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happening, which made a lee-shore between Haslar hospital and the point near Jillkecker, we went from the Centaur with the longboat and barge towards that shore. Our disposition was this ; the longboat was anchored about a quarter of a mile from the shore; part of the company were landed behind the point (a place more sheltered from the sea) who came round and placed themselves opposite to the longboat, where they might observe the surf, and note if any change occurred in it upon using the oil. Another party, in the barge, plied to windward of the longboat, as far from her as she was from the shore, making trips of about half a mile each, pouring oil continually out of a large stone bottle, through a hole in the cork, some· what bigger than a goose-quill. The experiment had not, in the main point, the success we wished, for no material difference was observed in the height or force of the surf upon the shore; but those who were in the longboat could observe a tract of smoothed water, the whole of the distance in which the barge poured the oil, and gradually spreading in breadth towards the long-boat. I call it smoothed, not that it was laid level; but because, though the swell continued, its surface was not roughened by the wrinkles, or smaller waves, before mentioned; and none or very few white caps (or waves whose tops turn over in foam) appeared in that whole space, though to windward and leeward of it there were plenty; and a wherry, that came round
; the point under sail, in her way to Portsmouth, seemed to turn into that tract of choice, and to use it from end to end, as a piece of turnpike road.
It may be of use to relate the circumstances of an experiment that does not succeed, since they may give hints of amendment in future trials; it is therefore I have been thus particular. I shall only add what I apprehend may have been the reason of our disappointment.
I conceive, that the operation of oil on water is, first, to prevent the raising of new waves by the wind; and, secondly, to prevent its pushing those before raised with such force, and consequently their continuance of the same repeated height, as they would have done, if their surface were not oiled. But oil will not prevent waves being raised by another power, by a stone, for instance, falling into a still pool; for they then rise by the mechanical impulse of the stone, which the greasiness on the surrounding water cannot lessen or prevent, as it can prevent the winds catching the surface and raising it into waves. Now waves once raised, whether by the wind or any other power, have the same mechanical operation, by which they continue to rise and fall
, as a pendulum will continue to swing a long time after the force ceases to act by which the motion was first produced; that motion will, however, cease in time; but time is necessary. Therefore, though oil spread on an agitated sea may weaken the push of the wind on those waves whose surfaces are covered by it, and so, by receiving less fresh impulse, they may gradually subside; yet a considerable time, or a distance through which they will take time to move, may be necessary to make the effect sensible on any shore in a diminution of the surf; for we know, that, when wind ceases suddenly, the waves it has raised do not as suddenly subside, but settle gradually, and are not quite down till after the wind has ceased. So, though we should, by oiling them, take off the effect of wind on waves already raised, it is not to be expected that those waves should be instantly levelled. The motion they have received will, for some time, continue; and, if the shore is not far distant, they arrive there so soon, that their effect upon it will not be visibly diminished. Possibly, therefore, if we had begun our operations at a greater distance, the effect might have been more sensible. And perhaps we did not pour oil in sufficient quantity. Future experiments may determine this.
I was, however, greatly obliged to Captain Bentinck, for the cheerful and ready aids he gave me; and I ought not to omit mentioning Mr. Banks, Dr. Solander, General Carnoc, and Dr. Blagden, who all assisted at the experiment, during that blustering, unpleasant day, with a patience and activity that could only be inspired by a zeal for the improvement of knowledge, such especially as might possibly be of use to men in situations of distress.
I would wish you to communicate this to your ingenious friend, Mr. Farish, with my respects; and believe me to be, with sincere esteem, dear Sir, Your most obedient humble servant,
TO HUMPHRY MARSHALL.
On the Spots in the Sun; Dr. Wilson's Nero
London, 14 February, 1773. SIR, A considerable time after its arrival, I received the box of seeds you sent me the beginning of last year, with your observations on spots of the sun. The seeds I distributed among some of my friends who are curious; accept my thankful acknowledgments for them. The observations I communicated to our astronomers of the Royal Society, who are much pleased with them,
and hand them about from one to another; so that I have had little opportunity of examining them myself, they not being yet returned to me.
Here are various opinions about the solar spots. Some think them vast clouds of smoke and soot arising from the consuming fuel on the surface, which at length take fire again on their edges, consuming and daily diminishing till they totally disappear. Others think them spots of the surface, in which the fire has been extinguished, and which by degrees is rekindled. It is however remarkable, that, though large spots are seen gradually to become small ones, no one has observed a small spot gradually become a large one; at least I do not remember to have met with such an observation. If this be so, it should seem they are suddenly formed of their full size; and perhaps, if there were more such constant and diligent observers as you, some might happen to be observing at the instant such a spot was formed, when the appearances might give some ground of conjecture by what means they were formed.
The professor of astronomy at Glasgow, Dr. Wilson, has a new hypothesis. It is this; that the sun is a globe of solid matter, all combustible, perhaps, but whose surface only is actually on fire to a certain depth, and all below that depth unkindled, like a log of wood, whose surface to half an inch deep may be burning coal, while all within remains wood. Then he supposes, by some explosion similar to our earthquakes, the burning part may be blown away from a particular district, leaving bare the unkindled part below, which then appears a spot, and only lessens as the fluid burning matter by degrees flows in upon it on all sides, and at last covers or rekindles it. He founds this opinion on certain appearances of
the edges of the spots as they turn under the sun's disk, or emerge again on the other side; for, if there are such hollows in the sun's face as he supposes, and the bright border round their edges be the fluid burning matter flowing down the banks into the hollow, it will follow, that, while a spot is in the middle of the sun's disk, the eye looking directly upon the whole, may discern that border all round; but when the hollow is moved round to near the edge of the disk, then, though the eye which now views it aslant can see full the farthest bank, yet that which is nearest is hidden, and not to be distinguished; and when the same spot comes to emerge again on the other side of the sun, the bank which before was visible is now concealed, and that concealed which before was visible, gradually changing, however, till the spot reaches the middle of the disk, when the bank all round may be seen as before. Perhaps your telescope may be scarce strong enough to observe this. If it is, I wish to know whether you find the same appearances. When your observations are retumed to me, and I have considered them, I shall lodge them among the papers of the Society, and let you know their sentiments. With great esteem and regard, I am, &c.
* In the Philosophical Transactions (Vol. LXIV. p. 194) is a letter from Humphry Marshall to Dr. Franklin on solar spots.