To John Pringle
MS not found; reprinted from Benjamin Franklin, Experiments and Observations on Electricity (London, 1769), pp. 438–40.9
Philadelphia, Dec. 1, 1762.
During our passage to Madeira,1 the weather being warm, and the cabbin windows constantly open for the benefit of the air, the candles at night flared and run very much, which was an inconvenience. At Madeira we got oil to burn, and with a common glass tumbler or beaker, slung in wire, and suspended to the cieling of the cabbin, and a little wire hoop for the wick, furnish’d with corks to float on the oil, I made an Italian lamp, that gave us very good light all over the table. The glass at bottom contained water to about one third of its height; another third was taken up with oil; the rest was left empty that the sides of the glass might protect the flame from the wind. There is nothing remarkable in all this; but what follows is particular. At supper, looking on the lamp, I remarked that tho’ the surface of the oil was perfectly tranquil, and duly preserved its position and distance with regard to the brim of the glass, the water under the oil was in great commotion, rising and falling in irregular waves, which continued during the whole evening. The lamp was kept burning as a watch light all night, till the oil was spent, and the water only remain’d. In the morning I observed, that though the motion of the ship continued the same; the water was now quiet, and its surface as tranquil as that of the oil had been the evening before. At night again, when oil was put upon it, the water resumed its irregular motions, rising in high waves almost to the surface of the oil, but without disturbing the smooth level of that surface. And this was repeated every day during the voyage.
Since my arrival in America, I have repeated the experiment frequently thus. I have put a pack-thread round a tumbler, with strings of the same, from each side, meeting above it in a knot at about a foot distance from the top of the tumbler. Then putting in as much water as would fill about one third part of the tumbler, I lifted it up by the knot, and swung it to and fro in the air; when the water appeared to keep its place in the tumbler as steadily as if it had been ice. But pouring gently in upon the water about as much oil, and then again swinging it in the air as before, the tranquility before possessed by the water, was transferred to the surface of the oil, and the water under it was agitated with the same commotions as at sea.2
I have shewn this experiment to a number of ingenious persons. Those who are but slightly acquainted with the principles of hydrostatics, &c. are apt to fancy immediately that they understand it, and readily attempt to explain it; but their explanations have been different, and to me not very intelligible. Others more deeply skill’d in those principles, seem to wonder at it, and promise to consider it.3 And I think it is worth considering: For a new appearance, if it cannot be explain’d by our old principles, may afford us new ones, of use perhaps in explaining some other obscure parts of natural knowledge. I am, &c.
9. Letter XLV in the 1769 edition of Exper. and Obser. and also in the 1774 edition, where it is printed on pp. 448–50.
1. The convoy with which BF sailed to Philadelphia stopped at Madeira for three days.
2. In a letter to William Brownrigg, Nov. 7, 1773 (Royal Soc.), BF recorded all he had “heard, and learnt, and done” about the interaction of oil and water, a matter which had interested him since boyhood when he had read Pliny’s account of seamen stilling waves in a storm by pouring oil into the sea.
3. In May 1763 Pringle reported that he had read this letter to a group of friends and that everyone was “agog about this new property of fluids.” See below, p. 269.