[Sidenote: The Depth below the Clouds gives no Idea of Distance.] 157. Nor does the Depth _below the Clouds_ give an Idea of _Distance_. On the contrary, the _smooth chequered Lawns_ which form the Surface of the Earth, are presented to the Eye, as on a _Level_ with the _Clouds_ themselves: _at least_ +come up+ to their +undersides+, and appear so much a Part of _them_; that the _Clouds_ occupy the Place of _Earth_: and the Aironaut seems able to descend from the _Car_ upon the _Clouds_, and to walk from Side to Side over the _empty_ Space, as over a Sheet of _transparent_ Ice, across a _River_, whose Depth is equal to the _small_ but indefinite Thickness of the Clouds. 158. It is from _frequent_ +experience+ only that the _Diminution of Objects_ presuppose their _Distance_. CHAPTER XXVII USEFUL CONCLUSIONS. [Sidenote: Change in the _Form_ of the Balloon while descending: with Conclusions drawn from the Change.] Section 159. It was remarkable that, the lower Parts of the Balloon regularly adopted a _similar_ Form at each Descent: not unlike a _Ship’s Bottom_; looking up, at the Head or Prow, while on the Stocks: the _Neck_ of the Balloon forming a beautiful _central_ Pillar; in Shape like that of a _Speaking Trumpet_ inverted. [Sidenote: Time of Descent discovered by the Form of the Balloon.] And hence may be derived a Piece of _useful_ Information: as the _precise_ Time of descending is discovered by bare _Inspection_ of the Machine. [Sidenote: Balloon adopting the Form of an elliptic Solid.] 160. Another Conclusion seems likewise deducible from the above, that if the Balloon is so burdened, as to _descend_ while it retains the Form of an +elliptic solid+;[44] it will descend more rapidly, than if it contained less Gass: the Force of Descent in both Cases being supposed the same. For if the Diminution of Gass be so great as _not_ to fill the upper Hemisphere of the _Balloon_; the Resistance of the atmospheric Air _below_ woud probably give _it_ the Appearance of a _Concave_ or Umbrella, which woud greatly _check_ the Descent: viz. in Proportion to the Square of the Number of Feet of which the Surface was composed. [Sidenote: An equatorial Hoop prefered to a Parashute.] 161. Hence also the evident Utility of an +equatorial hoop+ for Balloons: in Preference to a Parashute, which woud be only an Incumbrance. CHAPTER XXVIII. [Sidenote: An uncommon Sound in the Air.] Section 162. At 40 Minutes past III, when the Balloon was _apparently_ some Miles above the Level and Summit of the Clouds; a +sudden+ and uncommon _Sound_ was heard for three or four Seconds only. A Sort of _hollow_ Wind seemed issuing from a Plain of Clouds in the North-East Quarter, greatly below the Balloon: which as +suddenly+ ceased. [Sidenote: An unusual Motion communicated to the Balloon.]