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.

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 1⁄2: required the upper Height in English Feet.”

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, 29​1⁄2.

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]