Chapter IV: Appendix (3)

As to the quantity of this Motion it is almost impossible to define it, both from the Nature of this kind of Observation, which cannot be very accurately perform'd, as also from the small time these Variations have been observ'd, and their Change discover'd. It appears by all Circumstances, that its Period is of many Centuries of Years, and as far as may be collected from the Change of the Place, where there was no Variation, by reason of the Equilibre of the two Southern Magnetical Poles, _viz._ from _Cape d' Agulhas_ to the Meridian of St. _Helena_ (which is about 23 _degr._ in about ninety Years) and of the place where the Westerly Variation is in its ἀκμὴ or greatest Deflection, being about half so much, _viz._ from the Isle of _Diego Roiz_ to the South West Parts of _Madagascar_. We may with some Reason conjecture, that the _American_ Pole has mov'd Westwards forty six Degrees in that time, and that the whole Period thereof is perform'd in seven hundred Years, or thereabouts; so that the nice Determination of this, and of several other Particulars in the Magnetick System is reserv'd for remote Posterity; all that we can hope to do, is to leave behind us Observations that may be confided in, and to propose an Hypothesis which after Ages may examine, amend or refute. Only here I must take leave to recommend to all Masters of Ships, and all others, Lovers of Natural Truths, that they use their utmost Diligence to make, or procure to be made, Observations of these Variations in all parts of the World, as well in the North as South Latitude (after the laudable Custom of our _East India_ Commanders) and that they please to communicate them to the _Royal Society_, in order to leave as compleat a History as may be to those that are hereafter to compare all together, and to compleat and perfect this abstruse Theory.

And by the way it will not be amiss to amend a receiv'd Error in the Practice of observing the Variation, which is, to take it by the Amplitude of the Rising and Setting Sun, when his Center appears in the visible Horizon; whereas he ought to be observ'd when his under Limb is still above the Horizon about ⅔ of his Diameter, or twenty Minutes, upon the score of the Refraction, and the height of the Eye of the Observer above the Surface of the Sea: Or else they are to work the Amplitudes as they do the Azimuth, reckoning the Suns Distance from the Zenith 90° 36': This, though it be of little consequence near the Æquinoctial, will make a great Error in high Latitudes, where the Sun rises and sets obliquely.

But to return to our Hypothesis, In order to explain the Change of the Variations, we have adventur'd to make the Earth hollow, and to place another Globe within it; and I doubt not but this will find Opposers enough. I know 'twill be Objected, That there is no Instance in Nature of the like thing; that if there was such a middle Globe it would not keep its place in the Center, but be apt to deviate therefrom, and might possibly chock against the Concave Shell, to the ruin, or at least endammaging thereof; That the Water of the Sea would perpetually leak through, unless we suppose the Cavity full of Water; That were it possible, yet it does not appear of what use such an inward Sphere can be of, being shut up in Eternal Darkness, and therefore unfit for the Production of Animals or Plants; with many more Objections, according to the Fate of all such new Propositions.

To these, and all other that I can foresee, I briefly Answer, That the Ring environing the Globe of _Saturn_ is a notable Instance of this kind, as having the same common Center, and moving along with the Planet, without sensibly approaching him on one side more than the other. And if this Ring were turn'd on one of its Diameters, it would then describe such a Concave Sphere as I suppose our External one to be. And since the Ring, in any Position given, would, in the same manner, keep the Centre of _Saturn_ in its own, it follows, that such a Concave Sphere may move with another included in it, having the same common Centre. Nor can it well be suppos'd otherwise, considering the Nature of Gravity; for should these Globes be adjusted once to the same common Centre, the Gravity of the parts of the Concave would press equally towards the Centre of the inner Ball, which equality must necessarily continue till some External Force disturb it, which is not easie to imagine in our Case. This perhaps I might more intelligibly express, by saying that the inner Globe being posited in the Centre of the Exteriour, must necessarily ascend which way soever it move; that is, it must overcome the force of Gravity pressing towards the common Centre, by an impulse it must receive from some outward Agent; but all outward Efforts being sufficiently fenc'd against by the Shell that surrounds it, it follows, that this _Nucleus_ being once fixt in the common Centre, must always there remain.

As to the leaking of the Water through this Shell, when once a passage shall be found for it to run through, I must confess it is an Objection seemingly of weight; but when we consider how tightly great Beds of Chalk or Clay, and much more Stone do hold Water, and even Caves arch'd with Sand; no Man can doubt but the Wisdom of the Creator has provided for the Macrocosm by many more ways than I can either imagine or express, especially since we see the admirable and innumerable Contrivances wherewith each worthless Individual is furnish'd both to defend it self, and propagate its Species. What Curiosity in the Structure, what Accuracy in the Mixture and Composition of the parts, ought not we to expect in the Fabrick of this Globe, made to be the lasting Habitation of so many various Species of Animals, in each of which there want not many Instances that manifest the boundless Power and Goodness of their Divine Author; and can we then think it a hard Supposition, that the Internal Parts of this Bubble of Earth should be replete with such Saline and Vitriolick Particles as may contribute to Petrefaction, and dispose the transuding Water to shoot and coagulate into Stone, so as continually to fortifie, and, if need were, to consolidate any breach or flaw in the Concave Surface of the Shell.

And this perhaps may not without Reason be suppos'd to be the final Cause of the admixture of the Magnetical Matter in the Mass of the Terrestrial parts of our Globe, _viz._ To make good and maintain the Concave Arch of this Shell: For by what the Excellent Mr. _Newton_ has shewn in his _Principia Philosophiæ_, it will follow, that according to the general Principle of Gravity, visible throughout the whole Universe, all those Particles that by length of time, or otherwise, shall moulder away, or become loose on the Concave Surface of the External Sphere, would fall in, and with great force descend on the Internal, unless those Particles were of another sort of Matter capable by their stronger tendency to each other, to suspend the force of Gravity; but we know no other Substances capable of supporting each other by their mutual Attraction but the Magnetical, and these we see miraculously to perform that Office, even where the Power of Gravity has its full effect, much more within the Globe where it is weaker. Why then may we not suppose these said Arches to be lin'd throughout with a Magnetical Matter, or rather to be one great Concave Magnet, whose two Poles are the Poles we have before observ'd to be fixt in the Surface of our Globe.

Another Argument, favouring this Hypothesis, is drawn from a Proposition of the same Mr. _Newton_, where he determines the force wherewith the _Moon_ moves the _Sea_ in producing the _Tides_: His Words are, _Densitas Lunæ est ad densitatem Terra ut 680 ad 387 seu 9 ad 5 quamproximé. Est igitur corpus Lunæ densius ac magis terrestre quam Terra nostra_, p. 466. Now if the Moon be more solid than the Earth, as 9 to 5, why may we not reasonably suppose the Moon, being a small Body, and a secondary Planet, to be solid Earth, Water, Stone, and this Globe to consist of the same Materials, only four Ninths thereof to be Cavity, within and between the Internal Spheres; which I would render not improbable.

To those that shall enquire of what use these included Globes can be, it must be allow'd, that they can be of very little service to the Inhabitants of this outward World, nor can the Sun be serviceable to them, either with his Light or Heat. But since it is now taken for granted, that the Earth is one of the Planets, and they all are with Reason suppos'd Habitable, though we are not able to define by what sort of Animals; and since we see all the parts of the Creation abound with Animate Beings, as the Air with Birds and Flies, the Water with the numerous varieties of Fish, and the very Earth with Reptiles of so many sorts; all whose ways of Living would be to us incredible did not daily Experience teach us. Why then should we think it strange that the prodigious Mass of Matter, whereof this Globe does consist, should be capable of some other improvement than barely to serve to support its Surface? Why may not we rather suppose that the exceeding small quantity of solid Matter, in respect of the fluid Æther, is so dispos'd by the Almighty Wisdom, as to yield as great a Surface for the use of living Creatures, as can consist with the conveniency and security of the whole? We our selves, in Cities where we are pressed for Room, commonly build many Stories one over the other, and thereby accommodate a much greater multitude of Inhabitants.

But still it will be said, That without Light there can be no living, and therefore all this _apparatus_ of our inward Globes must be useless: To this I Answer, That there are many ways of producing Light which we are wholly ignorant of; the Medium it self may be always luminous after the manner of our _Ignes fatui_. The Concave Arches may in several places shine with such a Substance as invests the Surface of the Sun; nor can we, without a boldness unbecoming a Philosopher, adventure to assert the impossibility of peculiar Luminaries below, of which we have no sort of _Idea_. I am sure the Poets _Virgil_ and _Claudian_ have gone before me in this Thought, inlightning their _Elysian Fields_ with Sun and Stars proper to those infernal, or rather internal Regions. _Virg. Æneid. 6._

_Largior hic compos Æther & lumine vestit
Purpureo; Solemque suum sua Sidera norunt._

And _Claudian lib 2. De Raptu Proserpinæ._

_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.

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.