of heat, and the condensation produced by cold; the air has no access to it. An explanation of it would, therefore, be irrelevant to our present subject.

Emily. I have been reflecting, that since it is the weight of the atmosphere which supports the mercury in the tube of a barometer, it would support a column of any other fluid in the same manner.

Mrs. B. Certainly; but as mercury is heavier than all other fluids, it will support a higher column of any other fluid; for two fluids are in equilibrium, when their height varies inversely as their densities. We find the weight of the atmosphere is equal to sustaining a column of water, for instance, of no less than 32 feet above its level.

Caroline. The weight of the atmosphere is, then, as great as that of a body of water the depth of 32 feet?

Mrs. B. Precisely; for a column of air of the height of the atmosphere, is equal to a column of water of 32 feet, or one of mercury of 28 inches.

The common pump is constructed on this principle. By the act of pumping, the pressure of the atmosphere is taken off the water, which, in consequence, rises.

The body of a pump consists of a large tube or pipe, whose lower end is immersed in the water which it is designed to raise. A kind of stopper, called a piston, is fitted to this tube, and is made to slide up and down it by means of a metallick rod fastened to the centre of the piston.

Emily. Is it not similar to the syringe, or squirt, with which you first draw in, and then force out water? Mrs. B. It is; but you know that we do not wish to force the water out of the pump at the same end of the pipe at which we draw it in. The intention of a pump is to raise water from a spring or well; the pipe is therefore placed perpendicularly over the water which enters it at the lower extremity, and it issues at a horizontal spout towards the upper part of the pump. The pump

631. Will the weight of the atmosphere support other fluids than mercury?682. What fluid is heaviest ?- -683. When are two fluids of different density in equilibrium?684. How high a column of water will the weight of the atmosphere sustain ?

-685. What instrument in common use is constructed on this principle?686. What causes the water to rise in a pump ?~ 687. How is a common pump constructed?

therefore, is rather a more complicated piece of machinery than the syringe.

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

688. How would you explain the pump, by reference to fig. 4, plate XIV. ?689. Is the power of suction, and that which causes water to rise in a pump, the same?- -690. What is the power of suction ?-691. In what consists the action of sucking Fiquid through a straw or any small tube?

a vacuum. In suction, the muscular powers answer the purpose of the piston and valves.

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, plate 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

692. What in suction answer the purpose of the piston and valves of the pump ? -693. Can water be raised in a pump more than 32 feet 694. How can it, if the weight of the atmosphere is only equal to a column of water of that height? 695. Of what does the forcing pump consist ?- -696. Which figure represents the forcing pump? -697. How would you explain the forcing pump by the figure?

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 elastick 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 Sonorous Bodies; Of Musica Sounds; Of Concord or Harmony, and Melody.

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 forcing pump, 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.

I have promised to-day to give you some account of the nature of wind. 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 neces

698. What is wind?- -699. How is the air put in motion so as to produce wind?

sarily follows a motion of the surrounding air towards that part, in order to restore it; this spot, therefore, 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

*Fill a large dish with cold water; into the middle of this put a waiter, filled with warm water. The first will represent the ocean and the other an island, rarefying the air above it. Blow out a wax eandle, and if the air be still, on applying it successively to every side of the dish, the smoke will be seen to move towards the plate. -Again, if the ambient water be warmed and the plate be filled with cold water, let the wick of smoking candles be held over the plate, and the contrary will happen.

700. What illustration of wind produced by change of temperature is given in the note ?- 701. What is the consequence when winds from different quarters meet or interfere?-702. Where does this mostly happen?-703. Why does this mostly happen in the torrid zone? -704. What regular wind prevails about the equator?-705. Why is there a regular east wind at and near the equator?