Art. VII.—On Local Variations of Atmospheric Pressure dependent on the Strength of Winds.
[Read before the Otago Institute, 22nd July, 1872.]
On the 2nd of December last the Field Naturalists’ Club should have met for an excursion to the saddle between the Water of Leith valley and Blueskin.
There being no attendance at the appointed hour, nor for long after it, the indefatigable honorary secretary of the club, not caring for a lonely walk, abandoned the excursion. Mr. Blair and the writer arriving late, and each supposing the usual party to be ahead, undertook the ascent, and they presently joined one another. Mr. Blair, who has a keen eye for possible railway tracks wherever he goes, was, when I overtook him, engaged in noting the height of the barometer, with the view of estimating the elevation of the point he had reached. Similar observations were continued throughout our walk, which extended to a mile or so beyond the summit, and were repeated during the descent at most of the points adopted during the upward journey.
Coming down we noticed an increasing discrepancy between the two sets of observations, the new ones showing decreased atmospheric pressure. We were, of course, prepared to find that the barometer at the sea level had fallen during the afternoon, and expected that the reduction of our observations would give us much trouble in consequence, and probably be far from reliable as measurements of height. As we began to emerge from the narrower part of the gorge through which the Water of Leith flows, we found the difference between the first and second observations began to decrease, and finally, when we reached the sawmills, the two coincided, which was again the case at the intersection of Castle-street and Albany-street, where Mr. Blair made his first observation of the barometer. This curious phenomenon caused us some perplexity. After consideration I am inclined to attribute it to the effect of a fairly strong wind which was blowing from the S.W. when we started, but which subsided during the afternoon. In a valley so completely shut in as that of the Leith during its upper course, and descending so rapidly from the saddle, which lies at about 1,100 feet above the level of the sea, it is certain that a strong breeze blowing almost directly into it at the lower end must cause increased pressure in the lower strata of air.
The following is a list of our observations:—
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|Points of Observation.||During Ascent.||During Descent.||Difference.|
|N.W. corner, Section 7||29.42||29.36||.06|
|Ford of Leith||29.10||29.11||.08|
|Section peg .42/59||28.78||28.74||.04|
Since the aneroid barometer came into use barometrical measurements of altitude have become very common, but I am not aware of any scientific work in which the subject is treated at all fully.
The officers of the United States Survey, engaged on the survey of the western slopes of the North American continent, are reported to have made careful and elaborate investigations, and to have constructed hypsometrical tables suitable for all altitudes above the sea-level, but I have not been able to obtain any work containing an account of the results they have arrived at. Whether facts similar to those I have detailed above have been previously noted I have not been able to discover, and my chief object in presenting these notes to the Society is, if possible, to elicit information on the subject.