LETTER THE EIGHTH.
Of Time and Space, with regard to the known Objects of Immensity and Duration.
SIR,
T
The Opportunity you gave me in your last Visit, of shewing you my general Scheme of the Universe, I find, besides the Pleasure it then gave, is now attended with many useful Advantages.
I now not only hope to be better understood for the future, but have reason to expect what I now write will merit your Attention more, and have some Title to your Approbation. The Ideas I have fram'd of Time and Space, will now more gradually fill your Imagination both with Wonder and Delight, before they can arise so high as to be lost in an Eternity and the Infinity of Space. And I am fully perswaded your farther Inquiries into these vast Properties of the Deity, will here be answered intirely to your Satisfaction. You must allow me now to be in some measure a Judge of what I think will please you most, from the Observations you have made upon my general System, or otherwise you would have reason to think me perhaps too presuming: But I flatter myself the great Difficulty is now over; and what remains to be said, will all so naturally follow from what has gone before, that this Letter, I guess, will go near to furnish you with all the Ideas you wish to form upon the Subject. To what you have said of my having left out my own Habitation in my Scheme of the Universe, having travell'd so far into Infinity as both to lose sight of, and forget the Earth, I think I may justly answer as Aristotle did when Alexander, looking over a Map of the World, enquir'd of him for the City of Macedon; 'tis said the Philosopher told the Prince, That the Place he sought for was much too small to be there taken Notice of, and was not without sufficient Reason omitted.
The System of the Sun compar'd but with a very minute Part of the visible Creation, takes up so small a Portion of the known Universe, that in a very finite View of the Immensity of Space, I judg'd the Seat of the Earth to be of very little Consequence, could I have possibly represented it, as not only being one of the smallest Objects in our Regions, but in a manner infinitely less than even her own annual Orbit, and had nothing to do with my main Design, which was to represent all our planetary Worlds as one collective Body, and begin my comparative Scale of Magnitude from the Sun only and his Sphere of activity; as the smallest Object I could with any Propriety pretend to express in such a Plan.
In some Measure to convince you that I have committed no Error in this, I will try by some less mathematical Method than that of meer Numbers, to imprint an Idea in your Mind of the true Extent of the solar System, and the Magnitude of all its moving Bodies, by natural Objects most familiar to your Senses. When we endeavour to form any Idea of Distance, Magnitude, or Duration, by Numbers only, we so soon exceed the Limits of Conception, that this way we find our Faculties of reasoning as finite as our Senses; and no doubt 'tis right it should be so, Providence, as it were, having ordain'd that the first should only attend the last, in such an adequate Degree to a determin'd Distance; but what Distance or Degree of Knowledge is destin'd to human Nature, none but the Power that gave it can tell. 'Tis certain that beyond the third or fourth Place of our Nomenclator, we receive but very faint Impressions of the thing exprest, and can frame scarce any Notion at all of either Number, Distance, or Magnitude, signified beyond it: Hence Astronomers are frequently oblig'd to have recourse to mixt Ideas, and make Things of different Natures and Properties assist each other, to excite more adequate Ideas of what they would have conceived. Thus to express immense Distances and Magnitude, they frequently apply themselves to Time and Motion; and vice versa, to signify a long Duration, they have often recourse to Distance and Matter, removing, in Imagination, Worlds of Sand, Grain after Grain, to some remote known Region.
Hesiod,[AQ] to express his Idea of the Distance from his highest Heaven to Earth, and from Earth to Hell, or Tartarus, supposes an Anvil to be let fall from one to the other, which he says in nine natural Days would reach the Earth from Heaven, and in the same time would fall from the Earth to Hell. [AR]Homer makes his Vulcan fall from Heaven to the Island of Lemnos in much less Time, not exceeding one full artificial Day.
From the high Heaven a brazen Anvil cast,
Nine Nights and Days in rapid Whirls would last,
And reach the Earth the Tenth, whence strongly hurl'd;
The same the Passage to th' infernal World.
Hurl'd headlong downward from th' etherial Height;
Toss'd all the Day in rapid Circles round,
Nor till the Sun descended touch'd the Ground.
Modern Astronomers have made use of the swiftest Velocity of a Cannon-Ball as continued thro' the Space they would so describe, and in this Light, the Distance to the Sun has been by many compar'd to twenty-five Years Motion of a Cannon-Ball, supposing it to travel at the Rate of 100 Fathom in a Moment, i. e. the Pulse of an Artery; and that a Journey so performed to one of the nearest fix'd Stars, would take the same Body at least 100,000 Years before it could arrive there. But the Method I have chose to convey my Ideas of the Magnitude of the planetary Bodies, and the Extent of the visible Creation to you, I am willing to hope you will find still more familiar, comprehensive, and easy: And it only depends upon your Remembrance of a very few known Objects, and their neighbouring Distances, which may be presumed you are, or have been, very well acquainted with. You have not only very lately but very often been in London, and must, I think, retain some Idea of the Dome of St. Paul's, tho' I own I ought not to be sorry if you should chance to have forgot it, provided it might prove a Means of making your Visits more frequent. The Diameter of the Dome of this Church is 145 Feet: Now if you can imagine this to represent the Surface of the Sun, a spherical Body 18 Inches diameter, will justly represent the Earth in like Proportion; and another of only five Inches diameter, will represent the Moon. The Truths of these Proportions I have shewn in my Clavis Cœlestis; and the Reason why I have here fixt upon the Dome of this Church for my first Object of Comparison, will naturally appear from what follows.
From the Magnitude of the Earth on which we live, as from a known Scale with respect to its Parts compar'd with our own Bodies, we naturally frame our first Ideas of Extent, and fix our Rationale of Remoteness; by which we are sufficiently enabled to judge of all other sensible Distances within one finite View. And hence by the undoubted Principles of Geometry, having first given the Measurement of the Earth in any known Proportion with any other Quantity most familiar to our Senses, and the Angle of Appearance, or Parallax to any perceivable Object, we can easily find in homogenial Parts its true Distance from the Eye. And thus allowing for some small tho' unavoidable Errors, that may possibly arise from the Difficulties of Observation (especially small Angles and minute Quantities) we can always determine to a sufficient, and very frequently to a just Exactness, the relative Distance of all visible Bodies, remote or near, such as the Planets, Comets, and the Sun.
[AS]In this Manner Astronomers having procur'd a comparative Standard, reduc'd to some known Measure, as English Miles, Leagues, Semi-Orbs or Orbits, with all the Force of analogical Reasoning, clearly can demonstrate the Place and Distance of any Object within the Reach of Observation, and judge of Distances almost indefinite.
[AS] Parallax is the changeable Position of Bodies to different Situations of the Eye. First having found the Quantity of a Degree (i. e. a 60th Part of the Circumference) upon the Earth's Surface, Aratosthenes discover'd that the Magnitude of the whole was easily known; and then from the Moon's horizontal Parallax having given the Radius of the Earth, the Distance of the Moon is soon determined; next by the menstrual Parallax of the Lunar Orbit, the Distance of the Sun is found; and by the Elongation of the inferior Planets, their mutual Distance from each other; and, lastly, from the annual Parallax of the Earth's Orbit, all the other Orbits of the superior Planets are easily found.