CHAPTER LVI.

NEW MODE OF ASCENT, TO DETERMINE THE INSTANT THE BALLOON IS ARRIVED AT ANY GIVEN HEIGHT: TO MEASURE THE HEIGHTS: AND TO ESTIMATE THE DENSITIES OF THE AIR AT THE GIVEN HEIGHTS.

ALSO, A METHOD OF ASCENDING TO A FIXED BAROMETRIC HEIGHT: THERE TO REMAIN SUSPENDED IN EQUILIBRIO.

Section 299. Previous to the Ascent, provide a Cord, which shall have sufficient Strength to support twice its own Weight, when so great a Quantity of it is _coiled_ together, as, if extended, woud measure half a Mile or a Mile.

Weigh the whole _Coil_, or any Number of Yards, so as to obtain the whole Weight.

Mark the whole Length of the Cord, with different _coloured_ Worsted, or otherways, at the Distance of every eight Yards: as a _sounding_ Line.

Note the Marks in a Pocket-Book.

These Things being done; give the Balloon, by +inflation+, a Power of Levity _at least_ equal to the known Weight of the Cord: which may be easily obtained by throwing into the Car, already _ballasted_ and prepared, a Weight equal to the Aironaut, together with that of the _Cord_.

The Cord must also, previous to the Ascent, be rolled upon a Reel, (made fast in the Ground) whose Diameter shoud be +two+ Feet: each Turn of the Wheel may be called two Yards.

A Barometer with an attached Thermometer fixed in the same Frame, also a second or detached Thermometer placed at the Distance of a Yard from the Frame, shoud remain upon the Ground during the Inflation.

The same Apparatus of Barometer with attached and detached Thermometer, shoud be suspended in the Car.

The Instant the Balloon ascends, an Observer below is to note in a Book the _Point_ at which the Quicksilver stands in each of the +three+ Tubes of the lower Apparatus, also the Time of Ascent: the Aironaut the same.

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_.]

302. The Aironaut, may, at any Height, marked by looking at the Barometer, when at 24 Inches for Example, or as soon as he finds his Balloon sufficiently expanded, pull up the Rope over a Pulley; or, wind it upon a Reel of two Feet Diameter, within the Car; and continue to do so; till he finds that the Barometer begins to _rise_, (which is a Sign that the Balloon _descends_), by the additional _Weight_ of the Balancer just brought into the Car: on which, by preconcerted Agreement, he may throw out a +white+ Flag, prepared to hang a Yard below the Car.

On Sight of the Flag, the Person at the Reel _below_ is to cut the Rope: which Rope, or a Part of it, is to be drawn into the Car.

The Balloon will rise no higher; but remain in _Equilibrio_ in the Air, at that Height.