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 run out faster than it run in, it will of course sometimes be empty. And 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 bosom 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 I should now rather call it a 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 it origin to the Rhone, which passes through it if, when this 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

644. Why do rivers usually have their source in mountainous regions? -645. When a spring once issues from the surface of the earth what is its course?- -646. What is the consequence if a spring runs into a situation which is surrounded by higher ground? -647. How was lake Geneva probably formed? 648. Are artificial fountains of the nature of springs

which it flows; and it will rise nearly as high as the reservoir B, from whence it proceeds. (Plate XIV. figure 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 particles of water spout from the tube with an equal velocity, and as the pressure of the air upon the exteriour particles must diminish their velocity, they will, in some degree, strike against the under parts, and force 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 elastick fluid, differs in many respects from liquids.

649. Which figure represents an artificial fountain ?———650. Why in that representation does not the water rise as high as 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; Specifick Gravity of Air; Of Pumps; Description of the Sucking Pump; Description of the Forcing Pump.

MRS. B.

Ar our last meeting we examined the properties of fluids in general, and more particularly of such fluids as are called liquids.

There is another class of fluids, distinguished by the name of aeriform or elastick 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 elastick fluids in general.

Caroline.

And from whence arises this difference? Mrs. B. There is no attraction of cohesion between, the particles of elastick fluids; so that the expansive pow er of heat has no adversary to contend with but gravity any increase of temperature, therefore, expands elastick fluids prodigiously, and a diminution proportionally con

denses them.

The most essential point in which air differs from other fluids, is by its spring or elasticity; that is to say, its power of increasing or diminishing in bulk, according 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; (see p. 32.) 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 to about the distance

651. How are the fluids called air distinguished from liquids? -652. How do the other kinds of air differ from atmospherick air?653. Has the attraction of cohesion any influence upon the particles of elastick fluids ?-654. What effect does heat have on them? 655. What is to be understood by the elasticity of the atmosphere? 656. To what distance from the earth does the atmosphere extend?

of 45 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 external air, and renders us less sensible 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 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

*

The height to which the atmosphere extends has never been accurately ascertained; but at a greater distance than 45 miles it ceases to reflect the sun's rays.

657. What weight of air is a common sized man supposed to sustain ? -658. Why does not such a weight crush him to atoms? -659. What would be the consequence, if the weight of external air were removed from us?

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 both 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 p'iable 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; 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 re-action 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,

660. Why do not bodies of various weights in the atmosphere fall in the same time?661. What does the fact prove, that light bodies are retarded by the air in falling to the earth?-662. How may it be shown that the air has weight?

14 *