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 cla.s.s of fluids, distinguished by the name of aeriform, or elastic fluids, the princ.i.p.al 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 quant.i.ty: 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 dest.i.tute of weight, how could it support other bodies, or r.e.t.a.r.d their fall?

I shall now show you some other experiments, which ill.u.s.trate, in a striking manner, both the weight, and elasticity of air. I shall tie a piece of bladder over this gla.s.s 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.