« PreviousContinue »
lock by the fingers) another to that, and the third to blame second, by like threads. Turn the globe and you will see these locks extend themselves towards the table (as the lower small clouds do towards the earth) being attracit's by it: but on presenting a sharp point erect under te lowest, it will shrink up to the second, the second to 16
, first, and all together to the prime conductor, where thy will continue as long as the point continues under them May not, in like manner, the small electrised clout, whose equilibrium with the earth is soon restored by the point, rise up to the main body, and by that means ou casion so large a vacancy, as that the grand cloud can strike in that place?
These thoughts, my dear friend, are many of th crude and hasty; and if I were merely ambitious of ace' ing some reputation in philosophy, I ought to keep the by me,
till corrected and improved by time, and fart! experience. But since even short hints and imperfect ex periments in any new branch of science, being commun cated, have oftentimes a good effect, in exciting the intention of the ingenious to the subject, and so becor, the occasion of more exact disquisition, and more con plete discoveries, you are at liberty to communicate ti s paper to whom you please; it being more of importa.se that knowledge should increase, than that your frien should be thought an accurate philosopher.
TO DR, PRINGLE, IN LONDON. Relating a curious Instance of the Effect of
Oil on Water.
Philadelphia, Dec, 1, 1702. sir, Daring our passage to Madeira, the weather being warm, ar the cabin windows constantly, open for ti benefit of the air, the candles at night flared and um very much, which was an inconvenience. At Madeira, we got oil to burn; and with a common glass tumbier or beaker, slung in wire, and suspended to the cieling of the cabin, and a little wire hoop for the wick, furnished with corks to float on the oil, I made an Italian lar pa 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
CURIOUS EFFECT OF OIL ON WATER.
from the wind. There is nothing remarkable in all this; but what follows is particular. At supper, looking on the lamp, I remarked, that though 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 remained. In the morning I observed, that though the motion of the ship continued the same, the water was now qniet, 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 packthread round a tumbler, with strings of the same, from each side, meeting above it in a knot at about a foot distance from the toop 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 tranquillity 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.
I have shewn this experiment to a number of ingenious persons. Those who are but slightly acquainted with the principles of hydrostatics, etc. are apt to fancy immediately that they understand it, and readily attempt to explain it; but their explanations have been different, and to nie not very intelligible. Others, more deeply skilled in those principles, seem to wonder at it, and promise to consider it. And 'I think it is worth considering: for a new appearance, if it cannot be explained by our old principles, may afford us new ones, of use perhaps in explaining some other obscure parts of natural knowledge. I ani, etc.
TO DR. BROWNRIGG.
London, Nov. 7, 1773. I thank you for the remarks of your learned friend at Carlisle. I had, when a youth, read and smiled at Pliny's
account of a practice among the seamen of his time, to still the waves in a storm by pouring oil into the sea; which he mentions, as well as the use made of oil by the divers; but the stilling a tempest þy throwing vinegar into the air had escaped me. 'I think with your friend, that it has been of late too much the mode to slight the learning of the ancients. The learned, too, are apt to slight too much the knowledge of the vulgar. The cool. ing by evaporation was long an instance of the latter. This art of smoothing the waves by oil is an instance of both.
Perhaps you may not dislike to have an account of all I have heard, and learnt, and done in this way. Take it, if you please, as follows.
In 1757, being at sea in a flcet of 96 sail bound against Louisbourg, I observed the wakes of two of the ships to be remarkably smooth, while all the others were ruffled by the wind, which blew fresh. Being puzzled with the differing appearance, I at last pointed it out to our captain, and asked him the meaning of it. “The cooks, says he, "have, I suppose, been just emptying their greasy water through the scuppers, which has greased the sides of those ships a little;, and this answer he gave me with an air of soine little contempt, as to a person ignorant of what every body else knew. In my own mind I at first slighted his solution, though I was not able to think of another; but, recollecting what I had formerly read in Pliny, I resolved to make some experiment of the effect of oil on water, when I should have opportunity.
Afterwards being again at sea in 1762, I first observed the wonderful quietness of oil on agitated water, in the swinging, glass lamp I made to hang up in the cabin, as described in my printed papers *). This I was continually looking at and considering, as an appearance to me inexplicable. An old sca captain, then a passenger with me, thought little of it, supposing it an effect of the same kind with that of oil put on water to smooth it, which he said was a practice of the Bermudians when they would strike fish, which they could not see, if the surface of the water' was ruffled by the wind. This practice I had never before heard of, and was obliged to him for the information: though I thought him mistaken as to the sameness of the experiment, the operations being different as well as the effects. In one case, the water is smooth till the oil is put on, and then becomes agitated. In the other it is agitated before the oil is applied,
and then becomes smooth. The same gentleman told me, he had heard it was a practice with the fishermen of Lisbon when about to return into the river (if they saw before them too great a surf upon the bar, which they apprehended might fill their boats in passing) to empty a bottle
) See the preceding paper.
or two of oil into the sea, which would suppress the breakers, and allow them to pass safely. A confirmation of this I have not had an opportunity of obtaining: but dis. coursing of it with another per on, who had often been in the Mediterranean, was informed, that the divers there, who, when under water in their business, need light which the curling of the surface interrupts by the refractions of so many little waves, let a small quantity of oil now and then out of their months, which rising to the surface smooths it, and permits the light to come down to them. All these inforniations 1 at times revolved in my mind, and wondered to find no mention of them in our books of experimental philosophy,
At lenght being at Clapham, where there is, on the common, a large pond, which I observed one day to be very rough with the wind, I fetched out a cruet of oil, and dropt a little of it on the water. | saw it spread itself with surprising swiftness upon the surface: but the effect of smoothing the waves was not produced; for I had adplied it first on the leeward side of the pond, where the waves were largest, and the wind drove my oil back upon the sliore. I then went to the windward side where they began to form; and there the oil, though not more than tea spoonfal, produced an instant calm over a space several yards square, which spread amazingly, and extended itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half an acre, as smooth as a lookingglass.
After this I contrived to take with me, whenever I went into the country, a little oil in the upper hollow joint of my banıboo cane, with which I might repeat the experiment as opportunity should offer, and "I found it constantly to succeed.
In these experiments, one circumstance struck me with particular surprise. This was the sudden, wide, and forcible spreading of a drop of oil on the face of the water, which I do not know that any body has hitherto considered. If a drop of oil is put on a highly polished marble table, or on a looking glass that lies horizontally the drop remains in its place, spreading very little. But when put on water, it spreads instantly many feet round, becoming so thin as to produce the prismatic colours, for a considerable space, and beyond them so much thinner as to be invisible, except in its effect of smoothing the waves at a much greater distance. It seems as if a mutual repulsion between its particles took place as soon as it touched the water, and a repulsion 80 strong as to act on other bodies swimming on the surface, as straw, leaves, chips, etc. forcing them to recede every way from the drop, as froin a centre, leaving a large clear space. The quantity of this force, and the distance to which it will operate, I have not yet ascertained; but I think it a curious inquiry, and I wish to understand whence it arises.
In our journey to the north, when we had the pleasure of seeing you at Ormathwaite, we visited the celebrated Mr. Smeaton, near Leeds. Being about to show him the smoothing experiment on a little pond near his house, an ingenious pupil of his, Mr. Jessop, then present, told us of an odd appearance on that pond, which had lately occurred to him. He was about to clean a little cup in which he kept oil, and he drew upon the water some flies that had been drowned in the oil. These flies presently began to move, and turned round on the water very rapidly, as if they were vigorously alive, though on examination he found they were not so. I immediately concluded that the motion was occasioned by the power of the repulsion above mentioned, and that the oil issuing gradually from the spungy body of the fly continued the motion. He found some more flies drowned in oil, with which the experiment was repeated before us. To show that it was not any effect of life recovered by the flies, 1 imitated it by litte bits of oiled chips and paper cut in the form of a comma, of the size of a common Aly'; when the stream of repelling particles issuing from the point made the comma turn round the contrary way. This is not a chamber experiment, for it cannot be well repeated in a bowl or dish of water on the table. A considerable surface of water is necessary to give room for the expansion of a small quantity of oil. In a dish of wa!er, if the smallest drop of oil be let fall in the middle, the whole surface is presently covered with a thin greasy film proceeding from the drop; but as soon as that film has reached the sides of the dish, no more will issne from the drop, but it remains in the form of oil, the sides of the dish putting a stop to its dissipation by prohibiting the farther expansion of the film.
Our friend Sir John Pringle, being soon after in Scot. land, learned there that those employed in the herringfishery could at a distance see where the shoals of herr. ing were, by the smoothness of the water over them, which might possibly be occasioned, he thought, by some oiliness proceeding from their bodies.
A gentleman from Rhode - island told me, it had been remarked, that the harbour of Newport was ever smooth while any whaling vessels were in it;, which probably arose from hence, that the blubber which they sometimes bring loose in the hold, or the leakage of their barrels, might afford some oil, to mix with that water, which from time to time they pump out to keep their vessel free, and that some oil might spread over the surface of the water in the harbonr, and prevent the forming of any
This prevention I would thus endeavour to explain.
There seems to be no natural repulsion between water and air, such as to keep them from coming into contact with each other. Hence we find a quantity of air in water; and if we extract it by means of the air-pump, the same,