W 1 N 
Which the air itfelf, afted on by the fame force, will 
move in the fame time by this rule, 
s. As the fpecific gravity of air is to that of any 
other fluid, lo reciprocally is the fquare of the fpace 
which that fluid, impelled by any force, moves in 
any given time, to the fquare of the fpace, which 
the air, by the fame impulfe, will move in the fame 
time. 
Sup no Ping therefore, the ratio of the fpecific gravity 
of that other fluid to that of air, to be —b:ci the 
fpace defcribed by the fluid to be called /; and that 
which the air will defcribe by the fame impulfe -x : the 
rule gives us x—>/ (b s : c.) 
Hence, if we fuppofe water, impelled by the given 
force, to move two feet in a fecond of time ; then 
will f— 2 ; and fince the fpecific gravity of water to 
air is as 970 to 1, we fhall have b~ 970, and c~ 1 ; 
confequently x—»y 070. 4=^/3880 = 632 feet : the 
velocity of the Wind, therefore to that of water, 
moved by the fame power, will be as 623 to 2 ; i. e. 
if water move two feet in a fecond, the Wind will fly 
623 feet. 
2. Add, that f—j ( c x z : b 5) and therefore the 
fpace any fluid, impelled by any impreffion, moves 
in any time, is determined by finding a fourth pro- 
portional to the two numbers that exprefs the ratio of 
the fpecific gravity, and the fquare of the fpace the 
Wind moves in the given time. The fquare root of 
that fourth proportional is the fpace required. 
M. Mariotte, e. g. found, by various experiments, 
that a pretty ftrong Wind moves 24 feet in a fecond 
of time ; wherefore, if the fpace which the water, act- 
ed on by the fame force as the air, will defcribe in the 
fame time, be required, then will c— 1, *=24, £=97 o, 
and we (hall find /=/ (576: 970=44) 
3. The velocity of Wind being given, to determine 
the preflure required to produce that velocity, we 
have this rule. 
The fpace the Wind moves in one fecond of time, is 
to the height a fluid is to be raifed in an empty tube, 
in order to have a preflure capable of producing that 
velocity, in a ratio compounded of the fpecific gravi- 
ty of the fluid to that of air ; and of quadruple the al- 
titude a body defcends in the firft fecond of time, to 
the aforefaid fpace of air. 
Suppofe, e. gr. the fpace the air moves in a fecond 
a= 24 feet, or 288 inches; call the altitude of the 
third x , and the ratio of the mercury to the air b : 
£=13580: 1, J=i8i inches; x will be lefs than the 
number by one line, or W of an inch. And hence 
we fee why a fmall, but fudden change in the baro- 
meter, fliould be followed by violent Winds. 
The force of the Wind is determined experimentally, 
by a peculiar machine called an anemometer, or Wind 
meafurer ; which being moved by means of fails, like 
thofe of a Windmill, raife a weight, that ftill the 
higher it is raifed, receding farther from the center of 
motion, by Aiding along a hollow arm fitted on to 
the axis of the fails, becomes heavier and heavier, and 
preffes on the arm, till, being a counterpoife to the 
force of the Wind on the fails, it flops the motion of 
them. An index then, fitted upon the fame axis, at 
right angles with the arm, by its riflng or falling, 
points out the ftrength of the Wind, on a plane di- 
vided like a dial plate into degrees. 
Winds are either conftant or variable. The conftant 
Winds are up and down always at a certain time of 
the year, and in certain parts of the world; but the 
variable vary fo much, that they cannot be reduced 
to any rule. 
The conftant and periodical Winds are only in the 
wideft feas ; as in the Atlantic and Ethiopic feas, be- 
tween the tropics, there is generally an eafterly _Wind 
all the year long, without any conliderable variation, 
unlefs declining fame few points toward the north or 
fouth ; but all along the coafts of Guiney, for five 
hundred leagues, the foutherly and fouth-weft Winds 
are perpetual. 
In the Indian ocean the Winds are partly general, as 
in the Ethiopic ocean, and partly periodical ; that is, 
W I N 
they blow one way half the year, and upon the oppo- 
fite points the other half. 
This that is here faid delates to the fea Winds, at fom6 
diftance from the land ; for upon the land, and near 
thelhores, the land and fea breezes are almoft every 
where fenfible ; and the great variety that happens 
in their periods, force, and dire&ion, happens from 
the fituation of mountains, valleys* and woods, and 
from the various texture of the foil, more or lefs capa- 
ble of retaining or reflecting heat, or of exhaling or 
condenfing vapours. 
Of variable Winds, fome are common to all countries* 
others are more peculiar to fome certain parts. 
Of the latter fort, the moft famous are hurricanes* 
which chiefly infeft the Caribbee iflands, but are not 
anniverfary, nor equally frequent. 
Their fury is fo great, that they throw down all be- 4 
fore them, tear up trees, overturn houfes, tofs fhipg 
pi'Odigioufly, and blow about things of a vaft weight. 
They are not even continued Winds, but blow in 
gufts, which fuddenly come and go ; neither do they 
extend very wide, but are fometimes confined to a 
narrow compafs, and at other times take a larger fcope. 
As for their duration, it is but a few days, and fome- 
times only fora few hours. They are more commoil 
in America than any where elfe, but yet Europe and 
Afia are not altogether without them, as appears from 
hiftories and travels. 
The caufes of tempefts and hurricanes are hardly to' 
be accounted for in all particulars. However it may, 
in the firft place be noted, that the ratio of all liquids 
is much the fame, and therefore an extraordinary- 
motion may be excited in the air, by the fame way as 
in water. 
Now, if water fall from a high place, or if there be 
a confluence of feveral ftreams together, this gives a 
violent motion, and caufes a many whirlings and ed- 
dies in it. This is apparent in the torrents falling 
down the rocks, and the confluence of rivers. 
If therefore fomething analogous may happen in the 
air, there needs muft be furious tempefts of Wind 
raifed in it. And fuch a thing may happen, if any 
extraordinary quantity of vapours be drawn by the 
Wind, upon a certain place, which they cannot eafily 
get over, by reafon of mountains or contrary Winds, 
which oppofe them : for example ; fuppofe a Wind, 
upon fome point between north and eaft, carries a 
large collection of vapours out of Africa into the Ca- 
ribbee iflands ; this Wind lights upon the continent 
of America ; now, it is poflible, that not only the 
mountains and woods of Panama may refift the cur- 
rent of this Wind, and croud the vapours together 
there ; but a contrary Wind, upon a point between 
fouth and weft, may blow at the fame time upon the 
weftern fhore of America, which fhall force the va- 
pours back again. When fuch a rencounter happens, 
there muft be a wild uproar in the air about the Ca- 
ribbee iflands, and in all that traCt between South and 
North America, and the vapours in this circular mo- 
tion muft needs be furious on all fides, juft as it is in 
the water. 
For we fee in the confluence of two rivers, if their 
currents are rapid at the place where they fall in, they 
caufe violent eddies, whirl things about that are caft 
in them, fwallowing them up for a little time, and 
then throwing them back again. 
This fhews us the reafon, why heavy bodies are often 
tolled in the air by the whirling of hurricanes, and then 
dallied to the ground again ; for the air being a cir- 
cular motion, is with great fury tolled backwards and 
forwards, between the ground and the clouds ; for as 
the waters of the rolling fea do not run to the Ihores 
in an even ftream, but in fuch waves as dalh by fits 
and turns; fo the courfe of a violent Wind is in broken 
and diftinCt blafts. 
Such tempefts do not extend very far, though their 
bounds are uncertain, becaufe the neighbouring air 
giving way to them, they fpend themfelves in the 
progrefs of their motion. 
ThviS} 
