1 × ((t + t′ - 64) / ·900) × A.
| To lower the | barometer | from | 31 | in. to | 30 | = | 857 | feet must | be ascended. |
| ” | ” | ” | 30 | ” | 29 | = | 886 | ” | ” |
| ” | ” | ” | 29 | ” | 28 | = | 918 | ” | ” |
| ” | ” | ” | 28 | ” | 27 | = | 951 | ” | ” |
| ” | ” | ” | 27 | ” | 26 | = | 986 | ” | ” |
| ” | ” | ” | 26 | ” | 25 | = | 1025 | ” | ” |
| ” | ” | ” | 25 | ” | 24 | = | 1068 | ” | ” |
| ” | ” | ” | 24 | ” | 23 | = | 1113 | ” | ” |
| ” | ” | ” | 23 | ” | 22 | = | 1161 | ” | ” |
| ” | ” | ” | 22 | ” | 21 | = | 1216 | ” | ” |
| ” | ” | ” | 21 | ” | 20 | = | 1276 | ” | ” |
| ” | ” | ” | 20 | ” | 19 | = | 1341 | ” | ” |
| ” | ” | ” | 19 | ” | 18 | = | 1413 | ” | ” |
A very complex formula is given by mathematicians for finding very nearly the true height of a mountain from barometrical and thermometrical observations made at its base and summit. The following rule by Mr Ellis will be found to give very nearly the same results:—Multiply the difference of the barometric readings by 52,400, and divide by the sum of the barometric readings. If the result be 1000, 2000, 3000, 4000, or 5000, add 0, 0·2, 6, 14, respectively. Subtract 21⁄3rd times the difference of the temperature of the mercury. Multiply the remainder by a number obtained by adding 836 to the sum of the temperatures of the air and dividing by 900. A correction must also be given for latitude, which can be done by the annexed table.
| Latitude. | Factor. | Latitude. | Factor. |
| 80 | 0·99751 | 35 | 1·00090 |
| 75 | 0·99770 | 30 | 1·00265 |
| 70 | 0·99797 | 25 | 1·00170 |
| 65 | 0·99830 | 20 | 1·00203 |
| 60 | 0·99868 | 15 | 1·00230 |
| 55 | 0·99910 | 10 | 1·00249 |
| 50 | 0·99954 | 5 | 1·00261 |
| 45 | 1·00000 | 0 | 1·00265 |
| 40 | 1·00046 |
Fortin’s and Gay-Lussac’s barometers are employed for measuring heights. The aneroid can be used for altitudes reaching to 5000 feet. A delicate instrument will register for as small an ascent as 4 feet.
The Barometer as a Weather-glass.—Generally speaking when the mercurial column in the barometer falls, ‘rain’ is indicated, and ‘fair weather’ when it rises. When it continues steady, a continuance of the weather at the time is regarded as the forecast; when low, the weather is generally broken or bad; and when high, it is fair and settled. A storm is usually preceded by a sudden fall in the mercurial column, the violence of the storm being in proportion to the suddenness of the fall. An unsteady barometer indicates an unsettled condition of weather, whilst a gradual change in it indicates the approach of some permanent condition of it. The state and direction of the wind has also to be taken into consideration when studying the changes of the barometer, and forms an important element in the calculations of the meteorologist, each different wind indicating variations of weather. The connection between changes of weather and the pressure of the atmosphere does not seem to have been satisfactorily established.
One of the reasons assigned for the mercurial column in the barometer being lower in wet than in fine weather is that so long as aqueous matter remains in the air in the form of elastic vapour, its tension assists in supporting the barometric column, but that when this aqueous vapour is precipitated in the form of rain, this tension is lost or removed, and the column therefore falls.
The correspondence between wet and fine weather and an elevation and depression of the barometer seems, however, equally, if not more, dependent on the nature of the winds than on the preceding cause. “In western Europe, the south and south-western winds, which are the rain-bringing winds, are warm winds. Now, a column of warm air to be of the same weight as one of cold air must necessarily be higher, but this cannot well be the case in the atmosphere, for no sooner does the warm column rise by its lightness above the surrounding level of the upper surface of the aërial ocean, than it flows over and becomes nearly of the same height as the cold air around it. The interchange taking place less interruptedly, and consequently less slowly, in the higher strata than in those near the ground, it is some time before the equilibrium, thus disturbed, is restored; and meanwhile the barometer keeps low under the pressure of a rarer atmospheric column. On the other hand, the northerly and easterly winds, being comparatively cold and dry, are accompanied by fair weather and a high barometer. It is thus to the warmth, and not to the moisture of these winds, that the pressure is to be ascribed.”[108]
[108] Chambers’s ‘Encyclopædia.’
Barometer, An′eroid. An instrument invented, or at least perfected, by M. Vidi, of Paris, in which the pressure of the atmosphere is measured without the employment of a fluid, as in the ordinary barometer.[109] Externally, it