact, and they are rarely quite pure; two pieces of iron, for instance, although of the same size, might not, from the causes mentioned, weigh exactly alike.

_Caroline._ But, Mrs. B., if you compare the weight, of equal quantities of different bodies, they will all be alike. You know the old saying, that a pound of feathers, is as heavy as a pound of lead?

_Mrs. B._ When therefore we compare the weight of different kinds of bodies, it would be absurd to take quantities of equal _weight_, we must take quantities of equal _bulk_; pints or quarts, not ounces or pounds.

_Caroline._ Very true; I perplexed myself by thinking that quantity referred to weight, rather than to measure. It is true, it would be as absurd to compare bodies of the same size, in order to ascertain which was largest, as to compare bodies of the same weight, in order to discover which was heaviest.

_Mrs. B._ In estimating the specific gravity of bodies, therefore, we must compare equal bulks, and we shall find that their specific gravity, will be proportional to their weights. The body which has been adopted as a standard of reference, is distilled, or rain water.

_Emily._ I am surprised that a fluid should have been chosen for this purpose, as it must necessarily be contained in some vessel, and the weight of the vessel, will require to be deducted.

_Mrs. B._ You will find that the comparison will be more easily made with a fluid, than with a solid; and water you know can be every where obtained. In order to learn the specific gravity of a solid body, it is not necessary to put a certain measure of it in one scale, and an equal measure of water into the other scale: but simply to weigh the body under trial, first in air, and then in water. If you weigh a piece of gold, in a glass of water, will not the gold displace just as much water, as is equal to its own bulk?

_Caroline._ Certainly, where one body is, another cannot be at the same time; so that a sufficient quantity of water must be removed, in order to make way for the gold.

_Mrs. B._ Yes, a cubic inch of water, to make room for a cubic inch of gold; remember that the bulk, alone, is to be considered; the weight, has nothing to do with the quantity of water displaced, for an inch of gold, does not occupy more space, and therefore will not displace more water, than an inch of ivory, or any other substance, that will sink in water.

Well, you will perhaps be surprised to hear that the gold will weigh less in water, than it did out of it?

_Emily._ And for what reason?

_Mrs. B._ On account of the upward pressure of the particles of water, 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.

_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.

_Mrs. B._ Springs which do not constantly flow, are called intermitting, 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.