CHAPTER LXX.

METHODS TO ASCERTAIN THE TRUE HEIGHT.

Section 350. Methods to be pursued on taking and comparing Heights, in order to ascertain the true Height of any Station in the Atmosphere, by the Barometer and Thermometers.

For this Purpose it is necessary, 1st, to provide a Barometer, (whose Bulb or Cistern is _large_ enough to contain all the Quicksilver in the Tube;)—into the Frame of which, a Thermometer, on _Farenheit’s_ Scale, is to be fixed or _attached_.

The Use of the _attached Thermometer_ is to point out the Temperature of the Barometer.

2d. A second or _detached Thermometer_ is also to be provided.[117]

This is to be hung in the Shade at the Distance of a Yard (or two) from the other:—to shew the _general_ Temperature of the Air at the same Time and Place: and may be called the _Air Thermometer_.

A proper Person, on the Ground, having a good Watch, with Pen Ink and Paper at Hand, is to attend the Instruments _below_ every ten Minutes, (or at any other _preconcerted_ Intervals of Time,) putting down,

1st. The Time of each Observation.

2d. The Point at which the Quicksilver stands in the Barometer.

3d. The Degree of Temperature of the _attached_ Thermometer.

4th, and lastly, the Degree of Temperature of the _detached_ or _Air_-Thermometer.

This Employment is to be carefully attended to; during the Time, that _similar_ Observations, by _preconcerted_ Agreement, are making, with three other _similar_ Instruments, on the Top of the Mountain, or any elevated Station in the Atmosphere, by Means of the _Balloon_; and to be written with a _red Lead_ Pencil, in a Patent Asses Skin Pocket Book.

[Sidenote: The Instruments to be compared on Return from the Mountain, or upper Station.]

Each single Observation, made with one Set of Instruments _below_, is to be compared with each single corresponding Observation, made with the other Set _above_.

And two Observations are said to _correspond_, when both are made _nearly_ at the _same_ Time, the one _below_, and the other _above_.

351. Take Shuckburgh’s first Example, (Ph. Tr. for 1777, 2d Part, Page 577.) viz.

“Let the Point at which the Quicksilver stands in the Barometer, on the Ground, be 29 Inches 4 tenths: the attached Thermometer 50 Degrees of Temperature, and the Air Thermometer, or general Temperature of the Air 45°: at the same Time, that at the Top of the Mountain, or other elevated Station in the Atmosphere, the Barometer stands at 25 Inches 19 Tenths, the attached Thermometer at 46°, and the Air Thermometer at 39° and ½: required the upper Height in English Feet.”

[Sidenote: Rules for the Work: and Practice of the first Example.]

352. The Work is divided into three Stages.

The End proposed in this first Stage is to bring the colder Barometer, to the same _Expansion_ or _Temperature_ with the _other_.

353. 1st. Step. First, write down the Observation made on the Ground, or at the Bottom of the Mountain, thus:

+Below.+ Barometer, 29 Inches 4 Tenths. attached Thermometer, 50 Degrees. Air Thermometer, 45°.

354. 2d. Step. Secondly, write down the Observation made at the Top of the Mountain, or upper Station in the Atmosphere, thus:

+Above.+ Barometer, 25 Inches, .19 Tenths. attached Thermometer, 46°. Air Thermometer, 29½.

355. 3d Step. Subtract the _colder attached_ Thermometer, from the other attached Thermometer, thus: 46 colder from 50 warmer, and there remains 4° warmer, viz. the Number of Degrees of Temperature to which the _colder_ Barometer must be _expanded_, before it becomes equal in Temperature to the _warmer_ Barometer: each Barometer being always supposed _equal_ in Temperature with its _attached_ Thermometer.

356. 4th Step. Give the _colder_ Barometer the same _Temperature_ with the warmer: or, which amounts to the same, give the _colder_ Barometer that _Expansion_ which is communicated by the Addition of 4 Degrees of Temperature.

Both Barometers will then have the same _Temperature_, or _Expansion_, viz. an Expansion equal to the warmer Barometer.

This is to be done by referring to the first Table, for the Application of which there are separate Instructions: see the Explanation of the first Table.[118]