Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 32

_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 pa.s.sage, 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 pa.s.sage 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 pa.s.s. 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 acc.u.mulating 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 pa.s.sage 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 pa.s.sage 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 pa.s.sage 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 gla.s.s, called Tantalus's cup; it consists of a goblet, containing a small figure of a man, and whatever quant.i.ty 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 gla.s.s A, it must rise in the syphon B, in proportion as it rises in the gla.s.s; and when the gla.s.s 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 gla.s.s 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.

[Ill.u.s.tration: PLATE XIV.]

_Mrs. B._ This has, however, no a.n.a.logy 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 a.s.sure you, that there is a fine lake on the summit of Mount Cenis, the highest mountain we pa.s.sed 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 pa.s.sage 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 acc.u.mulates, 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 pa.s.ses 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 acc.u.mulate, 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 polis.h.i.+ng, 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?