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 metallic 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; to effect this, there are, besides the piston, two contrivances called valves. The pump, therefore, is rather a more complicated piece of machinery, than the syringe.

Caroline. Pray, Mrs. B., is not the leather, which covers the opening, in the lower board of a pair of bellows, a kind of valve? Mrs. B. It is, valves are made in various forms; any contrivance, which allows a fluid to pass in one direction, and prevents its return, is called a valve; that of the bellows, and of the common pump, resemble each other, exactly. You can now, I think, understand the structure of the pump.

Its various parts, are delineated in this figure: (fig. 4. plate 14.) 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, is a similar valve, placed lower down in the body of the pump; H is the handle, which in this model, serves to work the piston.7

When the pump is in a state of inaction, the two valves are closed by their own weight; but when, by working 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 26. How does the common pump, raise water from a well? 27. What is meant by a piston? 28. Describe the construction, and use, of a valve. 29. What are the parts of the pump, as represented, fig. 4, plate 14?

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. The rod to which the piston is attached, must be made sufficiently long, to allow the piston to be within 32 feet of the surface of the water in the well, however deep it may be. 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 causing it to descend, 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 the descent of the piston.

Mrs. B. Figure 5, plate 14, 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 unfurn

30. How do these parts act, in raising the water? 31. In what does that which is commonly called suction, consist? 32. How must the piston be situated in the pump? 33. What other kind of pump is described?

ished 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 upwards, admits the water to pass, 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.

Emily. Does not the air pump, which you used in the experiments, on pneumatics, operate upon the same principles as the sucking pump?

Mrs. B. Exactly. The air pump which I used (plate 1, fig. 2,) has two hollow, brass cylinders, called barrels, which are made perfectly true. In each of those barrels, there is a piston; these are worked up, and down, by the same handle; the pistons, are furnished with valves, opening upwards, like those of the common pump: there are valves also, placed at the lower part of each barrel, which open upwards; there are therefore two pumps, united to produce the same effect: two tubes, connect these barrels with the plate, upon which I placed the receivers, which were to be exhausted.

Emily. I now understand how the air pump acts; the receiver contains air, which is exhausted, just as it is by the common pump, before the water begins to rise.

Mrs. B. Having explained the mechanical properties of air, I think 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.

34. How is the forcing pump constructed, as shown in plate 14, fig. 5? 35, Describe the construction and operation of the air pump, (fig. 2, plate 1.)

N

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 MUSICAL 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 as in a forcing pump, by raising the handle, you force the water into the smaller pipe, 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 air in consequence rises, and the equilibrium is destroyed. When this happens, there necessarily 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 most commonly 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

1. What is wind, and how is it generally produced? 2. How do the winds blow, around the place where the air becomes rarefied? 3. What effect is likely to be produced where the winds meet?

there, than in any other part of the globe, is lighter, and consequently, ascends; whilst the air from the north and south, is continually flowing in, 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?

Mrs. B. Just so, my dear. The composition of the two winds, north and east, produces a constant north-east wind; and that of the two winds, south and east, produces a regular southeast wind; these winds extend to about thirty degrees on each side of the equator, the regions further distant from it, experiencing only their respective northerly and southerly winds.

Caroline. But, Mrs. B., if the air is constantly flowing from the poles, to the torrid zone, there must be a deficiency of air, in the polar regions?

Mrs. B. The light air about the equator, which expands, and rises into the upper regions of the atmosphere, ultimately flows from thence, back to the poles, to restore the equilibrium: if it were not for this resource, the polar, atmospheric regions, would soon be exhausted by the stream of air, which, in the lower strata of the atmosphere, they are constantly sending towards the equator.

Caroline.

There is then a sort of circulation of air in the at

4. In what part of the globe is the air most rarefied, and what is the con sequence? 5. How do these winds change their direction as they approach the equator? 6. How are the trade-winds produced, and how far do they extend? 7. How is the equilibrium in the air restored?