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

Chap. IV. M E T E O
Winds. Perhaps it may be a confirmation of this conjefture,
' '' that the fouth weft winds generally extend over a great¬
er tract of country than nioft other winds which blow
in the temperate zones. What has been faid of fouth-
•weft winds holds equally with regard to north-weft
winds in the fouth temperate zone.
After fouth-weft winds have blown for fome time, a
great quantity of air will be accumulated at the pole,
at leaft if they extend over all the northern hemifphere j
and it appears, from comparing the tables kept by iome
of our late navigators in the northern Pacific ocean with
fimilar tables kept in this illand, that this is fometimes
the cafe fo far as relates to the Atlantic and Pacific
oceans. When this accumulation becomes great, it
muft, from the nature of fluids, and from the clafticity
of the air, prefs with a confiderable and increafing force
on the advancing air ; fo that in time it becomes ftrong-
er than the fouth-weft wind. This will occafion at firft
a calm, and afterwards a north wind, which will be¬
come gradually eafterly as it advances foutlwards, from
its not afluming immediately the velocity of the earth.
The mafs of the atmofphere will be increafed in all thofe
places over which this north-eaft wind blowrs 5 this is
confirmed by the almoft conftant rife of the barometer
during a north-eaft wind.
Whatever tends to increafe the bulk of the atmo¬
fphere near the pole, muft tend alfo to increafe the fre¬
quency of north-eaft rvinds } and if there be any feafon
when this increafe takes place more particularly, that
feafon will be molt liable to thefe winds. During win¬
ter the northern parts of Europe are covered with fnow,
■which is melted in the beginning of fummer, when the
heat of the fun becomes more powerful. Great quanti¬
ties of vapour are during that time raifed, which will
augment both the bulk and weight of the atmofphere,
efpecially if the conjetture about the converfion of va¬
pour into air has any foundation. Hence north-eaft
winds are moft prevalent during May and June.
But it will be faid, if this hypothefis were true, the
fouth-weft and north-eaft winds ought to blow alter¬
nately, and continue each of them for a ftated time ;
whereas the fouth-weft wind blows fometimes longer
and fometimes fhorter, neither is it always followed by
a north-eaft wind.
If the conjefture about the decompofition of vapour
in the torrid zone be true, the hydrogen which formed
a part of it will afcend from its lightnefs, and form a
ftratum above the atmofpherical air, and gradually ex¬
tend itfclf, as additional hydrogen rifes, towards the
north and fouth, till at laft it reaches the poles. The
lightnefs of hydrogen is owing to the great quantity of
heat which it contains •, as it approaches the poles it
muft lofe a great part of this heat, and may in confe-
quence become heavy enough to mix with the atmo-
f >here below. Oxygen makes a part of the atmo-
fphere ; and its proportion near the poles may fome¬
times be greater than ordinary, on account of the addi¬
tional quantity brought thither from the torrid zone.
Mr Cavendilh mixed oxygen and hydrogen together in
a glafs jar and upon making an electrical fpark pafs
through them, they immediately combined and formed
water.
That there is eleftric matter at the poles, cannot be
doubted. The abbe Chappe informs us, that he faw
thunder and lightning much more frequently at Tobol-
R O L O G T. 727
Iki and other parts of Siberia, than in any other part Winds.
of the world. In the nonh oi Europe, the air, during 1
very cold weather, is exceedingly elefiric ; iparks can
be drawm from a per Ion’s hands and face, by combing
his hair, or even powdering him with a puff. Aipinus
was an eye-witnefs to this laft, and to Itill more afto-
nifhing proofs of the eleftricity of the atmofphere during
great colds.
May not the appearance of the aurora borealis be
owing to the union of oxygen and hydrogen by the in¬
tervention of the eleftrie fluid ? Thai it is an eleftrieal
phenomenon, at leaft, can hardly be doubted. Artifi¬
cial eleftricity is much ftrengthened during an aurora,
as M. Volta and Mr Canton have obferved 5 and the
magnetic needle moves wflth the fame irregularity dur¬
ing an aurora that has been obferved in other eleftrical
phenomena. This faft we learn from Bergman and
De la Lande. Many philofophers have attempted to
demonftrate that aurorae boreales are beyond the earth’s
atmofphere 5 but the very different refults of their cal¬
culations evidently prove that they were not pofieffed of
fufficient data.
If this conjefture be true, part of the atmofphers
near the poles muft at times be converted into water.
This would account for the long continuance of fouth-
weft vdnds at particular times ; when they do fo, a de¬
compofition of the atmofphere is going on at the pole.
It would render this conjefture more probable, if the
barometer fell always when a fouth-weft wind continues
long. 79
If this hypothefts be true, a fouth-w’eft wind ought South-weft
always to blow after aurorae boreales 5 and we are Jn. winds very
formed by Mr Winn, that this is aftually the cafe. ^Turora;"
This he found never to fail in 23 inftances. He ob- boreales.
ferved alfo, that when the aurora wTas bright, the gale
came on within 24 hours, but did not laft long : but if
it was faint and dull, the gale was longer in beginning,
and lefs violent, but it continued longer. This looks
like a confirmation of our conjefture. Bright aurorae
are probably nearer than thofe which are dull. Now,
if the aurora borealis be attended with a decompofition
of a quantity of air, that part of the atmofphere which
is neareft muft firft rufh in to fupply the diftant parts.
Juft as if a hole were bored in the end of a long vefiel
filled with water, the water neareft the hole would flow
out immediately, and it would be fome time before the
water at the other end of the veffel began to move.
The nearer w'e are to the place of precipitation, the
fooner will we feel the fouth-weft wind. It ought
therefore to begin fooner after a bright aurora, becaufe
it is nearer than a dull and faint one. Precipitations
of the atmofphere at a diftance from the pole cannot be
fo great as thofe which take place near it ; becaufe the
cold will not be fufficient to eondenfe fo great a quantity
of hydrogen ; fouth-weft winds, therefore, ought not
to laft fo long after bright as after dull aurorae. Winds
are more violent after bright aurorse, becaufe they are
nearer the place of precipitation ; juft as the water near
the hole of the veffel runs fwifter than that which is at
a confiderable diftance.
If thefe eonjeftures have any foundation in nature, Probable
there are’two fources of fouth-weft winds 5 the firft has caufes of
its origin in the trade-winds, the fecond in precipita-^
lions of the atmofphere near the pole. When they ori-w u 5‘
ginate from the firft caufe, they will blow in countries
farther