This section is from the book "A Library Of Wonders And Curiosities Found In Nature And Art, Science And Literature", by I. Platt. Also available from Amazon: A library of wonders and curiosities.
This wonderful property, though well known to the ancients, as appears from the writings of Pliny, was for many ages either quite unnoticed, or treated as fabulous by succeeding philosophers, till Dr. Franklin again attracted the attention of the learned to this subject; though it appears, from some anecdotes, that seafaring people have always been acquainted with it. Mr. Pennant, in his British Zoology, vol. iv. under the article
Seal, takes notice, that when these animals are devouring a a very oily fish, which they always do under water, the waves above are remarkably smooth; and by this the fishermen know where to find them. Sir Gilbert Lawson, who served long in the army at Gibraltar, assured Dr. Franklin, that the fishermen in that place are accustomed to pour a little oil on the sea, in order to still its motion, that they may be enabled to see the oysters lying at its bottom, which are there very large, and which they take up with a proper instrument. A similar practice is followed among fishermen in various other parts; and Dr. Franklin was informed by an old sea captain, that the fishermen of Lisbon, when about to return into the river, if they saw too great a surf upon the bar, would empty a bottle or two of oil into the sea, which would suppress the breakers, and allow them to pass freely. The Doctor having revolved in his mind all these pieces of information, became impatient to try the experiment himself. At last, having an opportunity of observing a large pond very rough with the wind, he dropped a small quantity of oil upon it. But having at first applied it on the lee side, the oil was driven back again upon the shore. He then went to the wind-ward side, and poured on about a tea-spoonful of oil; this produced an instant calm over a space several yards square, which spread amazingly, and extended itself gradually till it came to the lee-side; making all that quarter of the pond, perhaps half an acre, as smooth as glass. This experiment was often repeated in different places, and was always attended with success.