The Words On The Scale
The following words are usually engraved on the scales of these barometers, although they are not now considered of so much importance as formerly:—
| At | 31 | inches | Â | Very dry. |
| " | 30·5 | " |  | Settled fair. |
| " | 30 | " | Â | Fair. |
| " | 29·5 | " |  | Changeable. |
| " | 29 | " | Â | Rain. |
| " | 28·5 | " |  | Much rain. |
| " | 28 | " | Â | Stormy. |
The French place upon their barometers a similar formula:—
| At | 785 | millimètres |  | Très-sec. |
| " | 776 | " | Â | Beau-fixe. |
| " | 767 | " | Â | Beau temps. |
| " | 758 | " | Â | Variable. |
| " | 749 | " | Â | Pluie ou vent. |
| " | 740 | " | Â | Grande pluie. |
| " | 731 | " |  | Tempête. |
Manufacturers of barometers have uniformly adopted these indications for all countries, without regard to the elevation above the sea, or the different geographical conditions; and as it can readily be shown that the height and variations of the barometer are dependent on these, it follows that barometers have furnished indications which, under many circumstances, have been completely false. Even in this country, and near the sea-level, storms are frequent with the barometer not below 29; rain is not uncommon with the glass at 30; even fine weather sometimes occurs with a low pressure; while it is evident that at an elevation of a few thousand feet the mercury would never rise to 30 inches; hence, according to the scale, there should never be fair weather there. If tempests happened as seldom in our latitude as the barometer gets down to 28 inches, the maritime portion of the community at least would be happy indeed. These words have long been ridiculed by persons acquainted with the causes of the barometric fluctuations; nevertheless opticians continue to place them on the scales, evidently only because they appear to add to the importance of the instrument in the eyes of those who have not learned their general inutility. In different regions of the world, the indications of the barometer are modified by the conditions peculiar to the geographical position and elevation above the sea, and it is necessary to take account of these in any attempt to found rules of general utility in connection with the barometer as a weather guide. All that can be said in favour of these words is, that within a few hundred feet of the sea-level, when the column rises or falls gradually during two or three days towards “Fair” or “Rain,” the indications they afford of the coming weather are generally extremely probable; but when the variations are quick, upward or downward, they presage unsettled or stormy weather.
Admiral FitzRoy writes:—“The words on the scales of barometers should not be so much regarded, for weather indications, as the rising or falling of the mercury; for if it stands at Changeable, and then rises a little towards Fair, it presages a change of wind or weather, though not so great as if the mercury had risen higher; and, on the contrary, if the mercury stands above Fair and falls, it presages a change, though not to so great a degree as if it had stood lower; besides which, the direction and force of wind are not in any way noticed. It is not from the point at which the mercury stands that we are alone to form a judgment of the state of the weather, but from its rising or falling; and from the movements of immediately preceding days as well as hours, keeping in mind effects of change of direction and dryness, or moisture, as well as alteration of force or strength of wind.”
13. Correction due to Capacity of Cistern.—These barometers, having no adjustment for the zero of the scale, require a correction for the varying level of the mercury in the cistern, when the observations are required for strict comparison with other barometric observations, or when they are registered for scientific purposes; but for the common purpose of predicting the weather, this correction is unnecessary. The neutral point, and the ratio of the bore of the tube to the diameter of the cistern, must be known (see p. 3). Then the capacity correction, as it is termed, is found as follows:—Take the fractional part, expressed by the capacity ratio, of the difference between the observed reading and the height of the neutral point; then, if the mercury stand below the neutral point, subtract this result from the reading; if it stand above, add it to the reading.
For example, suppose the neutral point to be 29·95 inches, and the capacity ratio 1â„50, required the correction when the barometer reads 30·78.
| Here |  | 30·78 - 29·95 | = | 0·83 | ||
| Correction |  | = | 0·83 | = | +0·02 | nearly. |
| Â | 50 | |||||
| Scale reading |  | 30·78 | ||||
| Correct reading |  | 30·80 | ||||
Of course the correction could as easily be found to three decimal places, if desirable. It is evident that the correction is more important the greater the distance of the top of the mercury from the neutral point.
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