CHAPTER XXXX.
ON THE EXCESSIVE DIMINUTION OF OBJECTS ON THE SURFACE OF THE EARTH, TO A SPECTATOR SITUATED ABOVE THE REGION OF CLOUD, AT THE BAROMETRIC HEIGHT OF NEAR A MILE AND HALF, PERPENDICULAR.
[Sidenote: Recapitulation of the Scenery below.]
Section 223. The Earth’s Surface was presented to the Eye throu’ a _circular_ Opening as already described.
This Opening discovered a _Plain_, smooth and level as a Die: a Sort of _shining_ Carpet, enriched with an endless Variety of Figures depicted _without_ Shadow, as on a Map: what was really Shadow forming a separate Colour, and not considered at the Time, as _Shadow_. The Objects were distinctly marked, and perfectly known to be Miniatures of the Face of Nature.
All was _Colouring_: no Outline: yet each Appearance curiously defined by a striking Contrast of simple Colours, which served to distinguish the respective Boundaries with most exact Precision, and inconceivable Elegance.
+Red+ Rivers, +yellow+ Roads, Enclosures +yellow+ and _light_ +green+, Woods and Hedges _dark_ +green+, were the only Objects clearly distinguishable, and their Colouring extremely vivid. The Sun’s Rays reflected from the Surface of the Sea, and other Waters, dazzled the Sight.
+All+ living Creatures were invisible.
224. The Area of each Inclosure, computed to contain a certain Number of Acres, was seen from above under the Form of a Miniature Picture of a certain Magnitude or visible Extension, perpetually diminishing, as the Eye recedes to a greater Distance.
And the Case is similar, whether the Miniature be seen from _above_, or _along_ the Ground.
The Miniature also lessens as the Distance encreases, according to a certain Proportion so exactly;[49] That,
1. If the _Distance_ and _Magnitude_ of a tangible Object be known by Mensuration; a Judgment is formed, and Laws laid down, for its corresponding _Miniature_ on the Eye.
2. If the _Miniature_ be seen, and _Distance_ known by Mensuration; the Mind forms a Judgment of its tangible _Magnitude_.
3. And lastly, if the _Miniature_ be seen, and _Magnitude_ of a tangible Object is known by Mensuration; the Mind makes an Effort, to the Estimation of its Distance from the Eye.
These are some, among many Modes of Comparison, by which the Mind acquires a tolerable Degree of Proficiency, in estimating _Distances_ of familiar Objects, _known_ from the Appearance of their respective Miniatures on the Fund or Bottom of the Eye.
And so far most Theories agree.
But such _ocular_ Test is only true, while the Comparison is made in _nearly_ the same Medium.
For an Object, if seen at the same Distance _along_ the Ground, will appear less as it rises above it; and least in the Zenith; as the Sun and Moon, at Setting or Rising, appear _large and oval_; but at their greatest Elevation, are _small_ and _round_: because being seen, when passed out of a Medium impregnated with Vapours, which in some Measure intercept the Rays of Light: for the +fainter+[50] a distant Object appears, the _greater_ it is apprehended to be.[51]
Possibly indeed an Object at the same Distance, if brighter at one Time than another, will _contract the Pupil_ in Proportion to its Brightness: which may have the same Effect, as if the Object had made a _smaller_ Miniature on the Retina; and will regularly strike the Mind with an Idea of _Magnitude_, _only_ equal to its corresponding _Contraction_; i.e. less, when the Object is bright, and greater when faint.
225. If a like Reasoning be applied to the Ascent of Balloons; and it be said that they do not rise so high as is imagined, because their Magnitude is diminished, merely from being elevated into a Portion of the Atmosphere _least_ impregnated with Vapours; it will follow, that to a Spectator in the Balloon; known Objects on the Surface of the Earth below,—being seen from a rarer into a denser Medium, also into one which contains a great Quantity of Vapours;—shoud appear _larger_, than when seen along the Ground, at a Distance equal to its Height in the Balloon: all which is contrary to Matter of Fact: particularly if the Barometer gives a proper Estimate of the Height, of which there is little Doubt: a proper Allowance being made, _in certain Cases_, on Account of the Refraction: for, as before mentioned, (Section 44) Objects seen from the Balloon at a Mile and Half _barometric_ Height, continued, with invariable Uniformity, to suggest the Idea of at least seven Miles.
226. By a general Comparison of Enclosures, and of separate Buildings when they coud be distinguished from the Balloon above the Region of Cloud, with the most distant Extremities, (on the horizontal Level) of Fields or Houses situated along the Sides of Hills or Mountains, at a known Distance by Miles, making Allowance for their being seen in a straight Line;—the latter seemed at least five Times _larger_ than the _former_: supposing them at equal Distances.
To give an Instance. Supposing the most distant Extremities of a known Building or Enclosure, situated on the Side of a Hill or Mountain, presented a Miniature of a _familiar_ Magnitude to the Eye of the Spectator on the Ground, at the known Distance of a Mile and Half; the same Object when seen from the Balloon at the same _barometric_ Height, appeared full five Times less.
This Comparison was made by Memory, the Morning after the Excursion, tho’ suggested while in the Balloon, from the wonderful Minuteness of all Objects then presented to the Eye.
The Author being likewise familiarized to judge of Heights; having been on several of the chief Mountains in Europe: also, of comparative Distances, from his Situation near a large City, in a populous, enclosed Country; on a high Plain, within View of the Sea, Mountains, Hills, Enclosures, Buildings, and Objects whose Magnitude and Distances were known.
227. The Balloon itself, a Globe twenty-five Feet in Diameter, was seen in the Air on the Day of Ascent, at the Distance of 19 Miles.
[Sidenote: The Magnitude of Objects seen from the Balloon compared with those of the Sun or Moon near the Meridian, when seen from below.]
228. The Reason already given, for the Solution of the famous Question concerning the apparent Magnitude of the horizontal Moon, seems no less applicable to Objects on the Earth’s Surface, when seen from the Balloon: which _Diminution_ of Objects _below_ confirms the Defect of Dr. Smith’s Hypothesis.
For, as they appeared _extremely bright_; being shone on by the Sun, and seen throu’ the Air in a perpendicular Line, containing the least possible Quantity of Vapour; the Brightness must have exceeded that of the same Objects, when seen along the Ground: and consequently the Miniatures of the former must have been less than the latter, and also their respective Distances _seem greater_.