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    Re: Sextant with an umbrella
    From: Bob Goethe
    Date: 2015 Oct 2, 18:10 -0700

    In the sextantbook.com reference, there is a sentence that caught my eye:  "...discussion of the matter then moved on (28 March) to how to determine whether there is abnormal dip of the horizon, a condition likely to occur when there is warm air over cool water, which is particularly common and severe in arctic regions."

    This situation put me in mind of a case study that T. S. Lecky described.  Except in his case, it was not warm air over cool water in the arctic, but scorching air over warm water in the vicinity of the Arabian penninsula.  The quote below is from the 1918 edition, pp. 94ff.  (Emphasized words were made so by Lecky.)

    Bob

    ___________________________

    "In the Red Sea and Persian Gulf, the horizon is very liable to displacement from the hot winds coming off the scorching deserts, and the refraction in the day time is generally in excess of the tabular value.

    "The writer has been enabled to practically demonstrate this to the complete satisfaction of his brother officers during a voyage to Calcutta and back.  It was alleged by those on board, who had repeatedly passed certain islands in the Red Sea known as the Zebayir and Hanish groups, that they were not shewn relatively in their proper positions on the chart, and to determine the correctness of this statement the writer devoted some considerable time and labour.

    "The Zebayir Islands lie, roughly speaking, about 70 miles to the northward and westward of the Hanish group, and both of them directly in the track of steamers passing up and down.  The distance between them is such that if one group should be passed about sight time in the morning, the other group will be passed about sight time in the afternoon.

    "It was found on the outward passage, when sights were taken in the morning off the Zebayir group, that they were apparently marked too far west on the chart; and when similar observations were made in the afternoon, the Hanish group appeared to be shewn too far east on the chart.  This was a serious business, as the relative bearing of the two groups of islands was thereby materially altered.  The question, moreover, was one independent of the correctness of the chronometers, as the islands were shewn relatively to be out of place some 7 or 8 miles.

    "After a very careful discussion of all the data in connection with the subject (including observations on previous voyages by other observers), the writer came to the conclusion, that in all probability the altitudes of the sun had been vitiated by excessive refraction.  To test this, on the passage home, sights were again taken off the Hanish Islands, which this time happened to be passed in the morning, and similar observations made off the Zebayir Islands, which were passed in the afternoon, thus reversing the conditions of the outward voyage.  The result fully justified the writer's expectations, as the Hanish group were now shewn too far west on the chart, and the Zebayir Islands too far east, while on the outward passage just the opposite had been the case.  So that all this bother and undertainty as to the relative position of two important groups of islands was unmistakably proved to be due in part, if not wholly, to errors of observation arising from excessive refraction.

    "In 1895 special attention was paid to Red Sea refraction by Lieutenant W. A. Marshall, U.S.N., of the U.S.S. Detroit, and this observer came to the conclusion that the effect of excessive refration when taking the sun in the Red Sea is a more probable cause of departure from the beaten steamer track than cross currents....

    "These things point strongly to the necessity for great caution in the navigation of a ship.  Nothing 'slapdash' should be allowed in connection with it, nor too much taken for granted.  Who can tell how many wrecks might be traced to this cause which at the time were ignorantly set down to some extraordinary 'jump' of the compasses, or some unlooked for current?  Seamen would do well to give this important subject the attention it merits."

       
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