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 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 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 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.

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 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 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 ?

Mrs B. Just so, my dear. The composition of the two win is north and east, produces a constant northeast wind; and that of the two winds south and east, produces a regular south-east wind: these winds extend to about thirty degrees on each side of the equator, the regions further distant from it experiencing only their respective north and south 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 atmosphere; the air in the lower strata flowing from the poles towards the equator, and in the upper strata, flowing back from the equator towards the poles.

Mrs. B. Exactly. I can show you an example of this circulation on a small scale. The air of this room being more rarefied than the external air, a wind or current of air is pouring in from the crevices of the windows and doors, to restore the equilibrium; but the light air with which the room is filled must find some vent, in order to make way for the heavy air which enters. If you set the door a-jar, and hold a candle near the upper part of it, you will find that the flame will be blown outwards, showing that there is a current of air flowing out from the upper part of the room. Now place the candle on the floor close by the door, and you will perceive, by the inclination of the flame, that there is also a current of air setting into the room.

Caroline. It is just so; the upper current is the warm light air, which is driven out to make way for the stream of cold dense air which enters the room lower down.

Emily. I have heard, Mrs. B., that the periodical winds are not so regular on land as at sea: what is the reason of that ?

Mrs. B. The land reflects into the atmosphere a much greater quantity of the sun's rays than the water ; therefore, that part of the atmosphere which is over the land, is more heated and rarefied than that which is