The Trees woud therefore spread a +shade+ _across_ the Road. The +tops+ of the _Houses_ likewise, being Part of them in the Shade; and either _thatched_ with Straw, or covered with Slates of a _dusky_ Hue; woud prevent their _throwing off_ any _striking_ Colour. Possibly the _Encrease_ of Shade _alone_, might give the Face of the Country _below_, a _dark-green_ Cast. It is certain that the Height of the Balloon must have been very great, to prevent the Sight of public and _Turnpike-Roads_, _above_ which it _frequently_ passed, and which had been +plainly+ seen _before_ the _Re-ascent_. For suppose the Road but 5 Yards wide, which is less than the Truth; if it be allowed that an Object may be distinguished by a _sharp_-sighted Person, when its _Distance_ from the Eye does not _exceed_ 5156 Times the Diameter of the Object; i.e. when the Object does not subtend a _less_ Angle at the Eye than _30 Seconds_ of a Circle, (Smith’s Optics, Article 97) which is the _smallest_ visible Point, and equal to the 8000th Part of an Inch on the _Retina_;—by multiplying 5 Yards, viz. the Diameter of the public Road, into 5156 (or, in round Numbers, into 5000) Times its Distance from the Eye in the Balloon; the Product is 25000 Yards: which Product being divided by 1760, the Number of Yards in a Mile, amounts to 14 Miles, and 360 Yards. Supposing farther, that a _common_ Eye can _only_ see an Object at _half_ that Distance; the Height woud _then_ be 7 Miles. The _Improbability_, therefore, (on Account of the _Warmth_ of the Air at that Height, viz. 60°;) of having _soared_ to so great an Altitude, seems to point out, that the +shadows+ must have contributed a _principal_ Share, in preventing a Sight of the public and _Turnpike_ Roads. [49] The +magnitude+ of an Object _decreases_, as the +squares+ of its Distance from the Eye _increase_. At whatever Distance, for Example, the Eye can see any Object clearly; as at the Distance of a Foot, or a Yard, if the Object be removed to _twice_ that Distance; it will appear 4 Times smaller than it did before: 2 multiplied into 2, equals 4, which is the Square of 2: in the same Manner, if the Object be removed to thrice the Distance from the Eye, it will appear 9 Times as small, as at the first Distance: for 3 into 3 gives 9, the Square of 3: and so of any farther Distance. [50] See “Berkeley’s New Theory of Vision, Section 67.” [51] Dr. Smith having Recourse to _intervening Objects_; the Writer cannot assent to the Validity of his Argument, illustrated by a well-known Figure, to solve the Appearance of the _horizontal Moon_. See “Priestley’s History of Light and Colours, Page 712.” [52] Phil. Trans. for 1785, Part 1, Page 287. [53] Cavallo’s Treatise on Air, Page 576. Vitriolic Acid Air, Alkaline Air, and other elastic Fluids, are instantly +absorbed+ by _Water_; (Page 673.) Inflammable Air, and fixed Air, are likewise +absorbed+ by +water+. (Page 434).