MECHANIC S. 
738 
from the north towards the equator will not continue 
upon the fame meridian, but it will be left behind; that 
is, in refpeft to the earth’s furface, it will have a motion 
from the eaft; and thefe two motions combined produce 
a north-eaft wind on the north fide of the equator. And 
in like manner, there mult be a foutli-eaft wind on the 
fouth fide. The air which is thus continually moving 
from the poles to the equator, being rarefied when it comes 
there, afcends to the top of the atmofphere, and then re¬ 
turns back to tlie poles. This folution is given by Mr. 
Hadley, in the Phil. Tranf. vol. xxxix. 
The periodical winds are fuppofed to be owing to the 
courfe of the fun northward and fouthward of the equa¬ 
tor. Dr. Halley explains them- thus : 
“ Seeing that fo great continents do interpofe and break 
the continuity of the ocean, regard rnuft be had to the 
nature of the foil and the pofition of the high mountains, 
which I,fuppofe the two principal caufes of the feveral 
variations of the winds from the former general rule: 
for, if a country lying near the fun prove to be fiat, 
fandy, low land, fuch as the Deferts of Libya are ufually 
reported to be, the heat occafioned by the reflection of 
the fun’s beams, and the retention thereof in the find, is 
incredible to thofe that have not felt it; whereby the air 
being exceedingly rarefied, it is neceflary that the cooler 
and more denfe air (hould run thitherward to reftore the 
equilibrium. This I take to be the caufe why near the 
coaft of Guinea the wind always fets in upon the land, 
blowing wefterly in Head of eafterly ; there being fufficient 
reafon to believe, that the inland parts of Africa are pro- 
digioufly hot, (nice the northern borders thereof were fo 
intemperate as to give the ancients caufe to conclude that 
all beyond the tropic was made uninhabitable by excefs 
of heat. From the fame caufe it happens that there are 
fuch confiant calms in that part of the ocean, called the 
JRaii/s. For, this trad: being placed in the middle, be¬ 
tween the wefterly winds blowing on the coaft of Guinea 
and the eafterly trade-winds blowing to the weftwards 
thereof, the tendency of the air here is indifferent to 
either, and fo (lands in equilibrio between both; and the 
weight of the incumbent atmofphere being diminifhed by 
the continual contrary winds blowing from hence, is the 
reafon that the air here holds not the copious vapour it 
receives, but lets it fall in fo frequent rains, 
“ As the cool and denfe air, by reafon of its greater 
gravity, preffes upon the hot and rarefied, it is demonttra- 
tive that this latter mull afcend in a continual ftream as 
fail as it is rarefied ; and that, being afcended, it mull 
difperfe itfelf to preferve the equilibrium, that is, by a 
contrary current, the upper air mud move from thofe 
parts where the greateft heat is: fo, by a kind of cir¬ 
culation, the north-eaft trade-wind below will be at¬ 
tended with a fouth-weft above, and the fouth-eaft with 
a north-weft wind above. And that this is more than a 
bare conjecture, the almoft inftantaneous change of the 
wind to tlie oppofite point, which is frequently found in 
palling the limits of the trade-winds, feems to alfure us ; 
but that which above all confirms this hypothecs, 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 confi- 
dered, that to the northward of the Indian Ocean there 
is every-where land within the ufual limits of the lati¬ 
tude of 30 0 ; viz. Arabia, Perfia, India, See. which, for 
the fame reafon as the mediterranean parts of Africa, are 
fubjeft to unfufferable heats when the fun is to the north, 
palling nearly vertical, but yet are temperate enough 
when the fun is removed towards the other tropic; be- 
caufe of a ridge of mountains at fome diltance within the 
land, faid to be frequently in winter covered with fnow, 
over which the air, as it palfes, muft needs be much 
chilled. Hence it comes to pals, that the air coming, 
according to the general rule, out of the north-eaft in the 
Indian feas, 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, as is clear from the times wherein thefe 
winds fet in, viz. in April, when the fun begins to warm 
thofe countries to the north, the fouth-weft monfoon be¬ 
gins, and blows during the heats till October, when, the 
fun being retired, and all things growing cooler north¬ 
ward, and the heat increafing to the fouth, the north-eaft 
winds enter, and blow all the winter till April again. 
And it is undoubtedly from the fame principle, that to 
the fouthward of the equator, in part of the Indian Ocean, 
the north-weft wind lucceeds the fouth-eaft when the fun 
draws near the tropic of Capricorn. But I mutt confefs 
that in this latter occurs a difficulty not well to be ac¬ 
counted for; which is, why this change of the monfoons 
(hould be any more in this ocean than in the fame lati¬ 
tudes in the Ethiopic, where there is nothing more cer¬ 
tain than a fouth-eaft wind all the year. It is likewife 
very hard to conceive, why the limits of the trade-wind 
ftiould he fixed about the 30th degree of latitude all round 
the globe; and that they (hould fo feldom tranfgrefs or 
fall Ihort of tiiofe bounds; as alfo that in the Indian Sea, 
only, the northern part (hould be fubject to-the change¬ 
able monfoons, and in the fouthern there he a conftant 
fouth-eaft.” 
We may further add, that the caufes mentioned in the 
laft article muft here alfo operate. There may perhaps 
be fome cafes of thefe periodical winds, which we cannot 
fee altogether a correct folution of; but, if all the cir- 
cumftances of fituation, heat, cold, See. were known, there 
is no reafon to doubt but that they might be accounted 
for from the principles here delivered. 
We may further obferve, in refpeft to the direftion in 
which winds blow, that, if a current fet off in any one 
direction, north-eaft for inftance, and move in a great 
circle, it will not continue to move on that point of the 
compafs, becaufe a great circle will not meet all the meri¬ 
dians at the fame angle, the meridians not being parallel. 
This circumftance muft therefore enter into our confede¬ 
ration in eflimating the direction of the wind. High 
mountains are alfo obferved to turn the winds into a par¬ 
ticular courfe. On the lake of Geneva there are only 
two winds; that is, eithetup or down the valley. And 
the like is known to happen at other fuch places. 
The conftant and periodical winds blow only at fea ; at 
land the wind is always variable. 
Befides the winds already mentioned, there are others, 
called land and fea breezes. The air over the land being 
hotter during the day than the air over the fea, a current 
of air will fet in from the fea to the land by day; but, 
the air over the land being colder than that over the fea 
at night, the current at that time will be from the land 
to the fea. This is very remarkable in iflands fituated 
between the tropics. 
Mr. Clare exemplifies this by the following experiment: 
In the middle of a veflel of water place a water-plate of 
warm water; the water in the veflel reprefenting the ocean, 
and the plate the ifland rarefying the air over it. Then 
hold a lighted candle over the cold water, and blow it 
out, and the fmoke will move towards the plate. But if 
the plate be cold, and the ambient fluid be warm, the 
fmoke will move in the contrary direction. 
Dr, Derham, from repeated obfervations upon the mo¬ 
tion of light downy feathers, found that the greateft ve¬ 
locity of wind was not above 60 miles in an hour. But 
Mr. Brice jultly obferves, that fuch experiments muft he 
fubjecl to great inaccuracy, as the feathers cannot pro¬ 
ceed in a ffraight line; he therefore eftimates the velocity 
by means of the fliadow of a cloud over the earth ; by 
which he found, that in a great ftorm the wind moves 
63 miles in an hour; when it blows a frefh’gale, at the 
rate of 21 miles an hour; and in a fin all breeze, at the 
rate of about io miles in an hour. But this method 
takes 
