_Amissum ne crede diem, sunt altera nobis
Sidera, sunt orbes alii, luménque videbis
Purius, Elysiumque magis mirabere Solem._
And though this be not to be esteem'd as an Argument, yet I may take the
liberty I see others do, to quote the Poets when it makes for my purpose.
Lastly, To explain yet farther what I mean, I have adventur'd to adjoin
the following Scheme, (_Tab. 1. Fig. 3_) wherein the Earth is
represented by the outward Circle, and the three inward Circles are made
nearly proportionable to the Magnitudes of the Planets _Venus_, _Mars_
and _Mercury_, all which may be included within the Globe of Earth, and
all the Arches more than sufficiently strong to bear their weight. The
Concave of each Arch, which is shaded differently from the rest, I
suppose to be made up of Magnetical Matter; and the whole to turn about
the same common Axis _pp_, only with this difference, that the Outer
Sphere still moves somewhat faster than the Inner. Thus the Diameter of
the Earth being about eight thousand _English_ Miles, I allow five
hundred Miles for the thickness of its Shell, and another space of five
hundred Miles for a Medium between, capable of an immense Atmosphere for
the use of the Globe of _Venus_: _Venus_ again I give a Shell of the
same thickness, and leave as great a space between her Concave and
_Mars_; so likewise from _Mars_ to _Mercury_, which latter Ball we will
suppose solid, and about two thousand Miles Diameter. Thus I have shew'd
a possibility of a much more ample Creation, than has hitherto been
imagin'd; and if this seem strange to those that are unacquainted with
the Magnetical System, it is hop'd that all such will endeavour, first,
to inform themselves of the Matter of Fact, and then try if they can
find out a more simple Hypothesis, at least a less absurd, even in their
own Opinions. And whereas I have adventur'd to make these Subterraneous
Orbs capable of being Inhabited, 'twas done designedly for the sake of
those who will be apt to ask _cui bono_, and with whom Arguments drawn
from _Final Causes_ prevail much. If this short Essay shall find a kind
Acceptance, I shall be encourag'd to enquire farther, and to Polish this
rough Draft of a Notion till hitherto not so much as started in the
World, and of which we could have no Intimation from any other of the
_Phænomena_ of Nature.
Since this was written, a Discovery I have made in the Cœlestial
Motions, seems to render a farther Account of the Use of the Cavity of
the Earth, _viz._ To diminish the Specifick Gravity thereof, in respect
of the Moon; for I think I can demonstrate that the Opposition of the
Æther to the Motions of the Planets in long time becomes sensible; and
consequently the greater Body must receive a less Opposition than the
smaller, unless the Specifick Gravity of the smaller do proportionably
exceed that of the greater, in which case only they can move together;
so that the Cavity I assign in the Earth, may well serve to adjust its
weight to that of the Moon, for otherwise the Earth would leave the Moon
behind it, and she become another Primary Planet.
_An Historical Account of the _Trade Winds_ and _Monsoons_, observable
in the Seas between and near the _Tropicks_, with an attempt to
assign the Physical Cause of the said Winds, by Mr. _Ed. Halley_._
An exact Relation of the constant and periodical Winds, observable in
several Tracts of the Ocean, is a part of Natural History not less
desireable and useful, than it is difficult to obtain, and its
_Phænomena_ hard to explicate: I am not ignorant that several Writers
have undertaken this Subject, and although _Varenius_ (_Lib. 1. Chap.
21. Geo. Gen._) seems to have endeavour'd after the best information
from _Voyagers_, yet cannot his Accounts be admitted for accurate, by
those that shall attentively consider and compare them together, and
some of them are most evident Mistakes; which, as near as I can, I shall
attempt to rectify, having had the opportunity of conversing with
Navigators, acquainted with all parts of _India_, and having liv'd a
considerable time between the _Tropicks_, and there made my own Remarks.
The Substance of what I have collected is briefly as follows.
The Universal _Ocean_ may most properly be divided into three Parts,
_viz._ 1. The _Atlantick_ and _Æthiopick-Sea_. 2. The _Indian Ocean_.
3. The Great _South Sea_, or the _Pacifick Ocean_; and though these Seas
do all communicate by the South, yet as to our present purpose of the
_Trade Winds_, they are sufficiently separated by the interposition of
great Tracts of _Land_; the first lying between _Africa_ and _America_,
the second between _Africa_ and the _Indian Islands_, and _Hollandia
Nova_; and the last between the _Phillipine Isles_, _China_, _Japan_ and
_Hollandia Nova_ on the _West_, and the Coast of _America_ on the
_East_. Now following this natural division of the Seas, so will we
divide our History into three parts in the same order.
I. In the _Atlantick_ and _Æthiopick_ Seas between the _Tropicks_, there
is a general _Easterly Wind_ all the Year long, without any considerable
Variation, excepting that it is subject to be deflected therefrom, some
few Points of the Compass towards the _North_ or _South_, according to
the Position of the place. The Observations which have been made of
these Deflections, are the following.
1. That near the Coast of _Africa_, as soon as you have pass'd the
_Canary Isles_, you are sure to meet a fresh Gale of _North East_ Wind,
about the Latitude of 28 Degrees _North_, which seldom comes to the
_Eastwards_ of the _East North-East_, or passes the _North North-East_.
This Wind accompanies those bound to the Southward, to the Latitude of
ten North, and about a hundred Leagues from the _Guinea_ Coast, where,
till the fourth Degree of North Latitude, they fall into the Calms and
Tornadoes; of which more hereafter.
2. That those bound to the _Caribbee Isles_, find, as they approach the
_American_ side, that the aforesaid _North-East_ Wind becomes still more
and more Easterly, so as sometimes to be _East_, sometimes _East_ by
_South_, but yet most commonly to the _Northward_ of the _East_ a Point
or two, seldom more. 'Tis likewise observ'd, that the strength of these
Winds does gradually decrease, as you sail to the _Westwards_.
3. That the limits of the Trade and variable Winds, in this Ocean, are
farther extended on the _American_ side than the _African_; for whereas
you meet not with this certain Wind till after you have pass'd the
Latitude of twenty eight Degrees on this side; on the _American_ side it
commonly holds to thirty, thirty one, or thirty two Degrees of Latitude;
and this is verified likewise to the Southwards of the Æquinoctial, for
near the _Cape_ of _Good-Hope_ the limits of the Trade Winds, are three
or four Degrees nearer the Line, than on the Coast of _Brazile_.
4. That from the Latitude of four Degrees _North_, to the aforesaid
Limits on the _South_ of the Æquator, the Winds are generally and
perpetually between the _South_ and _East_, and most commonly between
the _South-East_ and _East_, observing always this Rule, That on the
_African_ side they are more _Southerly_, on the _Brazilian_ more
_Easterly_, so as to become almost due _East_, the little deflection
they have being still to the _Southwards_. In this part of the Ocean it
has been my fortune to pass a full Year, in an Employment that oblig'd
me to regard more than ordinary the Weather, and I found the Winds
constantly about the _South-East_, the most usual Point _S E b E_; when
it was Easterly, it generally blew hard, and was gloomy, dark, and
sometimes rainy Weather; if it came to the Southwards it was generally
Serene, and a small Gale next to a Calm, but this not very common. But I
never saw it to the Westwards of the South, or Northwards of the East.
5. That the Season of the Year has some small effect on these Trade
Winds, for that when the Sun is considerable to the Northwards of the
Æquator, the South-East Winds, especially in the Straight of this Ocean
(if I may so call it) between _Brazile_ and the Coast of _Guinea_, do
vary a Point or two to the Southwards, and the North-East become more
Easterly; and on the contrary, when the Sun is towards the Tropick of
_Capricorn_ the South-Easterly Winds become more Easterly, and the
North-Easterly Winds on this side the Line veere more to the Northwards.
6. That as there is no general Rule that admits not of some Exception,
so there is in this Ocean a Tract of Sea wherein the Southerly and
South-West Winds are perpetual, _viz._ all along the Coast of _Guinea_,
for above five hundred Leagues together, from _Sierra Leona_ to the Isle
of St. _Thomas_; for the South-East Trade Wind having pass'd the Line,
and approaching the Coast of _Guinea_ within eighty or 100 Leagues,
inclines towards the Shore, and becomes S. S. E. and by Degrees, as you
come nearer, it veeres about to South, S. S. W. and in with the Land
South-West, and sometimes West South-West; which Variation is better
express'd in the Mapp hereto annexed, (_Vide Plate 2_) than it can well
be in Words. These are the Winds which are observ'd on this Coast when
it blows true, but there are frequent Calms, violent sudden Gusts call'd
_Tornado's_, from all Points of the Compass, and sometimes unwholsome
foggy Easterly Winds, call'd _Hermitaa_ by the Natives, which too often
infest the Navigation of these parts.
7. That to the Northwards of the Line, between four and ten Degrees of
Latitude, and between the Meridians of _Cape Virde_, and of the
Eastermost Islands that bear that Name, there is a Tract of Sea wherein
it were improper to say there is any Trade Wind, or yet a Variable; for
it seems condemn'd to perpetual Calms, attended with terrible Thunder
and Lightning, and Rains so frequent, that our Navigators from thence
call this part of the Sea the _Rains_; the little Winds that are, be
only some sudden uncertain Gusts, of very little Continuance and less
Extent; so that sometimes each Hour you shall have a different Gale,
which dies away into a Calm before another succeed, and in a Fleet of
Ships in sight of one another, each shall have the Wind from a several
Point of the Compass; with these weak _Breezes_ Ships are oblig'd to
make the best of their way to the Southward through the aforesaid six
Degrees, wherein 'tis reported some have been detain'd whole Months for
want of Wind.
From the three last Observables is shewn the Reason of two notable
Occurents in the _East-India_ and _Guinea_ Navigations: The one is, why,
notwithstanding the narrowest part of the Sea between _Guinea_ and
_Brazile_ be about five hundred Leagues over, yet Ships bound to the
Southward, sometimes, especially in the Months of _July_ and _August_,
find a great difficulty to pass it. This happens because of the
South-East Winds, at that time of the Year commonly extending some
Degrees beyond the ordinary limit of four Degrees North Latitude, and
withal they come so much Southerly, as to be sometimes South, sometimes
a Point or two to the West; there remains then only to ply to Windward,
and if on the one side they stand away W. S. W. they gain the Wind still
more and more Easterly; but there is danger of not weathering the
_Brazilian_ Shoar, or at least the Shoals upon that Coast. But if upon
the other Tack they go away E. S. E. they fall into the Neighbourhood of
the Coast of _Guinea_, from which there is no departing without running
Easterly, as far as the Isle of St. _Thomas_, which is the constant
practice of all the _Guinea_ Ships, and which may seem very strange,
without the consideration of the sixth Remark, which shews the Reason of
it: For being in with the Coast, the Wind blows generally at S. W. and
W. S. W. with which Winds they cannot go to the Northward for the Land;
and on the other Tack they can lie no nearer the Wind than S. S. E. or
South; with these Courses they run off the Shoar, but in so doing they
always find the Winds more and more contrary; so that when near the
Shoar they could lie South, at a greater distance they can make their
way no better than S. E. and afterwards E. S. E. with which Courses they
fetch commonly the Isle of St. _Thomas_ and Cape _Lopez_, where finding
the Winds to the Eastward of the South, they keep them favourable, by
running away to the Westward in the South Latitude, of three or four
Degrees, where the S. E. Winds are perpetual.
For the sake of these general Winds, all those that use the
_West-Indian_ Trade, even those bound to _Virginia_, count it their best
Course to get as soon as they can to the Southwards, that so they may be
certain of a fair and fresh Gale to run before it to the Westwards; and
for the same Reason those homewards bound from _America_, endeavour to
gain the Latitude of thirty Degrees, as soon as possible, where they
first find the Winds begin to be variable; tho' the most ordinary Winds
in the Northern part of the _Atlantick_ Ocean come from between the
South and West.
As to those furious Storms call'd _Hurricanes_, which are, as it were,
peculiar to the _Caribbee_ Isles; and which so dreadfully afflict them
in the Month of _August_, or not much before or after, they do not so
properly belong to this place, both by Reason of their small continuance
and extent, as likewise because they are not Anniversary, some Years
having more than one, and sometimes for several Years together there
being none at all. But their Violence is so unconceivable, and their
other _Phænomena_ so surprising, that they merit well to be consider'd
apart.
What is here said, is to be understood of the Sea Winds at some distance
from the Land; for upon and near the Shoars, the Land and Sea Breezes
are almost every where sensible; and the great Variety which happens in
their Periods, Force and Direction, from the situation of the Mountains,
Vallies and Woods, and from the various Texture of the Soil, more or
less capable of retaining and reflecting Heat, and of exhaling or
condensing Vapours, is such, that it were an endless task, to endeavour
to account for them.
II. In the _Indian_ Ocean, the Winds are partly general, as in the
_Æthiopick_ Ocean, partly Periodical; that is, half the Year they blow
one way, and the other half near upon the opposite Points; and these
Points and Times of shifting are different in different parts of this
Ocean; the limits of each Tract of Sea, subject to the same Change or
_Monsoon_, are certainly very hard to determine, but the diligence I
have used to be rightly inform'd, and the care I have taken therein,
has, in a great measure, surmounted that Difficulty; and I am perswaded
that the following Particulars may be relied upon.
1. That between the Latitudes of ten Degrees and thirty Degrees South,
between _Madagascar_ and _Hollandia Nova_, the general Trade Wind about
the S. E. by E. is found to blow all the Year long, to all Intents and
Purposes after the same manner as in the same Latitudes in the
_Æthiopick_ Ocean, as it is describ'd in the fourth _Remark_ aforegoing.
2. That the aforesaid S. E. Winds extend to within two Degrees of the
_Æquator_, during the Months of _June_, _July_, _August_, &c. to
_November_; at which time between the _South Latitudes_ of three and ten
Degrees, being near the Meridian of the North end of _Madagascar_, and
between two and twelve South Latitude, being near _Sumatra_ and _Java_,
the contrary Winds from the N. W. or between the North and West, set in
and blow for half the Year, _viz._ from the beginning of _December_ till
_May_; and this _Monsoon_ is observ'd as far as the _Molucca_ Isles, of
which more anon.
3. That to the Northward of three Degrees South Latitude, over the whole
_Arabian_ or _Indian-Sea_ and Gulph of _Bengall_, from _Sumatra_ to the
Coast of _Africa_, there is another _Monsoon_, blowing from _October_ to
_April_ upon the North East Points; but in the other half Year, from
_April_ to _October_, upon the opposite Points of S. W. and W. S. W. and
that with rather more force than the other, accompanied with dark rainy
Weather, whereas the N. E. blows clear; 'tis likewise to be noted, that
the Winds are not so constant, either in strength or point in the Gulph
of _Bengall_, as they are in the _Indian-Sea_, where a certain and
steady Gale scarce ever fails. 'Tis also remarkable, that the S. W.
Winds in these Seas are generally more Southerly on the _African_ side,
more Westerly on the _Indian_.
4. That as an _Appendix_ to the last describ'd _Monsoon_, there is a
Tract of Sea to the Southwards of the Æquator, subject to the same
Changes of the Winds, _viz._ near the _African_ Coast, between it and
the Island _Madagascar_ or St. _Lawrence_, and from thence Northwards as
far as the Line; wherein from _April_ to _October_ there is found a
constant fresh S. S. W. Wind, which, as you go more Northerly, becomes
still more and more Westerly, so as to fall in with the W. S. W. Winds,
mention'd before, in those Months of the Year to be certain to the
Northward of the Æquator: What Winds blow in these Seas, for the other
half Year, from _October_ to _April_, I have not yet been able to obtain
to my full satisfaction, for that our Navigators always return from
_India_ without _Madagascar_, and so are little acquainted in this
Matter; the Account that has been given me is only this, that the Winds
are much Easterly hereabouts, and as often to the North of the true East
as to the Southwards thereof.
5. That to the Eastward of _Sumatra_ and _Malacca_, to the Northwards of
the Line, and along the Coast of _Cambodia_ and _China_, the _Monsoons_
blow North and South, that is to say, the N. E. Winds are much
Northerly, and the S. W. much Southerly: This Constitution reaches to
the Eastwards of the _Philippine_ Isles, and as far Northerly as
_Japan_. The Northern _Monsoon_ setting in, in these Seas, in _October_
or _November_, and the Southern in _May_, blowing all the Summer Months:
Here it is to be noted, That the Points of the Compass, from whence the
Wind comes in these Parts of the World, are not so fixt as in those
lately describ'd; for the Southerly will frequently pass a Point or two
to the Eastwards of the South, and the Northerly as much to the
Westwards of the North, which seems occasion'd by the great quantity of
Land, which is interspers'd in these Seas.
6. That in the same _Meridians_, but to the Southwards of the Æquator,
being that Tract lying between _Sumatra_ and _Java_ to the West, and
_New Guinea_ to the East, the same Northerly _Monsoons_ are observ'd,
but with this difference, that the inclination of the Northerly is
towards the N. West, and of the Southerly towards the S. E. but the
_plagæ venti_ are not more constant here than in the former, _viz._
variable five or six Points; besides the times of the Change of these
Winds, are not the same as in the _Chinese_ Seas, but about a Month or
six Weeks later.
7. That these contrary Winds do not shift all at once, but in some
places the time of the Change is attended with Calms, in others with
variable Winds; and it is particularly remarkable, that the end of the
Westerly _Monsoon_ on the Coast of _Coromandel_, and the two last Months
of the Southerly _Monsoon_ in the Seas of _China_, are very subject to
be tempestuous: The violence of these Storms is such, that they seem to
be of the Nature of the _West-India Hurricanes_, and render the
Navigation of these parts very unsafe about that time of the Year. These
Tempests are by our Seamen usually term'd, _The breaking up of the
Monsoons_.
By reason of the shifting of these Winds, all those that sail in these
Seas, are oblig'd to observe the Seasons proper for their Voyages, and
so doing they fail not of a fair Wind and speedy Passage; but if so be
they chance to out-stay their time, till the contrary _Monsoon_ sets in,
as it frequently happens, they are forc'd to give over the hopes of
accomplishing their intended Voyages, and either return to the Port from
whence they came, or else put in to some other Harbour, there to spend
the time till the Winds shall come favourable.
III. The third Ocean call'd _Mare Pacificum_, whose Extent is equal to
that of the other two (it being from the West Coast of _America_ to the
_Philippine_ Islands, not less than 150 Degrees of Longitude) is that
which is least known to our own or the Neighbour Nations; that
Navigation that there is on it, is by the _Spaniards_, who go Yearly
from the Coast of _New Spain_ to the _Manilha's_, but that but by one
beaten track; so that I cannot be so particular here as in the other
two. What the _Spanish_ Authors say of the Winds they find in their
Courses, and what is confirm'd by the old Accounts of _Drake_ and
_Cavendish_; and since by _Schooten_, who sail'd the whole breadth of
this Sea in the Southern Latitude of fifteen or sixteen Degrees, is,
that there is a great conformity between the Winds of this Sea, and
those of the _Atlantick_ and _Æthiopick_; that is to say, that to the
Northwards of the Æquator, the predominant Wind is between the East and
North-East; and to the Southwards thereof there is a constant steady
Gale between the East and South-East, and that on both sides the Line
with so much constancy, that they scarce ever need to attend the Sails,
and Strength, that it is rare to fail of crossing this vast Ocean in ten
Weeks time, which is about 130 Miles _per diem_; besides, 'tis said that
Storms and Tempests are never known in these parts: So that here is the
very best of Sailing; no want of a fresh fair Wind, and yet no danger of
having too much: Wherefore some have thought it might be as short a
Voyage to _Japan_ and _China_, to go by the Streights of _Magellan_, as
by the _Cape_ of _Good Hope_.
The Limits of these general Winds are also much the same as in the
_Atlantick_ Sea, _viz._ about the thirtieth Degree of Latitude on both
sides; for the _Spaniards_ homewards bound from the _Manilha's_, always
take the advantage of the Southerly _Monsoon_, blowing there in the
Summer Months, and run up to the Northwards of that Latitude, as high as
_Japan_, before they meet with variable Winds, to shape their Course to
the Eastwards. And _Schooten_ and others that have gone about by the
_Magellan_ Streights, have found the Limits of S. E. Winds, much about
the same Latitude to the Southwards; besides a farther _Analogy_ between
the Winds of this Ocean, and the _Æthiopick_, appears in that, upon the
Coast of _Peru_, they are always much Southerly, like as they are found
near the Shoars of _Angola_.
Thus far Matter of Fact, wherein if the information I have receiv'd be
not in all parts Accurate, it has not been for want of inquiry from
those I conceiv'd best able to instruct me; and I shall take it for a
very great Kindness if any Master of a Ship, or other Person, well
inform'd of the Nature of the Winds, in any of the aforemention'd parts
of the World, shall please to communicate their Observations thereupon;
so that what I have here Collected may be either confirm'd or amended,
or by the addition of some material Circumstances enlarg'd. It is not
the work of one, nor of few, but of a multitude of Observers, to bring
together the Experience requisite to compose a perfect and compleat
History of these Winds; however I am not much doubtful that I have err'd
in, or omitted any of the principal Observables, whatever lesser
Particulars may have escaped my Knowledge.
To help the Conception of the Reader in a manner of so much difficulty,
I believ'd it necessary to adjoin a Scheme, (_Plate 2._) shewing at one
view all the various Tracts and Courses of these Winds; whereby 'tis
possible the thing may be better understood, than by any verbal
Description whatsoever.
The Limits of these several Tracts are design'd every where by prickt
Lines, as well in the _Atlantick_ and _Æthiopick_, where they are the
boundaries of the Trade and variable Winds, as in the _Indian_ Ocean,
where they also shew the Extent of the several _Monsoons_. I could think
of no better way to design the Course of the Winds on the Map, than by
drawing rows of stroaks in the same Line that a Ship would move going
always before it; the sharp end of each little stroak pointing out that
part of the Horizon, from whence the Wind continually comes; and where
there are _Monsoons_, the rows of the stroaks run alternately backwards
and forwards, by which means they are thicker there than elsewhere. As
to the great South Sea, considering its vast Extent, and the little
Variety there is in its Winds, and the great _Analogy_ between them, and
those of the _Atlantick_ and _Æthiopick_ Oceans; besides, that the
greatest part thereof is wholly unknown to us; I thought it unnecessary
to lengthen the Map therewith.
In the foregoing History are contained several Problems, that merit well
the Consideration of our acutest Naturalists, both by reason of the
constancy of the Effect, and of the immense Extent thereof; near half
the Surface of the Globe being concerned. The chief of these Problems
are, 1. Why these Winds are perpetually from the East in the _Atlantic_
and _Æthiopick_; as likewise in the _Pacifick_ Ocean, between the
Latitudes of 30 North and South? 2. Why the said Winds extend no farther
with constancy than to the Latitude of 30 Degrees? 3. Why there should
be a constant South-Westerly Wind upon and near the Coast of _Guinea_?
4. Why in the North part of the _Indian_ Ocean, the Winds, which for one
half Year do agree with those of the other two Oceans, should change in
other half Year, and blow from the opposite Points; whilst the Southern
part of that Ocean follows the General Rule, and has perpetual Winds
about S. E? 5. Why in these General Trade-Winds it should be always
true, that to the Northward of the _Æquator_ it is inclin'd to the
Northwards of the East; and in South Latitudes, to the Southward
thereof? 6. Why in these Seas of _China_ there should be so great an
Inclination from the East to the North, more than elsewhere? with many
more, which it would be much easier to propose than answer.
But lest I should seem to propose to others, Difficulties which I have
not thought worth my own Time and Pains, take here the result of an
earnest Endeavour after the true reason of the aforesaid _Phænomena_;
wherein if I am not able to account for all Particulars, yet 'tis hoped
the Thoughts I have spent thereon, will not be judged wholly lost, by
the Curious in Natural Enquiries.
Wind is most properly defined to be the Stream or Current of the Air,
and where such a Current is perpetual and fixt in its Course, 'tis
necessary that it proceed from a permanent un-intermitting Cause.
Wherefore some have been inclin'd to propose the diurnal _Rotation_ of
the Earth upon its _Axis_, by which, as the _Globe_ turns Eastwards, the
loose and _fluid_ Particles of the Air, being so exceeding light as they
be, are left behind, so that in respect of the Earths _Surface_ they
move Westwards, and become a constant Easterly Wind. This Opinion seems
confirm'd, for that these Winds are found only near the _Æquinoctial_,
in those Parallels of Latitude where the diurnal Motion is swiftest; and
I should readily assent to it, if the constant Calms in the _Atlantick_
Sea, near the _Æquator_, the Westerly Winds near the Coast of _Guinea_;
and the _Periodical_ Westerly _Monsoons_ under the _Æquator_ in the
_Indian_ Seas, did not declare the insufficency of that _Hypothesis_.
Besides the Air being kept to the Earth by the Principle of _Gravity_,
would acquire the same degree of _Velocity_ that the Earths _Surface_
moves with, as well in respect of the diurnal _Rotation_, as of the
Annual about the Sun, which is about thirty times swifter.
It remains therefore to substitute some other Cause, capable of
producing a like constant Effect, not liable to the same Objections, but
agreeable to the known Properties of the Elements of Air and Water, and
the Laws of the Motion of fluid Bodies. Such an one is, I conceive, the
Action of the Sun Beams upon the Air and Water, as he passes every Day
over the Oceans, consider'd together with the Nature of the Soil, and
Situation of the adjoining Continents: I say therefore, first, that
according to the Laws of _Staticks_, the Air which is less rarified or
expanded by heat, and consequently more ponderous, must have a Motion
towards those parts thereof, which are more rarified, and less
ponderous, to bring it to an _Æquilibrium_; and secondly, That the
Presence of the Sun continually shifting to the Westwards, that part
towards which the Air tends, by reason of the Rarifaction made by his
greatest _Meridian_ Heat, is with him carried Westward, and consequently
the tendency of the whole Body of the lower Air is that way.
Thus a general Easterly Wind is formed, which being impressed upon all
the Air of a vast Ocean, the Parts impel one the other, and so keep
moving till the next return of the Sun, whereby so much of the Motion as
was lost, is again restored, and thus the Westerly Wind is made
perpetual.
From the same Principle it follows, that this Easterly Wind should on
the North side of the _Æquator_, be to the Northwards of the East, and
in South Latitudes to the Southwards thereof; for near the _Line_, the
Air is much more rarified, than at a greater distance from it; because
of the Sun twice in a Year Vertical, and at no time distant above 23
Degr. and a half; at which distance the Heat, being as the Sine of the
Angle of Incidence, is but little short of that of the perpendicular
Ray. Whereas under the Tropicks, though the Sun stay long Vertical, yet
he is as long 47 Degr. off; which is a kind of Winter, wherein the Air
so cools, as that the Summer-heat cannot warm it to the same degree with
that under the Æquator. Wherefore the Air to the Northwards and
Southwards, being less rarified than that in the middle, it follows,
that from both sides it ought to tend towards the Æquator: This Motion
compounded with the former Easterly Wind, answers all the _Phænomena_ of
the general Trade-winds; which, if the whole Surface of the Globe were
Sea, would undoubtedly blow all round the World, as they are found to do
in the _Atlantick_, and _Æthiopick_ Oceans.
But seeing that so great Continents do interpose, and break the
continuity of the Oceans, regard must be had to the Nature of the Soil,
and the Position of the high Mountains, which I suppose the two
principal Causes of the several Variations of the Winds, from the former
general Rule: For if a Country lying near the Sun, prove to be flat,
sandy, low Land, such as the Desarts of _Lybia_ are usually reported to
be, the Heat occasion'd by the Reflection of the Suns Beams, and the
retention thereof in the Sand, is incredible to those that have not felt
it; whereby the Air being exceedingly rarified, it is necessary that the
cooler and more dense Air should run thitherwards to restore the
_Æquilibrium_: This I take to be the cause, why near the Coast of
_Guinea_ the Wind always sets in upon the Land, blowing Westerly instead
of Easterly, there being sufficient Reason to believe, that the Inland
Parts of _Africa_ are prodigiously hot, since the Northern Borders
thereof were so intemperate, as to give the Ancients cause to conclude,
that all beyond the _Tropick_, was made uninhabitable by excess of Heat:
From the same Cause it happens, that there are so constant Calms in that
part of the Ocean, called the _Rains_, (described in the 7th Remark on
the _Atlantick_ Sea) for this Tract being placed in the middle, between
the Westerly Winds blowing on the Coast of _Guinea_, and the Easterly
Trade-winds, blowing to the Westwards thereof, the tendency of the Air
here, is indifferent to either, and so stands in _Æquilibrio_ between
both; and the weight of the incumbent Atmosphere being diminished by the
continual contrary Winds blowing from hence, is the reason that the Air
here holds not the copious Vapour it receives, but lets it fall into
frequent Rains.
But as the cool and dense Air, by reason of its greater Gravity, presses
upon the hot and rarified, 'tis demonstrative that this latter must
ascend in a continued Stream as fast it rarifies; and that being
ascended, it must disperse it self to preserve the _Æquilibrium_: that
is, by a contrary Current, the upper Air must move from those Parts
where the greatest Heat is: So by a kind of Circulation, the North-East
Trade-Wind below, will be attended with a South-Westerly above, and the
South-Easterly with a North-West Wind above; that this is more than a
bare Conjecture, the almost instantaneous Change of the Wind to the
opposite Point, which is frequently found in passing the limits of the
Trade-winds, seems to assure us; but that which above all confirms this
_Hypothesis_ is the _Phænomenon_ of the _Monsoons_, by this means most
easily solved, and without it hardly explicable.
Supposing therefore such a Circulation, as above, 'tis to be considered
that to the Northward of the _Indian_ Ocean there is every where Land
within the usual limit of the Latitude of 30, _viz._ _Arabia_, _Persia_,
_India_, _&c._ which for the same reason as the _Mediterranean_ Parts of
_Africa_, are subject to unsufferable Heats when the Sun is to the
North, passing nearly Vertical; but yet are temperate enough when the
Sun is removed towards the other _Tropick_; because of a ridge of
Mountains at some distance within the Land, said to be frequently in
Winter cover'd with Snow, over which the Air, as it passes, must needs
be much chill'd. Hence it comes to pass, that the Air coming according
to the general Rule, out of the N. E. in the _Indian_ Seas, is sometimes
hotter, sometimes colder, than that which by this Circulation is
return'd out of the S. W. and by consequence, sometimes the under
Current or Wind, is from the N. E. sometimes from the S. W.
That this has no other Cause, is clear from the times wherein these
Winds set in, _viz._ in _April_, when the Sun begins to warm those
Countries to the North, the S. W. _Monsoon_ begins, and blows during the
Heats till _October_; when the Sun being retir'd, and all things growing
cooler Northward, and the Heat increasing to the South, the North-East
Winds enter and blow all the Winter till _April_ again. And it is
undoubtedly from the same Principle that to the Southwards of the
Æquator, in part of the _Indian_ Ocean, the North-West Winds succeed to
the South-East, when the Sun draws near the _Tropick_ of _Capricorn_;
but I must confess, that in this latter occurs a difficulty, not well to
be accounted for, which is, why this Change of the _Monsoons_ should be
any more in this Ocean, than in the same Latitudes in the _Æthopick_,
where there is nothing more certain than a S. E. Wind all the Year.
'Tis likewise very hard to conceive why the limits of the Trade-wind
should be fixt, about the thirtieth Degree of Latitude all round the
Globe; and that they should so seldom transgress or fall short of those
bounds; as also that in the _Indian_ Sea, only the Northern Part should
be subject to the changeable _Monsoons_, and in the Southern there be a
constant S. E.
These are Particulars that merit to be consider'd more at large, and
furnish a sufficient Subject for a just Volume, which will be a very
commendable Task for such, who being us'd to Philosophick Contemplation,
shall have leisure to apply their serious Thoughts about it.
[Illustration: _Plate 2 pag. 80_
A new & Correct SEA CHART
of the
WHOLE WORLD
_Shewing the _Variations_ of yᵉ
_COMPASS_
as they were found Año 1700
with a View of the Generall
and Coasting _Trade Winds_
and _Monsoons_ or shifting
_Trade Winds_
by the Direction of
_Capᵗ. Edm. Halley_._]
_A Discourse of the Rule of the Decrease of the height of the _Mercury_
in the Barometer, according as Places are elevated above the Surface
of the Earth; with an Attempt to discover the true Reason of the
Rising and Falling of the _Mercury_, upon Change of Weather. By _Edm.
Halley_._
The Elastick Property of the Air has been long since made out, by
Experiments before the _Royal Society_, and elsewhere; and the
Resistance of its Spring is found to be nearly equal to the Weight or
Force that compresses it; as also, that the Spaces the same Air
occupies, under differing Pressures, are reciprocally as those
Pressures: It has been shewn likewise by undoubted Experiment, that the
specifick Gravity of the Air, near the Earth's Surface to that of Water,
was once as 1 to 840; again as 1 to 852; and a third time, in a very
large Vessel holding 10 Gallons, as 1 to 860; all which, considering the
Difficulty of the Experiment, agree well enough, the Mercury standing at
all those times about 29 Inches ¾: But by Reason 'twas Summer-weather,
and consequently the Air rarified, when all these were tried, we may
without sensible Error say in round numbers, that the Barometer standing
at 30 Inches, and in a mean State of Heat and Cold, the specifick
Gravity of the Air to Water, is as 1 to 800. By the like Trials the
weight of Mercury to Water, is as 13½ to 1, or very near it; so that
the weight of Mercury to Air, is as 10800 to 1; and a Cylinder of Air of
10800 Inches or 900 Feet, is equal to an Inch of Mercury; and were the
Air of an equal density like Water, the whole Atmosphere would be no
more than 5,1 Miles high, and in the Ascent of every 900 Feet the
Barometer would sink an Inch. But the Expansion of the Air increasing in
the same proportion as the incumbent weight of the Atmosphere decreases;
that is, as the Mercury in the Barometer sinks; the upper Parts of the
Air are much more rarified than the lower, and each Space answering to
an Inch of Quicksilver, grows greater and greater; so that the
Atmosphere must be extended to a much greater height. Now, upon these
Principles, to determine the height of the Mercury at any assigned
height in the Air; and _è contra_, having the height of the _Mercury_
given, to find the height of the Place where the Barometer stands, are
Problems not more difficult than curious; and which I thus resolve.
The Expansions of the Air being reciprocally as the heights of the
Mercury, it is evident, that by the help of the Curve of the _Hyperbola_
and its _Asymptotes_, the said Expansions may be expounded to any given
height of the _Mercury_: For by the 65th _Prop. lib. 2. Conic.
Mydorgii_, _the Rectangles_, _ABCE_, _AKGE_, _ALDE_, _&c._ (in _Plate 1.
Fig. 4._) are always equal, and consequently the sides, _CB_, _GK_,
_LD_, &c. are reciprocally as the sides _AB_, _AK_, _AL_, &c. If then
the Lines _AB_, _AK_, _AL_, be supposed equal to the heights of the
_Mercury_, or the pressures of the Atmosphere, the Lines _CB_, _GK_,
_LD_, answering thereto, will be as the Expansions of the Air under
those Pressures, or the Bulks that the same quantity of Air will occupy;
which Expansions being taken infinitely many, and infinitely little,
(according to the Method of Indivisibles) their Summ will give the
Spaces of Air between the several heights of the _Barometer_; that is to
say, the Summ of all the Lines between _CB_ and _KG_, or the _Area_
_CBKG_, will be proportioned to the Distance or Space intercepted
between the Levels of two Places in the Air, where the _Mercury_ would
stand at the heights represented by the Lines _AB_, _AK_; so then the
Spaces of Air answering to equal Parts of _Mercury_ in the _Barometer_,
are as the _Area's_ _CBKG_, _GKLD_, _DLFM_, &c. These _Area's_ again
are, by the Demonstration of _Gregory_ of St. _Vincent_, proportionate
to the _Logarithms_ of the Numbers expressing the _Rationes_ of _AK_ to
_AB_, of _AL_ to _AK_, of _AM_ to _AL_, &c. So then by the common Table
of _Logarithms_, the height of any Place in the _Atmosphere_, having any
assign'd height of the _Mercury_, may most easily be found: For the Line
_CB_ in the _Hyperbola_, whereof the _Area's_ design the _Tabular
Logarithms_, being 0,0144765; 'twill be, as 0,0144765, to the difference
of the _Logarithms_ of 30, or any other lesser Number, for 900 Feet, or
the Space answering to an Inch of _Mercury_, if the Air were equally
prest with 30 Inches of _Mercury_, and every where alike, to the height
of the _Barometer_ in the Air, where it will stand at that lesser number
of Inches: And by the Converse of this Proportion may the height of the
_Mercury_ be found, having the Altitude of the Place given. From these
Rules I deriv'd the following Tables.
_A Table shewing the Altitude,
to given heights of the _Mercury_._
Inch. Feet.
30 0
29 915
28 1862
27 2844
26 3863
25 4922
20 10947
15 18715
10 29662
5 48378
1 91831
0.5 110547
0.25 129262
0.1 29 mil. or 154000
0.01 41 m. or 216169
0.001 53 m. 278338
_A Table shewing the heights of the
_Mercury_, at given Altitudes._
Feet. Inch.
0 30 00
1000 28 91
2000 27 86
3000 26 85
4000 35 87
5000 feet 24 93
1 mile 24 67
2 20 29
3 16 68
4 13 72
5 11 28
10 4 24
15 1 60
20 0 95
25 0 23
30 0 08
40 0 012
Upon these Suppositions it appears, that at the height of 41 Miles the
Air is so rarified, as to take up 3000 times the Space it occupies here,
and at 53 Miles high it would be expanded above 30000 times; but it's
probable that the utmost Power of its Spring cannot exert it self, to so
great an Extension, and that no part of the _Atmosphere_ reaches above
45 Miles from the Surface of the Earth.
This seems confirm'd from the Observations of the _Crepusculum_, which
is observ'd commonly to begin and end when the Sun is about 18 Degrees
below the Horizon; for supposing the Air to reflect light from its most
rarified Parts, and that as long as the Sun illuminates any of its
_Atoms_, they are visible to an Eye not intercepted by the Curvity of
the Earth, it will follow from _Fig. 5. Plate 1._ that the proportion of
the height of the whole Air, to the Semi-diameter of the Earth, is much
about, as 1 to 90, or as the excess of the _Secant_ of about 8½
Degrees to the _Radius_. For if _E_ be the Eye of the Observer, _S_ a
Place where the Sun sets at the end of Twilight in _E_, and the Arch
_ECS_, or _TCA_, be found 18 Degrees, the excess of the _Secant_ of half
thereof _ECH_, would be the height of the Air, _viz._ _GH_: But the Beam
of the Sun _ASH_, and the Visual Ray _EH_, do each of them suffer a
Refraction of about 32 or 33 Minutes, whereby being bent inwards from
_H_ towards _G_, the height of the Air need not be so great as if they
went streight; and having from the Angle _ECS_ taken the double
Refraction of the _Horizontal Ray_, the half of the Remainder will be
8½ Degrees _circiter_, whose _Secant_ being 10,111, it follows, that
as 10000 to 111, so the Semi-diameter of the Earth supposed 4000 Miles,
to 44,4 Miles; which will be the height of the whole Air, if the Places
_E_, _S_, whose visible Portions of the _Atmosphere_ _ERZH_, and _SHKB_,
just touch one the other, be 18 Degrees asunder.
At this height the Air is expanded into above 3000 times the space it
occupies here, and we have seen the Experience of condensing it into the
60th part of the same Space, so that it should seem, that the Air is a
Substance capable of being compressed into the 180000th part of the
Space it would naturally take up, when free from pressure. Now what
Texture or Composition of Parts shall be capable of this great Expansion
and Contraction, seems a very hard Question; and which, I suppose, is
scarce sufficiently accounted for, by comparing it to Wool, Cotten, and
the like springy Bodies.
Hitherto I have only consider'd the _Air_ and _Atmosphere_, as one
unalter'd Body, as having constantly at the Earth's Surface the 800th
part of the weight of Water, and being capable of Rarifaction and
Condensation _in infinitum_; neither of which _Hypotheses_ are rigidly
true: For here in _England_ it is notoriously known, that the weight of
the whole _Atmosphere_ is various, being counterpoised sometimes by
28½ Inches of _Mercury_, and at other times by no less than 30½;
so that the under parts being pressed by about a 15th part, less weight,
the _specifick_ Gravity of the Air upon that score will sometimes be a
15th part lighter than another; besides Heat and Cold, does very
considerably dilate and contract the Air, and consequently alter its
Gravity; to which add the mixture of _Effluvia_, or steams arising from
almost all Bodies, which assimulating into the Form of Air, are kept
suspended therein, as Salts dissolv'd in Liquors, or Metals in corroding
_Menstrua_; which Bodies being all of them very much heavier than Air,
their Particles by their Admixture must needs encrease the weight of
that Air they lie incorporated withal, after the same manner as melted
Salts do augment the specifick Gravity of Water. The other Consideration
is, that the Rarifaction and Condensation of the Air is not precisely
according to the proportion here laid down; for the Experiment very
nearly agrees thereto, as may be seen in the 58th Chapter of Mr.
_Hook's_ _Micrography_; yet are the Condensations not possible beyond
certain degrees: For being compressed into an 800th part of the Space it
takes up here, its consistence would be equally dense with that of
Water; which yields not to any force whatsoever, as hath been found by
several Experiments tried here, and at _Florence_, by the _Academia del
Cimento_. Nor can the Rarifaction proceed _in infinitum_; for supposing
the Spring whereby it dilates it self, occasion'd by what Texture of
Parts you please, yet must there be a determinate Magnitude of the
natural State of each Particle, as we see it is in Wool, and the like,
whose Bodies being compressable into a very small Space, have yet a
determinate bulk which they cannot exceed, when free'd from all manner
of Pressure.
These Objections being true, do disturb the Geometrical Accuracy of
these Conclusions, drawn from the specifick Gravity of the Air observ'd
at any time; but the Method here shewn will compute by a like
Calculation, the heights of the Quick-silver, and the Rarifactions of
the Air from any assign'd height of the _Barometer_ at the Earth's
Surface, and any specifick Gravity given. As to the Condensation and
Rarifaction by Heat and Cold, and the various mixture of Aqueous and
other Vapours, these two Objections seem generally to compensate each
other; for when the Air is rarified by Heat, they are raised most
copiously; so that though the Air properly so call'd, be expanded, and
consequently lighter, yet the _Interstices_ thereof being crouded full
of Vapours of much heavier Matters, bulk for bulk, the weight of the
_Compositum_ may continue much the same, at least a most curious
Experiment made by the Ingenious Mr. _John Caswell_, of _Oxford_, upon
the top of _Snowdon_ Hill, in _Carnarvanshire_, seems to prove, that the
first Inches of _Mercury_ have their Portions of Air near enough to what
I now determine: For the height of the Hill being 1240 Yards, or very
near it, he found the _Mercury_ to have subsided to 25,6 Inches, or 4
inches below the mean Altitude thereof at the Level of the Sea, (which
is a greater difference than has been found in any of our former
Experiments,) and the Space answering to 4 Inches, by my Calculation,
should be 1288 Yards; and it agrees as well with the Observations in the
Appendix to Mr. _Pascall_'s Book, _del Equilibre des Liqueurs_, made on
the high Hill in _Auvergne_, call'd _le puy de Domme_. So that the
Rarifaction and Vapours seem not to have alter'd considerably, the
Gravity of the under Parts of the _Air_; and much above the height where
these Experiments were made, do few Vapours ascend, and the Cold is such
that the Snow lies continually, so that for the more elevated Parts of
the Sphere of _Air_, there is much less Reason to doubt.
But now we have had occasion to mention the difference there is between
the height of the _Mercury_ at one time, from the height thereof at
another, it may not be unacceptable to offer at some Reasons for the
said difference; which, at least to my self, seem to have some
appearance of Truth. _First_, Then it's undoubtedly demonstrable, that
the height of the Cylinder of _Mercury_ is equal to the weight of the
whole incumbent Air, and consequently that that whole is sometimes a
fifteenth more than at other times; which cannot otherwise be, but by
the access of new Matter when 'tis heavy, and its diminution when 'tis
light; that _Hypothesis_ therefore that shews how the Air shall be
encreased or diminished, in any particular place, will give a Reason for
the greater and lesser height of the _Mercury_ in the _Baroscope_: But
to direct us in the choice of the several Causes, which may be assign'd
for the Increase and Decrease of the Air, 'twill not be unnecessary to
enumerate some of the principal Observations made upon the _Barometer_,
most whereof are sufficiently known already to all those that are
curious in these Matters.
The _First_ is, That in calm Weather, when the Air is inclin'd to Rain,
the _Mercury_ is commonly low.