The Rope is, previous to the Ascent, to be tyed to a Center above the Car: and as soon as the Balloon has elevated the Car 100 Yards; the Observations, as before, are to be set down below, and by the Aironaut: and repeated at the Height of each 100 Yards: a Drum to beat; during the Time each Observation _below_ is _noting_ down; and the Balloon not suffered to rise, till the Drum has ceased. By such repeated _Notice_, and _Silence_; the Aironaut will know the _exact Height_, at which the Balloon is checked in its Elevation: and the _exact Time_ during which its Elevation is impeded. This Process is to continue, till the Rope is raised to its full Length. At which Instant a double-barrel Gun is to be fired: the exact Time noted _below_: and the Time of hearing the Sound noted above. These Notes are to be compared at the Aironaut’s arrival on Earth. 300. For such _nice_ Experiments the Aironaut shoud ascend half an Hour before +sunrise+, or _Sunset_: and the Day chosen by the foregoing Rules. The Air must be +quite calm+: but it is not necessary that it shoud be free from Clouds or Mist. When the Rope is at its full Extent, the Operator _below_ is to shorten it, by winding down the Balloon, 100 Yards: the Signals _below_, being repeated, till the Balloon is arrived within 100 Yards of the Ground. [Sidenote: To estimate the Densities at different Heights.] 301. While one Observer _below_ is writing down the Observation to be made the Instant the Balloon has risen exactly 100 Yards; another Operator is to weigh, by Hand, with Spring Steel-Yards, the Force of Levity already acquired, which is to be noted down by a third Bystander. This Process is to be repeated at every 100 Yards. The Levity, it is true, will encrease as the Balloon rises, (probably in a geometric Progression;)[100] yet the Cord, by rising with the Balloon, will greatly check it: if, however, it prove insufficient for that Purpose, and, lest the Cord shoud be in Danger of breaking; the Bottom of the Balloon must be opened, or the upper Valve drawn. If the _Cord_, _Rope_, or _Balancer_, be sufficiently strong; there will be no Necessity for the Aironaut to throw out Ballast occasionally; nor for the Observations in the former Part of this Section: the _Densities_ will likewise be more easily determined, by the _Weights_; which shew the _Encrease_ of Levity and Expansion of the Balloon, at each of the _given_ Heights: Allowance being made for the Weight of the _Balance Rope_, _raised_ by the Balloon. [Sidenote: Method of ascending to a fixed _barometric_ Height: there to remain suspended _in Equilibrio_.]