The Botanic Garden Volume I Part 30

2. Sometimes these south-east winds consist of a vertical eddy of north- east air, without any mixture of south-west air; in that case the barometer continues above 30, and the weather is dry or frosty for four or five days together.

It should here be observed, that air being an elastic fluid must be more liable to eddies than water, and that these eddies must extend into cylinders or vortexes of greater diameter, and that if a vertical eddy of north-east air be of small diameter or has pa.s.sed but a little way to the south of us before its return, it will not have gained the velocity of the earth's surface to the south of us, and will in consequence become a south-east wind.--But if the vertical eddy be of large diameter, or has pa.s.sed much to the south of us, it will have acquired velocity from its friction with the earth's surface to the south of us, and will in consequence on its return become a south-west wind, producing great cold.

NORTH-WEST WINDS.

There seem to be three sources of the north-west winds of this hemisphere of the earth. 1. When a portion of southern air, which was pa.s.sing over us, is driven back by acc.u.mulation of new air in the polar regions. In this case I suppose they are generally moist or rainy winds, with the barometer under 30, and if the wind had previously been in the south-west, it would seem to prognosticate a change to the north-east.

2. If a current of north wind is pa.s.sing over us but a few miles high, without any easterly direction; and is bent down upon us, it must immediately possess a westerly direction, because it will now move faster than the surface of the earth where it arrives; and thus becomes changed from a north-east to a north-west wind. This descent of a north- east current of air producing a north-west wind may continue some days with clear or freezing weather, as it may be simply owing to a vertical eddy of north-east air, as will be spoken of below. It may otherwise be forced down by a current of south-west wind pa.s.sing over it, and in this case it will be attended with rain for a few days by the mixture of the two airs of different degrees of heat; and will prognosticate a change of wind from north-east to south-west if the wind was previously in the north-east quarter.

3. On the eastern coast of North America the north-west winds bring frost, as the north-east winds do in this country, as appears from variety of testimony. This seems to happen from a vertical spiral eddy made in the atmosphere between the sh.o.r.e and the ridge of mountains which form the spine or back-bone of that continent. If a current of water runs along the hypothenuse of a triangle an eddy will be made in the included angle, which will turn round like a water-wheel as the stream pa.s.ses in contact with one edge of it. The same must happen when a sheet of air flowing along from the north-east rises from the sh.o.r.e in a straight line to the summit of the Apalachian mountains, a part of the stream of north-east air will flow over the mountains, another part will revert and circulate spirally between the summit of the country and the eastern sh.o.r.e, continuing to move toward the south; and thus be changed from a north-east to a north-west wind.

This vertical spiral eddy having been in contact with the cold summits of these mountains, and descending from higher parts of the atmosphere will lose part of its heat, and thus const.i.tute one cause of the greater coldness of the eastern sides of North America than of the European sh.o.r.es opposite to them, which is said to be equal to twelve degrees of north lat.i.tude, which is a wonderful fact, not otherwise easy to be explained, since the heat of the springs at Philadelphia is said to be 50, which is greater than the medium heat of the earth in this country.

The existence of vertical eddies, or great cylinders of air rolling on the surface of the earth, is agreeable to the observations of the constructors of windmills; who on this idea place the area of the sails leaning backwards, inclined to the horizon; and believe that then they have greater power than when they are placed quite perpendicularly. The same kind of rolling cylinders of water obtain in rivers owing to the friction of the water against the earth at their bottoms; as is known by bodies having been observed to float upon their surfaces quicker than when immersed to a certain depth. These vertical eddies of air probably exist all over the earth's surface, but particularly at the bottom or sides of mountains; and more so probably in the course of the south-west than of the north-east winds; because the former fall from an eminence, as it were, on a part of the earth where there is a deficiency of the quant.i.ty of air; as is shewn by the sinking of the barometer: whereas the latter are pushed or squeezed forward by an addition to the atmosphere behind them, as appears by the rising of the barometer.

TRADE-WINDS.

A column of heated air becomes lighter than before, and will therefore ascend, by the pressure of the cold air which surrounds it, like a cork in water, or like heated smoke in a chimney.

Now as the sun pa.s.ses twice over the equator for once over either tropic, the equator has not time to become cool; and on this account it is in general hotter at the line than at the tropics; and therefore the air over the line, except in some few instances hereafter to be mentioned, continues to ascend at all seasons of the year, pressed upwards by regions of air brought from the tropics.

This air thus brought from the tropics to the equator, would const.i.tute a north wind on one side of the equator, and a south wind on the other; but as the surface of the earth at the equator moves quicker than the surface of the earth at the tropics, it is evident that a region of air brought from either tropic to the equator, and which had previously only acquired the velocity of the earth's surface at the tropics, will now move too slow for the earth's surface at the equator, and will thence appear to move in a direction contrary to the motion of the earth. Hence the trade-winds, though they consist of regions of air brought from the north on one side of the line, and from the south on the other, will appear to have the diagonal direction of north-east and south-west winds.

Now it is commonly believed that there are superior currents of air pa.s.sing over these north-east and south-west currents in a contrary direction, and which descending near the tropics produce vertical whirlpools of air. An important question here again presents itself, _What becomes of the moisture which this heated air ought to deposit, as it cools in the upper regions of the atmosphere in its journey to the tropics?_ It has been shewn by Dr. Priestley and Mr. Ingenhouz that the green matter at the bottom of cisterns, and the fresh leaves of plants immersed in water, give out considerable quant.i.ties of vital air in the sun-shine; that is, the perspirable matter of plants (which is water much divided in its egress from their minute pores) becomes decomposed by the sun's light, and converted into two kinds of air, the vital and inflammable airs. The moisture contained or dissolved in the ascending heated air at the line must exist in great tenuity; and by being exposed to the great light of the sun in that climate, the water may be decomposed, and the new airs spread on the atmosphere from the line to the poles.

1. From there being no constant deposition of rains in the usual course of the trade-winds, it would appear that the water rising at the line is decomposed in its ascent.

2. From the observations of M. Bougner on the mountain Pinchinca, one of the Cordelieres immediately under the line, there appears to be no condensible vapour above three or four miles high. Now though the atmosphere at that height may be cold to a very considerable degree; yet its total deprivation of condensible vapour would seem to shew, that its water was decomposed; as there are no experiments to evince that any degree of cold hitherto known has been able to deprive air of its moisture; and great abundance of snow is deposited from the air that flows to the polar regions, though it is exposed to no greater degrees of cold in its journey thither than probably exists at four miles height in the atmosphere at the line.

3. The hygrometer of Mr. Sauffure also pointed to dryness as he ascended into rarer air; the single hair of which it was constructed, contracting from deficiency of moisture. Essais sur l'Hygromet. p. 143.

From these observations it appears either that rare and cold air requires more moisture to saturate it than dense air; or that the moisture becomes decomposed and converted into air, as it ascends into these cold and rare regions of the atmosphere.

4. There seems some a.n.a.logy between the circ.u.mstance of air being produced or generated in the cold parts of the atmosphere both at the line and at the poles.

MONSOONS AND TORNADOES.

1. In the Arabian and Indian seas are winds, which blow six months one way, and six months the other, and are called Monsoons; by the accidental dispositions of land and sea it happens, that in some places the air near the tropic is supposed to become warmer when the sun is vertical over it, than at the line. The air in these places consequently ascends pressed upon one side by the north-east regions of air, and on the other side by the south-west regions of air. For as the air brought from the south has previously obtained the velocity of the earth's surface at the line, it moves faster than the earth's surface near the tropic where it now arrives, and becomes a south-west wind, while the air from the north becomes a north-east wind as before explained. These two winds do not so quietly join and ascend as the north-east and south-east winds, which meet at the line with equal warmth and velocity and form the trade-winds; but as they meet in contrary directions before they ascend, and cannot be supposed accurately to balance each other, a rotatory motion will be produced as they ascend like water falling through a hole, and an horizontal or spiral eddy is the consequence; these eddies are more or less rapid, and are called Tornadoes in their most violent state, raising water from the ocean in the west or sand from the deserts of the east, in less violent degrees they only mix together the two currents of north-east and south- west air, and produce by this means incessant rains, as the air of the north-east acquires some of the heat from the south-west wind, as explained in Note XXV. This circ.u.mstance of the eddies produced by the monsoon-winds was seen by Mr. Bruce in Abyssinia; he relates that for many successive mornings at the commencement of the rainy monsoon, he observed a cloud of apparently small dimensions whirling round with great rapidity, and in few minutes the heavens became covered with dark clouds with consequent great rains. See Note on Canto III. l. 129.

2. But it is not only at the place where the air ascends at the northern extremity of the rainy monsoon, and where it forms tornadoes, as observed above by Mr. Bruce, but over a great tract of country several degrees in length in certain parts as in the Arabian sea, a perpetual rain for several months descends, similar to what happens for weeks together in our own climate in a less degree during the south-west winds. Another important question presents itself here, _if the climate to which this south-west wind arrives, it not colder than that it comes from, why should it deposit its moisture during its whole journey? if it be a colder climate, why does it come thither?_ The tornadoes of air above described can extend but a little way, and it is not easy to conceive that a superior cold current of air can mix with an inferior one, and thus produce showers over ten degrees of country, since at about three miles high there is perpetual frost; and what can induce these narrow and shallow currents to flow over each other so many hundred miles?

Though the earth at the northren extremity of this monsoon may be more heated by certain circ.u.mstances of situation than at the line, yet it seems probable that the intermediate country between that and the line, may continue colder than the line (as in other parts of the earth) and hence that the air coming from the line to supply this ascent or destruction of air at the northern extremity of the monsoon will be cooled all the way in its approach, and in consequence deposit its water. It seems probable that at the northern extremity of this monsoon, where the tornadoes or hurricanes exist, that the air not only ascends but is in part converted into water, or otherwise diminished in quant.i.ty, as no account is given of the existence of any superior currents of it.

As the south-west winds are always attended with a light atmosphere, an incipient vacancy, or a great diminution of air must have taken place to the northward of them in all parts of the earth wherever they exist, and a deposition of their moisture succeeds their being cooled by the climate they arrive at, and not by a contrary current of cold air over them, since in that case the barometer would not sink. They may thus in our own country be termed monsoons without very regular periods.

3. Another cause of TORNADOES independent of the monsoons is ingeniously explained by Dr. Franklin, when in the tropical countries a stratum of inferior air becomes so heated by its contact with the warm earth, that its expansion is increased more than is equivalent to the pressure of the stratum of air over it; or when the superior stratum becomes more condensed by cold than the inferior one by pressure, the upper region will descend and the lower one ascend. In this situation if one part of the atmosphere be hotter from some fortuitous circ.u.mstances, or, has less pressure over it, the lower stratum will begin to ascend at this part, and resemble water falling through a hole as mentioned above. If the lower region of air was going forwards with considerable velocity, it will gain an eddy by riling up this hole in the inc.u.mbent heavy air, so that the whirlpool or tornado has not only its progressive velocity, but its circular one also, which thus lifts up or overturns every thing within its spiral whirl. By the weaker whirlwinds in this country the trees are sometimes thrown down in a line of only twenty or forty yards in breadth, making a kind of avenue through a country. In the West Indies the sea rises like a cone in the whirl, and is met by black clouds produced by the cold upper air and the warm lower air being rapidly mixed; whence are produced the great and sudden rains called water-spouts; while the upper and lower airs exchange their plus or minus electricity in perpetual lightenings.

LAND AND SEA-BREEZES.

The sea being a transparent ma.s.s is less heated at its surface by the sun's rays than the land, and its continual change of surface contributes to preserve a greater uniformity in the heat of the air which hangs over it. Hence the surface of the tropical islands is more heated during the day than the sea that surrounds them, and cools more in the night by its greater elevation: whence in the afternoon when the lands of the tropical islands have been much heated by the sun, the air over them ascends pressed upwards by the cooler air of the incircling ocean, in the morning again the land becoming cooled more than the sea, the air over it descends by its increased gravity, and blows over the ocean near its sh.o.r.es.

CONCLUSION.

1. There are various irregular winds besides those above described, which consist of horizontal or vertical eddies of air owing to the inequality of the earth's surface, or the juxtaposition of the sea.

Other irregular winds have their origin from increased evaporation of water, or its sudden devaporation and descent in showers; others from the partial expansion and condensation of air by heat and cold; by the acc.u.mulation or defect of electric fluid, or to the air's new production or absorption occasioned by local causes not yet discovered. See Notes VII. and XXV.

2. There seem to exist only two original winds: one consisting of air brought from the north, and the other of air brought from the south. The former of these winds has also generally an apparent direction from the east, and the latter from the west, arising from the different velocities of the earth's surface. All the other winds above described are deflections or retrogressions of some parts of these currents of air from the north or south.

3. One fifteenth part of the atmosphere is occasionally destroyed, and occasionally reproduced by unknown causes. These causes are brought into immediate activity over a great part of the surface of the earth at nearly the same time, but always act more powerful to the northward than to the southward of any given place; and would hence seem to have their princ.i.p.al effect in the polar circles, existing nevertheless though with less power toward the tropics or at the line.

For when the north-east wind blows the barometer rises, sometimes from 281/2 inches to 301/2, which shews a great new generation of air in the north; and when the south-west wind blows the barometer sinks as much, which shews a great destruction of air in the north. But as the north- east winds sometimes continue for five or six weeks, the newly-generated air must be destroyed at those times in the warmer climates to the south of us, or circulate in superior currents, which has been shewn to be improbable from its not depositing its water. And as the south-west winds sometimes continue for some weeks, there must be a generation of air to the south at those times, or superior currents, which last has been shewn to be improbable.

4. The north-east winds being generated about the poles are pushed forwards towards the tropics or line, by the pressure from behind, and hence they become warmer, as explained in Note VII. as well as by their coming into contact with a warmer part of the earth which contributes to make these winds greedily absorb moisture in their pa.s.sage. On the contrary, the south-west winds, as the atmosphere is suddenly diminished in the polar regions, are drawn as it were into an incipient vacancy, and become therefore expanded in their pa.s.sage, and thus generate cold, as explained in Note VII. and are thus induced to part with their moisture, as well as by their contact with a colder part of the earth's surface. Add to this, that the difference in the sound of the north-east and south-west winds may depend on the former being pushed forwards by a pressure behind, and the latter falling as it were into a partial or incipient vacancy before; whence the former becomes more condensed, and the latter more rarefied as it pa.s.ses. There is a whistle, termed a lark-call, which consists of a hollow cylinder of tin-plate, closed at each end, about half an inch in diameter and a quarter of an inch high, with opposite holes about the size of a goose-quill through the centre of each end; if this lark-whistle be held between the lips the sound of it is manifestly different when the breath is forceably blown through it from within outwards, and when it is sucked from without inwards.

Perhaps this might be worthy the attention of organ-builders.

5. A stop is put to this new generation of air, when about a fifteenth of the whole is produced, by its increasing pressure; and a similar boundary is fixed to its absorption or destruction by the decrease of atmospheric pressure. As water requires more heat to convert it into vapour under a heavy atmosphere than under a light one, so in letting off the water from muddy fish-ponds great quant.i.ties of air-bubbles are seen to ascend from the bottom, which were previously confined there by the pressure of the water. Similar bubbles of inflammable air are seen to arise from lakes in many seasons of the year, when the atmosphere suddenly becomes light.

6. The increased absorptions and evolutions of air must, like its simple expansions, depend much on the presence or absence of heat and light, and will hence, in respect to the times and places of its production and destruction, be governed by the approach or retrocession of the sun, and on the temperature, in regard to heat, of various lat.i.tudes, and parts of the same lat.i.tude, so well explained by Mr. Kirwan.

7. Though the immediate cause of the destruction or reproduction of great ma.s.ses of air at certain times, when the wind changes from north to south, or from south to north can not yet be ascertained; yet as there appears greater difficulty in accounting for this change of wind for any other known causes, we may still suspect that there exists in the arctic and antarctic circles a BEAR or DRAGON yet unknown to philosophers, which at times suddenly drinks up, and as suddenly at other times vomits out one-fifteenth part of the atmosphere: and hope that this or some future age will learn how to govern and domesticate a monster which might be rendered of such important service to mankind.

INSTRUMENTS.

If along with the usual registers of the weather observations were made on the winds in many parts of the earth with the three following instruments, which might be constructed at no great expence, some useful information might be acquired.

1. To mark the hour when the wind changes from north-east to south-west, and the contrary. This might be managed by making a communication from the vane of a weatherc.o.c.k to a clock; in such a manner, that if the vane mould revolve quite round, a tooth on its revolving axis should stop the clock, or put back a small bolt on the edge of a wheel revolving once in twenty-four hours.

2. To discover whether in a year more air pa.s.sed from north to south, or the contrary. This might be effected by placing a windmill-sail of copper about nine inches diameter in a hollow cylinder about six inches long, open at both ends, and fixed on an eminent situation exactly north and south. Thence only a part of the north-east and south-west currents would affect the sail so as to turn it; and if its revolutions were counted by an adapted machinery, as the sail would turn one way with the north currents of air, and the contrary one with the south currents, the advance of the counting finger either way would shew which wind had prevailed most at the end of the year.

3. To discover the rolling cylinders of air, the vane of a weatherc.o.c.k might be so suspended as to dip or rise vertically, as well as to have its horizontal rotation.

RECAPITULATION.

NORTH-EAST WINDS consist of air flowing from the north, where it seems to be occasionally produced; has an apparent direction from the east owing to its not having acquired in its journey the increasing velocity of the earth's surface; these winds are a.n.a.logous to the trade-winds between the tropics, and frequently continue in the vernal months for four and six weeks together, with a high barometer, and fair or frosty weather. 2. They sometimes consist of south-west air, which had pa.s.sed by us or over us, driven back by a new acc.u.mulation of air in the north, These continue but a day or two, and are attended with rain. See Note XXV.

SOUTH-WEST WIND consists of air flowing from the south, and seems occasionally absorbed at its arrival to the more northern lat.i.tudes. It has a real direction from the west owing to its not having lost in its journey the greater velocity it had acquired from the earth's surface from whence it came. These winds are a.n.a.logous to the monsoons between the tropics, and frequently continue for four or six weeks together, with a low barometer and rainy weather. 2. They sometimes consist of north-east air, which had pa.s.sed by us or over us, which becomes retrograde by a commencing deficiency of air in the north. These winds continue but a day or two, attended with severer frost with a sinking barometer; their cold being increased by their expansion, as they return, into an incipient vacancy.

NORTH-WEST WINDS consist, first, of south-west winds, which have pa.s.sed over us, bent down and driven back towards the south by newly generated northern air. They continue but a day or two, and are attended with rain or clouds. 2. They consist of north-east winds bent down from the higher parts of the atmosphere, and having there acquired a greater velocity than, the earth's surface; are frosty or fair. 3. They consist of north- east winds formed into a vertical spiral eddy, as on the eastern coasts of North America, and bring severe frost.

SOUTH-EAST WINDS consist, first, of north-east winds become retrograde, continue for a day or two, frosty or fair, sinking barometer. 2. They consist of north-east winds formed into a vertical eddy not a spiral one, frost or fair.