Book II • Section II
Light is propagated through the etherial Medium as a real Substance, and has all the Properties of a material Body, but the Minuteness of its Particles are such, that one Grain of any solid Substance productive of it, being reduced to a Flame, is capable of filling a spherical Space of a League Radius, and that successively repeated for several Moments together. This is the nearest Approximation that can possibly be made in the human Mind, of the infinite Divisibility of Substances, towards conceiving, in the Imagination, an adequate Idea of the primitive Particles of Matter.
The Propagation of Light is not instantaneous, but is prov’d to be progressive, and not caus’d by the Agitation of any etherial Medium; its Velocity is found to be different thro’ different Mediums, and has been observed to come from the Sun to the Earth in about 7 or 8 Minutes; this determined by the Eclipses of Jupiter’s Satellites, compar’d near the Conjunction and Opposition of the Sun; and from these and many such like Observations, we conclude that the mean Velocity of Light,
- Is nearly 650,000,000 Miles per Hour.
- 10,800,000 Miles per Min.
- And 180,000 Miles per Second of Time.
But to treat of the various Reflections, Refractions, and Incurvations of Light, belongs properly to Opticks, and I may presume will not be expected here, so shall content myself at present with speaking of its relative Degrees of Heat.
Of Light and Heat to the Planets from the Sun &c.
Light and Heat to all equal Bodies, or the same Surfaces, as proceeding from any radiant Point, is reciprocally proportional to the Square of the Distance from that Point, (i.e. as the Density of the Rays) and as all the Parts of the Sun may be look’d upon as such, in respect of the Planets, this will evidently appear, from the equal Rays falling upon the Bodies A, B and C; their unequal Distances from the Luminary being as 2, 3 and 6, and their Light respectively, as 81, 36, and 9; i.e. 36 to 4, 36 to 9, and 9 to 4.
- Thus Light and Heat being supposed at the Earth as 1,
- To Venus it will be as 2,
- To Mercury as 6.
And to Bodies within one Diameter of the Earth from the Sun’s Surface, not less than 100,000,000.
To Mars in the same Proportion it will be
Nearly, as \(\frac{4}{10}\); to Jupiter, as \(\frac{1}{27}\); and to Saturn, as \(\frac{1}{90}\).
Hence it may be made appear, that the Comet of 1680, which in its Perihelion approached within less than a sixth Part of the Sun’s Diameter from him; nearly as 6 to 1000 in respect of our Earth, must have acquir’d an Heat, which to that of our Summer’s Sun, would be as 1,000,000 to 36, or as 28,000 to 1; and as the Heat of the driest Earth in Summer, is from Experience, found to be to that of the hottest boiling Water, as 1 to 4, and as the Heat of boiling Water, is strongly suspected to be 3 or 4 times less than that of red hot Iron, we may conclude that the Heat which such dry Earth in the Comet might have conceived from the Sun-beams in that Point, was very near 2000 times greater than that of red hot Iron.
By the same Way of Reasoning we may also infer, that the same Comet, when in its Aphelion, pass’d thro’ a Portion of Light 20,000 times less warm than when we receive from our Summer Sun; consequently as our Air 4 or 5 times less enlighten’d, is frequently in a freezing State, this Comet at that vast Distance from the Sun, must have indured a Degree of Cold, relatively 4000 times greater than that of freezing Ice.1
Sir Isaac Newton seems to have been of Opinion, that the above Comet must retain its Heat for an exceeding long Time, from imagining that a Globe of red hot Iron equal to our Earth, expos’d to the immediate Contact of the open ambient Air, would be 50,000 Years in cooling. But had he once reflected on the intense Regions of Cold, this Body was was to pass thro’, he would soon have reduce it to its mean or temporate State.
Tis hardly possible that these Globes, from the great Extreams of Cold and Heat, they are subject to pass through, (tho’ undoubtedly durable) should be inhabited Bodies, or that they should finally drop into the Sun; which Sir Isaac Newton intimates they must at last do; for tho’ we are almost positive, he has an attractive Power, yet we have many Reasons to believe that he has also a repelling one near his Surface; and if he has Power to throw vast Masses of Matter, from or out of his own Body into his Atmosphere, such as appear like Spots upon his Disk, which very few doubt of, no other Body can be supposed to have Power to approach him.
The only Cause of new Stars they cannot be. Vid. periodical Appearances. But how all this may really be, whether they are ordain’d to feed the other Parts of Nature’s Works with fresh Supplies of solid or of fluid Matter, or whether they are barren Bodies fram’d on purpose for a perpetual Change in Creation’s great Variety; the Cause of Conflagrations, Deluges, or total Catastophies, or be they Worlds themselves not quiet2 destroy’d, or new creating from their old Remains, will never be known, till some of the human Race have been Eye Witness of such a Dissolution, either by the Loss of some one of the known Planets we now see, or have upon the Earth experienced such a Change themselves.
This we may be sure of, it is as possible for the infinite Power to be perpetually creating, and perpetually destroying Worlds, as by Death and new Life he is ever ending and beginning the human Generations, &c. And as this is evident to our Senses, and obvious in our present Being, what may we not expect and hope for in a future State.
But to return. The Proportion of Light and Heat to all Parts of the same Globe, is very far from being equal in Degree, tho’ equal in Duration, i.e. all the different Regions of Earth, enjoy the Sun’s Light one half of the Year, and are deprived of it the other; yet the Proportion of Heat is vastly different.
Thus to the Globe at E (Tab. II.) five Beams are requir’d to fill the Spaces FG and GH, where the Sun is nearly vertical; when at the same Time one performs the like Office, covering an equal Space DE and IK, the Sun being then near the Horizon.
This is the Reason, why the Summer is to us in the North Parts of the World so much warmer than the Winter; the Mornings and Evenings so much colder than Mid-day; and is the Cause from whence proceeds the distinction of the Zones.3 Two being originally judg’d temperate, and the other three intemperate Extreams.
The Rays of Light, falling upon the Earth, coming from so great a Distance as the Sun, may be look’d upon as parallel; such are represented by the lines LE and NI, &c. and in this Case, the immediate Warmth, proceeding from the Sun, will be nearly respectively as the Difference of the natural Sines of his Distance or Declination from the Zenith.
How much the Heat in Summer, Winter and Spring, differ in the Middle of the temperate Zone, appear by the oblique Rays falling upon the Line RX.
Of Refraction. Tab. III
Of Refraction, and reflected Vision &c.
All Rays of Light, coming from any luminous Body, in passing thro’ different Mediums, are variously reflected and turn’d out of their original Direction, always deviating from the Point of Line of Direction towards a Perpendicular in the new Medium.
Thus the Ray R arriving at A, will appear, as if it came in the Direction of the Line S, and the Path described by any Particle of Light near the Horizon, will the most deviate from a right Line, as the Ray TGQOA will first in the medium ♂ describe the Line FG; then in the Medium ♀, the Line GH; afterwards in the Medium ☿, QO, and lastly in the Medium ☾, ☉A.
But as the Atmosphere is in every Point in Elevation, of a different Degree of Density, decreasing in a certain Ratio, nearly as the Square of the Distance from the Earth’s Surface increases, this Path will be a regular Curve of the parabolick Kind.
The Atmosphere, like the Earth itself, is of a spheroidical Figure, but something more oblate than the Earth; it is much lower under the Poles than in the Equator, and is ever varying its Density and Elevation, with the Sun’s Advance and Decline.
Its greatest Height has never been known to be above 60 Miles, and in this Degree of Expansion or Dilation, the uppermost Clouds of the Zenith of one Place, as at W, will appear to another 1000 Miles distant in the Horizon; as at X or at Y; but that same Cloud seen 300 Miles from the Zenith Station, will appear about 45deg. high; as Y from Z, &c.
In this Scheme, (Tab. III.) B represents the Horizon Point to an Observer at A; C the Place where the Sun produces the Aurora to A; and D his Depression below the Horizon at the End of the Crepusculum to the same Station; and where the Sun sinks no lower than this, there can be no real dark Night.
Of the Vicissitude of Seasons
As the Vicissitude of Day and Night is occasioned by the Earth’s Rotation round her Axis; so the Diversity of Seasons in like Manner caus’d by the various Position of the Poles, in respect of Light and Shade, arising from the annual Revolution round the Sun. For as the Plain of the Earth’s Equator never changes its Inclination to that of the Ecliptic; it follows, that the Poles of the World must always be directed to the same fix’d Star, and consequently the equatorial Plain in all Parts of Earth’s Orbit, must ever lie parallel to its primitive Position, and the diurnal Axis must always move parallel to itself. Hence the two Poles will alternately be enlighten’d by the Sun, and interchangeably immerse or emerge, to or from the obscure Hemisphere.
All this evidently appears in Tab. III. being a perspective View of the Earth in her Orbit; the Eye being supposed at an infinite Distance in the Plain of the Equator.
The several Projections are Sections of the Sphere, representing the Earth at the Sun’s Ingress to every Sign.
The primitive Circles represent in every Place the equinotial Colure, and the parallel Lines in the polar Directions the solstial one: The Points as PPP are the North Ecliptic Poles, or Poles of the Sun’s apparent Motion thro’ the Ecliptic; the Poles of the World are those, as at A and B the North ones, and C and D the South Ones; the Circles as VI♑︎W; represent an Archo of the Ecliptic, and ÆQ; ÆQ &c. the Equator; EF in the Tropic of Capricorn and G that of Cancer.
The Circles meeting at the Poles, are all horary Circles, of 15deg. each, and the oblique Circles crossing one another at the ecliptical Poles, are Circles of the Sun’s Longitude, of 30deg. each answering to the 12 Signs.
Now tis plain, the Earth being at H, and the Sun in Capricorn; the North Pole of the World will be in Darkness, and the South will be in Light. But when the Earth is removed to K, the South Pole will then be in the Shadow, and the North in removed to K, the South Pole will then be in the Shadow, and the North one enlighten’d; at M and N, the solar Horizon will pass thro’ both Poles, and consequently the Nights and Days will there be nearly equal to all Parts of the World, the Sun then rising and setting every where near the Hours of 6 and 6.
The Progress of Light, and the gradual Increase and Decrease of Day and Night, may be very easily trac’d in any given Latitude, to all Parts of the World, in the adjacent Schemes, which are each respectively, Projections of the Sphere, to every aforementioned Position of the Earth thro’ the Seasons. They represent the horary Meridians and Parallels of Latitude in Plano, concentric to the Poles; the double shaded Part represent Night; the single shaded Part, the Dawn or Twilight; and the unshaded Part Day.
By which, if proper Regard be had to the Hemisphere of the Place, the Light falling upon opposite Parts of the Globe, being ever alternately equal, and successively Suppliments to each other; the Time of Sun rise, Sun set, Length of the Day, Night, or Twilight; at any Time of the Year, and to any Part of the World will be nearly shown.
Thus London at it shortest Day, will first appear to the Sun at e, and enter the Cone of Night at d; the Sun will rise a little after 8 and set as much before 4. The Day will break about 6, whilst London is at c, and the Length of the Crepusculum will be about 2 Hours.
But at Midsummer the Sun will rise when London shews itself at b, a little before 4, and set a little after 8 when London comes to a, there will be no Night all, by reason London will not here reach the perfect Shadow.
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The Nature of Comets was not unknown in Milton’s Time, and it is very probable his Description of the dam’d, or State of the Fallen Angels might be taken from them.
Thither by harpy-footed Furies hail’d,
At certain Revolutions, all the Damn’d
Are brought, and feel by turns the bitter Change
Of fierce Extreams, Extreams by change more fierce;
From Beds of raging Fire, to starve in Ice
Their soft etherial Warmth, and their to pine,
Immovable, infix’d and frozen round;
Periods of Time, thence hurried back to Fire.Milton, Book II. L. 600.
- Quer. whether the solar Spots may not be broken Parts of Comets, tore to pieces by the excessive Heat of the Sun in their nearest Proximity.
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And as five Zones th’ etherial Regions bind;
Five correspondent are to Earth assign’d;
The Sun with Rays, directly darting down,
Fires all below, and fries the middle Zone;
The two beneath the distant Poles complain,
Of endless Winter and pepetual Rain;
Betwixt th’ Extreams two happier Climates hold
A Temper, that partakes of hot and cold.Dryden’s Ovid.
