Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 13, MAT-MIC

724 M E T E O
. Wm:!s- . _ A pretty good idea of the velocity of the wind, under
55 ' different circumftances, may be formed from the follow-
Velocity of table, which was drawn up by Mr Smeaton.
the winds.
61
Miles
per
Hour
1
2
3
4
5
10
20
25
30
35
40
45
5°
60
80
100
Feet
per
Second,
I.47
2-93
4.4
5-87
7-33
14.67
22.
29-34
36.67
44.01
5'-34
58.68
66.01
73-35
88.02
117.36
146.J
Perpendicular force on one fquare foot,
in Avoirdupois pounds and parts.
.005
.020
.044
.079
.123
•49 2
1.107
1.968
3-°75
4.429
6.027
7-S73
9-963
12.300
I7-7I5
35 -49°
49.200
/ Hardly perceptible,
f lull perceptible.
j- Gently pleafant.
^ Pleafant, brilk.
Very brifk.
High wind.
Very high wind.
Storm or tempeft.
Great ftorm.
Hurricane.
{Hurricane that tears up trees
and carries buildings before
it.
For the means of afcertaining the velocity of the
winds, fee Anemometer and Anemoscope.
We {hall now endeavour to explain the phenomena
that we have been defcribing, or to form a plaufible
theory of the winds.
The atmofphere is a fluid furrounding the earth, and
extending to an unknown height. Now all fluids tend
invariably to a level : if a quantity of water be taken
out of any part of a veffel, the furrounding water will
immediately flow in to fupply its place, and the furface
will become level as before -, or if an additional quan¬
tity of water be poured into any part of the veffel, it
will not remain there, but diffufe itfelf equally over the
whole. Such exactly would be the cafe with the at¬
mofphere. Whatever therefore deflroys the equilibrium
of this fluid, either by increafing or diminifhing its
bulk in any particular place, muff at the fame time
occafion a wind.
Air, befides its qualities in common with other fluids,
is alfo capable of being dilated and compreffed. Sup-
pofe a veffel filled with air : if half the quantity which
it contains be drawn out by means of an air-pump, the
remainder will ftill fill the veffel completely ; or if
twice or three times the original quantity be forced in
by a condenfer, the veffel will ftill be capable of hold-
ingit.
Rarefied air is lighter, and condenfed air heavier
than common air. When fluids of unequal fpeeific
gravities are mixed together, the heavier always deft end
and the lighter afcend. Were quickfilver, water,
and oil, thrown into the fame vefftl together, the quick-
filver would uniformly occupy the bottom ; the water
the middle, and the oil the top. Were water to be
thrown into a veffel of oil, it would immediately
defcend, becaufe it is heavier than oil. Exaftly the
fame thing takes place in the atmofphere. Were a
R O L O G Y. Chap. IV.
quantity of air, for inftance, to be fuddenly condenfed Winds,
at a didance from the furface of the earth, being now '
heavier than before, it would defcend till it came to
air of its own denfity; or, were a portion of the
atmofphere at the furface of the earth to be fuddenly
rarefied, being now lighter than the furrounding air, i’t
would immediately afcend.
If a bladder half filled with air be expofed to the c
heat of a fire, the air within will foon expand, and trade6- 9
diftend the bladder } if it be now removed to a cold winds,
place, it will foon become flaccid as before. This
fhews that heat rarefies, and that cold condenfes air.
1 he furface of the torrid zone is much more heated by
the rays of the fun than the frozen or temperate zones,
becaufe the rays fall upon it much more perpendicularly.
This heat is communicated to the air near the furface
of the torrid zone, which being thereby rarefied, afcends,
and its place is fupplied by colder air, which ruflies in
from the north and fouth.
The diurnal motion of the earth is greateft at the
equator, and diminifhes gradually as we approach the
poles, where it ceafes altogether. Every fpot of the
earth’s furface at the equator moves at the rate of 15
geographical miles in a minute j at 40° of latitude it
moves at about 11 miles and a half in a minute, and at
the 30° at nearly 13 miles. The atmofphere, by mov¬
ing continually round along with the earth, has ac¬
quired the fame degree of motion, fo that thofe parts
of it which are above the equator move falter than
thofe which are at a diftance. Were a portion of the
atmofphere to be tranfported in an inftant from latitude
30° to the equator, it would not immediately acquire
the velocity of the equator; the eminences of the earth,
therefore, would ftrike againft it, and it would affume
the appearance of an eaft wind. This is the cafe in a
fmaller degree with the air that flows towards the equa¬
tor, to fupply the place of the rarefied air which is con¬
tinually afeending ; and this, when combined with its
real motion from north to fouth, muft caufe it to affume
the appearance of a north-eafierly wind on this fide the
equator, and of a fouth-eafterly beyond it.
The motion weftward occafioned by this difference
in celerity alone, would be very fmall; but it is in-
creafed by another circumftance. Since the rarefa&ion
of the air in the torrid zone is owing to the heat de¬
rived from the contiguous earth, and fince this heat is
owing to the perpendicular rays of the fun, thofe parts
muft be hottefl where the fun is adtually vertical ; and
confequently the air above them muft be molt rarefied j
the contiguous parts of the atmofphere will therefore be
drawn moft forcibly to that particular fpot. Now, fince
the diurnal motion of the earth is from eaft to weft, this
hotteft fpot will be continually fhifting weftwards, and
this will occafion a current of the atmofphere in that
direftion. That this caufe really operates, appears
from a circumftance already mentioned : When the
fun approaches either of the tropics, the trade-wind on
the fame fide of the equator affumes a more eafterly di-
reftion, evidently from the caufe here mentioned, while
the oppofite trade-wind being deprived of this additional
impulfe, blows in a direflion more perpendicular to the
equator. What in-^
The wefterly dire61ion of the trade-wind is ftill we{terly di
farther increafed by another caufe. Since the attrac- reftiwi of
tion of the fun and moon produces fo remarkable antl,e tra£fr-
effe&wind"