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 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 exterior 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 elastic fluid, differs in many respects from liquids.

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 fluids as are called 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 proper

ties; 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? 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 prodigiously, and a diminution proportionally condenses 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. 42.) 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 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 the 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.