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4 ounce-weight of lava gave 20 ounce measures of air.
7 ............... basaltes .... 104 ......................
2 ............... toadstone .... 40 ......................
11/2 ............... granite .... 20 ......................
1 ............... elvain .... 30 ......................
7 ............... gypsum .... 230 ......................
4 ............... blue slate .... 230 ......................
4 ............... clay .... 20 ......................
4 ............... limestone-spar .... 830 ......................
5 ............... limestone .... 1160 ......................
3 ............... chalk .... 630 ......................
31/2 ............... white iron-ore .... 560 ......................
4 ............... dark iron-ore .... 410 ......................
1/2 ............... molybdena .... 25 ......................
1/2 ............... stream tin .... 20 ......................
2 ............... steatites .... 40 ......................
2 ............... barytes .... 26 ......................
2 ............... black wad .... 80 ......................
4 ............... sand stone .... 75 ......................
3 ............... coal .... 700 ......................

In this account the fixed air was previously extracted from the
limestones by acids, and the heat applied was much less than was
necessary to extract all the air from the bodies employed. Add to this
the known quantities of air which are combined with the calciform ores,
as the ochres of iron, manganese, calamy, grey ore of lead, and some
idea may be formed of the great production of air in volcanic eruptions,
as mentioned in note on Chunda, Vol. II. and of the perpetual
absorptions and evolutions of whole oceans of air from every part of the
earth.

But there would seem to be an officina aeris, a shop where air is both
manufactured and destroyed in the greatest abundance within the polar
circles, as will hereafter be spoken of. Can this be effected by some
yet unknown law of the congelation of aqueous or saline fluids, which
may set at liberty their combined heat, and convert a part both of the
acid and alcali of sea-water into their component airs? Or on the
contrary can the electricity of the northern lights convert inflammable
air and oxygene into water, whilst the great degree of cold at the poles
unites the azote with some other base? Another officina aeris, or
manufacture of air, would seem to exist within the tropics or at the
line, though in a much less quantity than at the poles, owing perhaps to
the action of the sun's light on the moisture suspended in the air, as
will also be spoken of hereafter; but in all other parts of the earth
these absorptions and evolutions of air in a greater or less degree are
perpetually going on in inconceivable abundance; increased probably, and
diminished at different seasons of the year by the approach or
retrocession of the sun's light; future discoveries must elucidate this
part of the subject. To this should be added that as heat and
electricity, and perhaps magnetism, are known to displace air, that it
is not impossible but that the increased or diminished quantities of
these fluids diffused in the atmosphere may increase its weight a well
as its bulk; since their specific attractions or affinities to matter
are very strong, they probably also possess general gravitation to the
earth; a subject which wants further investigation. See Note XXVI.

SOUTH-WEST WINDS.

The velocity of the surface of the earth in moving round its axis
diminishes from the equator to the poles. Whence if a region of air in
this country should be suddenly removed a few degrees towards the north
it must constitute a western wind, because from the velocity it had
previously acquired in this climate by its friction with the earth it
would for a time move quicker than the surface of the country it was
removed to; the contrary must ensue when a region of air is transported
from this country a few degrees southward, because the velocity it had
acquired in this climate would be less than that of the earth's surface
where it was removed to, whence it would appear to constitute a wind
from the east, while in reality the eminent parts of the earth would be
carried against the too slow air. But if this transportation of air from
south to north be performed gradually, the motion of the wind will blow
in the diagonal between south and west. And on the contrary if a region
of air be gradually removed from north to south it would also blow
diagonally between the north and east, from whence we may safely
conclude that all our winds in this country which blow from the north or
east, or any point between them, consist of regions of air brought from
the north; and that all our winds blowing from the south or west, or
from any point between them, are regions of air brought from the south.

It frequently happens during the vernal months that after a north-east
wind has passed over us for several weeks, during which time the
barometer has flood at above 301/2 inches, it becomes suddenly succeeded
by a south-west wind, which also continues several weeks, and the
barometer sinks to nearly 281/2 inches. Now as two inches of the mercury
in the barometer balance one-fifteenth part of the whole atmosphere, an
important question here presents itself, _what is become of all this
air_.

1. This great quantity of air can not be carried in a superior current
towards the line, while the inferior current slows towards the poles,
because then it would equally affect the barometer, which should not
therefore subside from 301/2 inches to 281/2 for six weeks together.

2. It cannot be owing to the air having lost all the moisture which was
previously dissolved in it, because these warm south-west winds are
replete with moisture, and the cold north-east winds, which weigh up the
mercury in the barometer to 31 inches, consist of dry air.

3. It can not be carried over the polar regions and be accumulated on
the meridian, opposite to us in its passage towards the line, as such an
accumulation would equal one-fifteenth of the whole atmosphere, and can
not be supposed to remain in that situation for six weeks together.

4. It can not depend on the existence of tides in the atmosphere, since
it must then correspond to lunar periods. Nor to accumulations of air
from the specific levity of the upper regions of the atmosphere, since
its degree of fluidity must correspond with its tenuity, and
consequently such great mountains of air can not be supposed to exist
for so many weeks together as the south west winds sometimes continue.

5. It remains therefore that there must be at this time a great and
sudden absorption of air in the polar circle by some unknown operation
of nature, and that the south wind runs in to supply the deficiency. Now
as this south wind consists of air brought from a part of the earth's
surface which moves faster than it does in this climate it must have at
the same time a direction from the west by retaining part of the
velocity it had previously acquired. These south-west winds coming from
a warmer country, and becoming colder by their contact with the earth of
this climate, and by their expansion, (so great a part of the
superincumbent atmosphere having vanished,) precipitate their moisture;
and as they continue for several weeks to be absorbed in the polar
circle would seem to receive a perpetual supply from the tropical
regions, especially over the line, as will hereafter be spoken of.

It may sometimes happen that a north-east wind having passed over us may
be bent down and driven back before it has acquired any heat from the
climate, and may thus for a few hours or a day have a south-west
direction, and from its descending from a higher region of the
atmosphere may possess a greater degree of cold than an inferior north
east current of air.

The extreme cold of Jan. 13, 1709, at Paris came on with a gentle south
wind, and was diminished when the wind changed to the north, which is
accounted for by Mr. Homberg from a reflux of air which had been flowing
for some time from the north. Chemical Essays by R. Watson, Vol. V. p.
182.

It may happen that a north-east current may for a day or two pass over
us and produce incessant rain by mixing with the inferior south-west
current; but this as well as the former is of short duration, as its
friction will soon carry the inferior current along with it, and dry or
frosty weather will then succeed.

NORTH-EAST WINDS.

The north-east winds of this country consist of regions of air from the
north, travelling sometimes at the rate of about a mile in two minutes
during the vernal months for several weeks together from the polar
regions toward the south, the mercury in the barometer standing above
30. These winds consist of air greatly cooled by the evaporation of the
ice and snow over which it passes, and as they become warmer by their
contact with the earth of this climate are capable of dissolving more
moisture as they pass along, and are thence attended with frosts in
winter and with dry hot weather in summer.

1. This great quantity of air can not be supplied by superior currents
passing in a contrary direction from south to north, because such
currents must as they arise into the atmosphere a mile or two high
become exposed to so great cold as to occasion them to deposit their
moisture, which would fall through the inferior current upon the earth
in some part of their passage.

2. The whole atmosphere must have increased in quantity, because it
appears by the barometer that there exists one-fifteenth part more air
over us for many weeks together, which could not be thus accumulated by
difference of temperature in respect to heat, or by any aerostatic laws
at present known, or by any lunar influence.

From whence it would appear that immense masses of air were set at
liberty from their combinations with solid bodies, along with a
sufficient quantity of combined heat, within the polar circle, or in
some region to the north of us; and that they thus perpetually increase
the quantity of the atmosphere; and that this is again at certain times
re-absorbed, or enters into new combinations at the line or tropical
regions. By which wonderful contrivance the atmosphere is perpetually
renewed and rendered fit for the support of animal and vegetable life.

SOUTH-EAST WINDS.

The south-east winds of this country consist of air from the north which
had passed by us, or over us, and before it had obtained the velocity of
the earth's surface in this climate had been driven back, owing to a
deficiency of air now commencing at the polar regions. Hence these are
generally dry or freezing winds, and if they succeed north-east winds
should prognosticate a change of wind from north-east to south-west; the
barometer is generally about 30. They are sometimes attended with cloudy
weather, or rain, owing to their having acquired an increased degree of
warmth and moisture before they became retrograde; or to their being
mixed with air from the south.

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 passed 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 passed 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
passing over us, is driven back by accumulation 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 passing 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 passing 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 shore 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 passes 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 shore 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 shore, 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 constitute one cause of the greater
coldness of the eastern sides of North America than of the European
shores opposite to them, which is said to be equal to twelve degrees of
north latitude, 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
quantity 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 passes 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 constitute
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
passing 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 quantities 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 analogy between the circumstance 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 circumstance 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 circumstances 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
quantity, 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 circumstances, 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 incumbent 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.

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