Its various parts are delineated in this figure: (fig. 4. plate XIV.) A B is the pipe or body of the pump, P the piston, V a valve, or little door in the piston, which opening upwards, admits the water to rise through it, but prevents its returning, and Y a similar valve in the body of the pump.

When the pump is in a state of inaction, the two valves are closed by their own weight; but when, by drawing down the handle of the pump, the piston ascends, it raises a column of air which rested upon it, and produces a vacuum between the piston and the lower valve Y, the air beneath this valve, which is immediately over the surface of the water, consequently expands, and forces its way through it; the water, then, relieved from the pressure of the air, ascends into the pump. A few strokes of the handle totally excludes the air from the body of the pump, and fills it with water, which, having passed through both the valves, runs out at the spout.

Caroline. I understand this perfectly. When the piston is elevated, the air and the water successively rise in the pump; for the same reason as the mercury rises in the barometer.

Emily. I thought that water was drawn up into a pump, by suction, in the same manner as water may be sucked through a straw.

Mrs. B. It is so, into the body of the pump; for the power of suction is no other than that of producing a vacuum over one part of the liquid, into which vacuum the liquid is forced, by the pressure of the atmosphere on another part. The action of sucking through a straw, consists in drawing in and confining the breath, so as to produce a vacuum in the mouth; in consequence of which, the air within the straw rushes into the mouth, and is followed by the liquid, into which the lower end of the straw is immersed. The principle, you see, is the same; and the only difference consists in the mode of producing a vacuum. In suction, the muscular powers answer the purpose of the piston and valve.

Emily. Water cannot, then, be raised by a pump above 32 feet; for the pressure of the atmosphere will not sustain a column of water above that height.

Mrs. B. I beg your pardon. It is true that there must never be so great a distance as 32 feet from the level of the water in the well, to the valve in the piston, otherwise the water would not rise through that valve; but when once the water has passed that opening, it is no longer the pressure of air on the reservoir which makes it ascend; it is raised by lifting it up, as you would raise it in a bucket, of which the piston formed the bottom. This common pump is, therefore, called the sucking, or lifting-pump, as it is constructed on both these principles. There is another sort of pump, called the forcing-pump: it consists of a forcing power added to the sucking part of the pump. This additional power is exactly on the principle of the syringe: by raising the piston you draw the water into the pump, and by descending it you force the water out.

Caroline. But the water must be forced out at the upper part of the pump; and I cannot conceive how that can be done by descending the piston.

Mrs. B. Figure 5. pl. XIV. will explain the difficulty. The large pipe A B represents the sucking part of the pump, which differs from the lifting-pump, only in its piston P being unfurnished with a valve, in consequence of which the water cannot rise above it. When, therefore, the piston descends, it shuts the valve Y, and forces the water (which has no other vent) into the pipe D: this is likewise furnished with a valve V, which, opening outwards, admits the water, but prevents its

return.

The water is thus first raised in the pump, and then forced into the pipe, by the alternate ascending and descending motion of the piston, after a few strokes of the handle to fill the pipe, from whence the water issues at the spout.

It is now time to conclude our lesson. When next we meet, I shall give you some account of wind, and of

sound, which will terminate our observations on elastic fluids.

Caroline. And I shall run into the garden, to have the pleasure of pumping, now that I understand the construction of a pump.

Mrs. B. And, to-morrow, I hope you will be able to tell me, whether it is a forcing or a common lifting pump.

CONVERSATION XIII.

ON WIND AND SOUND.

OF WIND IN GENERAL.-OF THE TRADE WIND. OF THE PERIODICAL TRADE WINDS. OF THE AERIAL TIDES.-OF SOUNDS IN GENERAL. OF SONOBOUS BODIES.-OF MUSICAL SOUNDS.-OF CONCORD OR HARMONY, AND HARMONY.

MRS. B.

WELL, Caroline, have you ascertained what kind of pump you have in your garden?

Caroline. I think it must be merely a lifting-pump, because no more force is required to raise the handle than is necessary to lift its weight; and in a forcingpump, by raising the handle, you force the water into the smaller pipe, and the resistance the water offers must require an exertion of strength to overcome it.

Mrs. B. I make no doubt you are right; for lifting pumps, being simple in their construction, are by far

the most common.

the nature of wind.

I have promised to day to give you some account of Wind is nothing more than the motion of a stream or current of air, generally produced by a partial change of temperature in the atmosphere; for when any one part is more heated than the rest, that part is rarefied; the equilibrium is destroyed, and the air in consequence rises. When this happens, there necessarily follows a motion of the surrounding air towards that part, in order to restore it; this spot there

fore, receives winds from every quarter. Those who live to the north of it experience a north wind; those to the south, a south wind:-do you comprehend this?

Caroline. Perfectly. But what sort of weather must those people have, who live on the spot where these winds meet and interfere?

Mrs. B. They have turbulent and boisterous weather, whirlwinds, hurricanes, rain, lightning, thunder, &c. This stormy weather occurs most frequently in the torrid zone, where the heat is greatest: the air being more rarefied there than in any other part of the globe, is lighter, and consequently ascends; whilst the air about the polar regions is continually flowing from the poles, to restore the equilibrium.

Caroline. This motion of the air would produce a regular and constant north wind to the inhabitants of the northern hemisphere; and a south wind to those of the southern hemisphere, and continual storms at the equator, where these two adverse winds would meet.

Mrs. B. These winds do not meet, for they each change their direction before they reach the equator. The sun, in moving over the equatorial regions from east to west, rarefies the air as it passes, and causes the denser eastern air to flow westwards, in order to restore the equilibrium; thus producing a regular east wind about the equator.

Caroline. The air from the west, then, constantly goes to meet the sun, and repair the disturbance which his beams have produced in the equilibrium of the atmosphere. But I wonder how you will reconcile these various winds, Mrs. B.: you first led me to suppose there was a constant struggle between opposite winds at the equator, producing storm and tempest; but now I hear of one regular invariable wind, which must naturally be attended by calm weather.

Emily. I think I comprehend it: do not these winds from the north and south combine with the easterly wind about the equator, and form what are called the trade-winds?