ilrative, that this latter muft afcend in a continual 
ftream as fail as it rarefies •, and that being afcended, 
it muft difperfe itfelf to preferve the equilibrium, that 
is, by a contrary current the upper air muft move 
from thofe parts where the greateft heat is ; fo, by a 
kind of circulation, the north-eaft trade Wind below 
will be attended with a fouth-wefterly above. That 
this is more than a bare conje&ure, the almoft inftan- 
taneous change of the Wind to the oppofite point, 
which is frequently found in paffing the limits of the 
trade Winds, feems to ajffure us •, but that which above 
all confirms this hypothefis, is the phenomenon of 
the Monfoons, by this means moft eafily folved, and 
without it hardly explicable. 
Suppofing therefore, fuch a circulation as above, it 
is to be confidered, that to the northward of the In- 
dian ocean, there is every where land within the ufu- 
al limits of the latitude of thirty degrees ; viz. Ara- 
bia, Perfia, India, &c. which for the fame reafon as 
the mediterranean parts of Africa, are fubjed to un- 
fufferable heats, when the fun is in the north, paffing 
nearly vertical •, but yet are temperate enough, when 
the fun is removed toward the other tropic, becaufe 
of a ridge of mountains at fome diftance within the 
land, laid to be frequently in winter covered with 
fnow, over which the air, as it pafies, muft needs be 
much chilled. 
Hence it comes to pafs, that the air, coming accord- 
ing to the general rule, out of the north-eaft in the 
Indian fea, is fometimes hotter, fometimes colder, than 
that which by this circulation is returned out of the 
fouth-weft ; and by confequence, fometimes the under 
current or Wind is from the north-eaft, fometimes 
from the fouth-weft. 
That this has no other caufe, is clear from the times 
•wherein thefe Winds fet,viz. in April, when the fun 
begins to warm thofe countries to the north, the 
fouth-weft Monfoons begin, and blow during the heat 
till October •, when the fun being retired, and all 
things growing cooler northward, and the heat in- 
creafing to the fouth, the north-eaft enter and blow 
all the winter, till April again. And it is undoubt- 
edly from the fame principle, that fouthward of the 
equator, in part of the Indian ocean, the north-eaft 
Winds fucceed the fouth-eaft, when the fun draws 
near the tropic of Capricorn. 
But in this latter occurs a difficulty not well to be ac- 
counted for, which is, why this change of the Mon- 
foons ffiould be any more in this ocean, than in the 
fame latitudes in the iEthiopic, where there is no- 
thing more certain than a fouth-eaft Wind all the 
year. * 
It is likewife very hard to conceive, why the limits 
of the trade Winds ffiould be fixed about the thirtieth 
decree of latitude all round the. globe ; and that they 
ffiould fo feldom tranfgrefs or fall ffiort of thofe 
bounds ; as alfo that in the Indian fea, only the north- 
ern part ffiould be fubjeft to the changeable Mon- 
foons, and in the fouthern there ffiould be a conftant 
fouth-eaft. 
This account of Wind is taken from the learned Dr. 
Halley’s difeourfe on this fubjed;, Philofoph. Tranf. 
N° 183. 
The Rev. Mr. Robinfon gives this account of the ori- 
gin of Wind ; that in the greateft probability it pro- 
ceeds from vaft fwarms of nitrous particles, which rife 
from the bottom of the fea, and have been put into 
motion, either by the central fire, or by that heat 
and fermentation which abounds in this huge body 
of the earth •, and therefore this firft commotion, which 
is excited by the faid fermentation, is called a bot- 
tom Wind, which is prefently difeovered by the por- 
poifes, and other fea fiffi, that delight to fport and 
play upon the waves of the fea, and by their playing, 
mve to the mariners the firft notice of an approaching 
0 
ftorm. 
When thefe nitrous fwarms are men toward the fur- 
face of the fea, they caufe, in a dark night, fuch a 
iliinino 1 light upon the. waves, as if the fea were on 
fire ; and being delivered from the brackiffi water* 
and received into the open air, thofe fiery and filming 
meteors, which fix upon malts and tides of the ffiips, 
are only nitrous particles condenfed by the circumam- 
bient cold, and like that which the chemifts call t>hof- 
phorus, or artificial glow-worm, mine' and calc alight, 
but have no heat. 
This gives the fecond notice to the mariners, that the 
ftorm is rifing *, for the fea begins to be rough upon 
the firft breaking out of the Wind, and the waves 
(well and rife, though the air at the fame time is calm 
and clear. 
This boiling fermentation of the fea canfes the va- 
pours to arife, which by the intenfenefs of the circum- 
ambient cold, are condenfed into thick clouds, and 
fall down in ftorms of Wind and rain , firft upon the 
fea, from whence they rofe •, and then the attraftive 
power of the mountain cold, by a iecret magnetifm 
between vapour and cold, attracts, the wateriffi va- 
pours, and intermixed with nitrous particles, they 
afcend to the high tops of mountains and hills, where 
they hang hovering in thick fogs, and watery mills, 
until the atmofpherical heat rarefies the nitrons part 
of the fog, which is almoft uppermoft, and appears 
white and tranflucent, into brifk gales of Wind. 
And the intenfenefs of the atmofpherical cold hav- 
ing attradled the vapours into the colder regions of 
the air, where they are condenfed into clouds, the 
Wind breaks, diffipates, and drives them before it, 
till they fall down in rain, and water the ftirface of 
the earth. 
And this feems to be the reafon why they have but 
little Wind, and lefs rain in Egypt, and thofe level 
countries where they have no mountains. 
Dr. Derham fays. Wind is a current of the air •, and 
that which excites or alters its current, may juftly be 
faid to be the caufe of Wind. 
An equipoife of the atmofphere produces a calm •, but 
if that equipoife be taken off, more or lefs, a ftream 
of air or Wind is accordingly thereby produced, either 
ftronger or weaker, fwifteror flower. 
And there are divers things that may caufe fuch al- 
terations in the[equipoife or balance of the atmofphere, 
viz. eruptions of vapours from the fea and land, ra- 
refactions and condenfations in one place more than 
in another, the falling of rain, preffure of the clouds, 
&c. 
It is obferved of caves, that they always emit Winds 
more or lefs ; and as great caves, fo great lakes alfo 
fend forth Winds ; but the moft univerfal and con- 
ftant alterations of the atmofphere are produced by 
heat and cold. 
This is manifeft in the general trade Winds, which, 
between the tropics, blow all the year from eaft to 
weft •, if the caufe of this be (according to the opinion 
of fome ingenious men) the daily progrefs of the fun 
round that part of the globe, and by his heat rarefy- 
ing one part of the air, while the cooler and heavier 
air behind preffes after. 
And fo the land and fea breezes and fo in our di « 
mate, the northerly and foutherly Winds (which are 
commonly efteemed the caufe of warm and cold wea- 
ther) are really the effeCls of the cold or Warm th of 
the atmofphere. 
Of this Dr. Derham fays, he has had fo many confir- 
mations, that he does not doubt of it ; and he pro- 
duces for an inftance of it, that it is not uncommon to 
fee a warm foutherly Wind, on a fudden change to 
the north, by the fall of fnow or hail •, and in a cold 
frofty morning, to fee the Wind north, and to wheel 
about toward the foutherly quarters, when the fun 
has well warmed the air •, and then again, in the cold 
evening, to turn northerly or eafterly. 
And hence alfo it is, that the Winds and clouds are 
oftentimes contrary to each other in thunder ffiowers 
(efpecially if hail falls ■,) the fultry weather below di- 
rects the Wind one way, and the cold above the clouds 
another way. 
And that he has obferved feveral times, that when 
the morning has been warm, and what Wind was 
flirting was weft - fouth-weft, that the clouds were 
u thick 
