NavList:

Alex, you wrote
"Unfortunately he does not say what was the SIGN of this difference"

Found this in Blackburne: Tables for Azimuths, Great Circle Sailing and Reduction to the Meridian, from 1916. The heading of the article below is "Exceptional Excessive Refraction near the Horizon", written by Logan:

Quote
?Lieutenant Koss and Ensign Thun-Hohenstein, of the Austrian Navy, while con-
ducting observations near Pola for finding the variation in the dip of the horizon, observed on a quiet day a rise of the apparent horizon above its computed position of 8' 47" at a height of 50ft., and of 9' 23" at a height of 33 ft. above water.?
...
?As a result of what has been set forth, the following brief summary may be given for the guidance of navigators : ?

(a.) The inaccuracy of tables showing the dip and the visibility of objects should always be suspected when there is a marked difference between the temperature of the air and that of the sea-water.

(b.) The errors will be largest in calm weather and when the eye is not far elevated above the sea, and will decrease as the wind increases and the eye is raised.

(c.) When the air is warmer than the water, the visible horizon is raised above its normal position ; the altitude corrected by the ordinary dip-table will be too small, ...

(d.) When the water is warmer than the air, the visible horizon is lowered below its normal position ; the altitude corrected by the ordinary dip-table will be too large, ...?
...
?A little more than a year before the time of writing this a very well authenticated case of undoubted excessive refraction was brought to the notice of the writer by an old pupil (Captain W. H. Sweny), then commanding the P. and O. s.s. "Mooltan," At about 6 p.m, on April 11th, 1910, he took observations of four different stars for a position ? viz, Procyon (ex-meridian) to northward, Rigel to north-westward, Canopus to south-westward, and ? Centauri to south-eastward, to cross with one another. The chief officer took an ex-meridian of Pollux to northward, and Rigel and ? Centauri for a cross. Shortly afterwards the fourth officer took Procyon, Canopus, ? Centauri, and Jupiter to eastward. Captain Sweny first sent the writer his own observations, asking him to work them out and let him know what he made the resulting position. This was done, and when the captain afterwards sent the results of his work both positions were in agreement and evidently not more than about 1' in error in either latitude or longitude. The captain also sent the worked out observations of the other two officers, and from all these observations the writer was able to deduce fairly accurate separate positions, and it was evident from these
observations that refraction was excessive all round the horizon, but greatest to the northward, where it was about 11.0', and in other parts of the horizon averaging about 6?', the altitudes being smaller by these amounts than they should have been by allowing the usual tabular corrections The height of eye when these observations were taken was 50 ft. If the captain had been satisfied with the ex-meridian observations of Procyon and Pollux, and the longitude of ? Centauri, he would have been 11' or 12' out in the latitude and 33' out in the longitude. By using the observations intelligently he was practically correct in his position, and made Rottnest Light nearly ahead at 2 a.m. His position at the time was about 30? 4' S. and 113? 47' E.

The above related experiences, though undoubtedly very exceptional, should tend to warn navigators not to trust too implicitly even in daylight to observations taken on one side only of the meridian, or prime vertical ; for though such excessive refraction is very rare, and may not be experienced in the lifetime of a frequent observer, it is probable that such amounts as 2' , difference from the tabulated value of refraction is not uncommon .?
Unquote

Here is a link to the book:

http://books.google.co.uk/books?id=7nw_Y6kgOXEC&pg=PA132&lpg=PA132&dq=%22abnormal+dip+of+horizon%22&source=bl&ots=IRRxTqy4Xh&sig=Forp7zy5dDKXksxwYPrVRfdABT4&hl=en&sa=X&ei=tKFsT5PoA6WC4gT_-vS_Ag&ved=0CCEQ6AEwAA#v=onepage&q=%22abnormal%20dip%20of%20horizon%22&f=false

Lars, 59N 18E
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