Chapter XLVI: 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 accumulation 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 principal 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 passage. 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 passage, 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 passes. 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 quantities 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 latitudes, and parts of the same latitude, so well explained by Mr. Kirwan.

7. Though the immediate cause of the destruction or reproduction of great masses 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 weathercock 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 passed 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 weathercock 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 analogous 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 passed by us or over us, driven back by a new accumulation 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 latitudes. 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 analogous 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 passed 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 passed 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.

NORTH WINDS consist, first, of air flowing slowly from the north, so that they acquire the velocity of the earth's surface as they approach, are fair or frosty, seldom occur. 2. They consist of retrograde south winds; these continue but a day or two, are preceded by south-west winds; and are generally succeeded by north-east winds, cloudy or rainy, barometer rising.

SOUTH WINDS consist, first, of air flowing slowly from the south, loosing their previous western velocity by the friction of the earth's surface as they approach, moist, seldom occur, 2. They consist of retrograde north winds; these continue but a day or two, are preceded by north-east winds, and generally succeeded by south-west winds, colder, barometer sinking.

EAST WINDS consist of air brought hastily from the north, and not impelled farther southward, owing to a sudden beginning absorption of air in the northern regions, very cold, barometer high, generally succeeded by south-west wind.

WEST WINDS consist of air brought hastily from the south, and checked from proceeding further to the north by a beginning production of air in the northern regions, warm and moist, generally succeeded by north-east wind. 2. They consist of air bent down from the higher regions of the atmosphere, if this air be from the south, and brought hastily it becomes a wind of great velocity, moving perhaps 60 miles an hour, is warm and rainy; if it consists of northern air bent down it is of less velocity and colder.

_Application of the preceding Theory to Some Extracts
from a Journal of the Weather._

_Dec. 1, 1790._ The barometer sunk suddenly, and the wind, which had been some days north-east with frost, changed to south-east with an incessant though moderate fall of snow. A part of the northern air, which had passed by us I suppose, now became retrograde before it had acquired the velocity of the earth's surface to the south of us, and being attended by some of the southern air in its journey, the moisture of the latter became condensed and frozen by its mixture mith the former.

_Dec. 2, 3._ The wind changed to north-west and thawed the snow. A part of the southern air, which had passed by us or over us, with the retrograde northern air above described, was now in its turn driven back, before it had lost the velocity of the surface of the earth to the south of us, and consequently became a north-west wind; and not having lost the warmth it brought from the south produced a thaw.

_Dec. 4, 5._ Wind changed to north-east with frost and a rising barometer. The air from the north continuing to blow, after it had driven back the southern air as above described, became a north-east wind, having less velocity than the surface of the earth in this climate, and produced frost from its coldness.

_Dec. 6, 7._ Wind now changed to the south-west with incessant rain and a sinking barometer. From unknown causes I suppose the quantity of air to be diminished in the polar regions, and the southern air cooled by the earth's surface, which was previously frozen, deposits its moisture for a day or two; afterwards the wind continued south-west without rain, as the surface of the earth became warmer.

_March 18, 1785._ There has been a long frost; a few days ago the barometer sunk to 291/2, and the frost became more severe. Because the air being expanded by a part of the pressure being taken off became colder. This day the mercury rose to 30, and the frost ceased, the wind continuing as before between north and east. _March 19._ Mercury above 30, weather still milder, no frost, wind north-east. _March 20._ The same, for the mercury rising shews that the air becomes more compressed by the weight above, and in consequence gives out warmth.

_April 4, 5._ Frost, wind north-east, the wind changed in the middle of the day to the north-west without rain, and has done so for three or four days, becoming again north-east at night. For the sun now giving greater degrees of heat, the air ascends as the sun passes the zenith, and is supplied below by the air on the western side as well as on the eastern side of the zenith during the hot part of the day; whence for a few hours, on the approach of the hot part of the day, the air acquires a westerly direction in this longitude. If the north-west wind had been caused by a retrograde motion of some southern air, which had passed over us, it would have been attended with rain or clouds.

_April 10._ It rained all day yesterday, the wind north-west, this morning there was a sharp frost. The evaporation of the moisture, (which fell yesterday) occasioned by the continuance of the wind, produced so much cold as to freeze the dew.

_May 12._ Frequent showers with a current of colder wind preceding every shower. The sinking of the rain or cloud pressed away the air from beneath it in its descent, which having been for a time shaded from the sun by the floating cloud, became cooled in some degree.

_June 20._ The barometer sunk, the wind became south-west, and the whole heaven was instantly covered with clouds. A part of the incumbent atmosphere having vanished, as appeared by the sinking of the barometer, the remainder became expanded by its elasticity, and thence attracted some of the matter of heat from the vapour intermixed with it, and thus in a few minutes a total devaporation took place, as in exhausting the receiver of an air-pump. See note XXV. At the place where the air is destroyed, currents both from the north and south flow in to supply the deficiency, (for it has been shewn that there are no other proper winds but these two) and the mixture of these winds produces so sudden condensation of the moisture, both by the coldness of the northern air and the expansion of both of them, that lightning is given out, and an incipient tornado takes place; whence thunder is said frequently to approach against the wind.

_August 28, 1732._ Barometer was at 31, and _Dec. 30_, in the same year, it was at 28 2-tenths. Medical Essays, Edinburgh, Vol. II. p. 7. It appears from these journals that the mercury at Edinburgh varies sometimes nearly three inches, or one tenth of the whole atmosphere. From the journals kept by the Royal Society at London it appears seldom to vary more than two inches, or one-fifteenth of the whole atmosphere. The quantity of the variation is said still to decrease nearer the line, and to increase in the more northern latitudes; which much confirms the idea that there exists at certain times a great destruction or production of air within the polar circle.

_July 2, 1732._ The westerly winds in the journal in the Medical Essays, Vol. II. above referred to, are frequently marked with the number three to shew their greater velocity, whereas the easterly winds seldom approach to the number two. The greater velocity of the westerly winds than the easterly ones is well known I believe in every climate of the world; which may be thus explained from the theory above delivered. 1. When the air is still, the higher parts of the atmosphere move quicker than those parts which touch the earth, because they are at a greater distance from the axis of motion. 2. The part of the atmosphere where the north or south wind comes from is higher than the part of it where it comes to, hence the more elevated parts of the atmosphere continue to descend towards the earth as either of those winds approach. 3. When southern air is brought to us it possesses a westerly direction also, owing to the velocity it had previously acquired from the earth's surface; and if it consists of air from the higher parts of the atmosphere descending nearer the earth, this westerly velocity becomes increased. But when northern air is brought to us, it possesses an apparent easterly direction also, owing to the velocity which it had previously acquired from the earth's surface being less than that of the earth's surface in this latitude; now if the north-east wind consists of air descending from higher parts of the atmosphere, this deficiency of velocity will be less, in consequence of the same cause, viz. The higher parts of the atmosphere descending, as the wind approaches, increases the real velocity of the western winds, and decreases the apparent velocity of the eastern ones.

_October 22._ Wind changed from south-east to south-west. There is a popular prognostication that if the wind changes from the north towards the south passing through the east, it is more likely to continue in the south, than if it passes through the west, which may be thus accounted for. If the north-east wind changes to a north-west wind, it shews either that a part of the northern air descends upon us in a spiral eddy, or that a superior current of southern air is driven back; but if a north-east wind be changed into a south-east wind it shews that the northern air is become retrograde, and that in a day or two, as soon as that part of it has passed, which has not gained the velocity of the earth's surface in this latitude, it will become a south wind for a few hours, and then a south-west wind.

The writer of this imperfect sketch of anemology wishes it may incite some person of greater leizure and ability to attend to this subject, and by comparing the various meteorological journals and observations already published, to construct a more accurate and methodical treatise on this interesting branch of philosophy.

NOTE XXXIV.--VEGETABLE PERSPIRATION.

_And wed the enamoured Oxygene to Light._