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, B. FRANKLIN. TO HUMPHRY MARSHALL. On the Spots in the Sun; Dr. Wilson's New SIR, Hypothesis. London, 14 February, 1773. 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 returned 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. — EDITOR. FROM M. DE SAUSSURE TO B. FRANKLIN.* Conductors of Lightning. - Project of the Royal Society to ascertain the Attraction of Mountains. — Volcanic Eruptions. - Experiments on the Torpedo. SIR, TRANSLATION. Naples, 23 February, 1773. I have received with the greatest pleasure the two letters, which you did me the honor to write to me; the one of October 8th, the other of December 1st. As they were both addressed to me at Geneva, and as I left that place in the beginning of October to come to Italy to pass the winter, they reached me very late, and I have thus been debarred the privilege of showing you by a prompt reply, how much I feel flattered by the honor of your correspondence. The letter on the action of pointed conductors, and the accompanying Essay, contain experiments and reasonings perfectly conclusive, and which leave no doubt as to the utility of these ingenious preservatives.t If I had been acquainted with these new experiments, I should have made use of them with great advantage, in a short apologetic memoir, which I published in October, 1771, for the information of some people, who were terrified at a conductor, which I had erected at Geneva before the house I lived in. This Memoir, however, met with the desired success. It M. De Saussure was the well-known professor at Geneva, celebrated throughout Europe for his philosophical writings, and particularly for his interesting account of his ascent of Mont Blanc.-Editor. + Franklin's letters to M. de Saussure have not been preserved. The papers here alluded to were probably in part those relating to the powder magazines at Purfleet. See Vol. V. pp. 427-445.- EDITor. reassured everybody, and I had the pleasure of watching the electricity from the clouds during the whole course of the last summer. Several persons even followed this example, and raised conductors either upon their houses or before them. M. de Voltaire was one of the first. He does the same justice to your theory, that he did to that of the immortal Newton. The project of the Royal Society is well worthy of the zeal of that illustrious body for the advancement of useful knowledge; and I should be much pleased, if I could in any way aid them in the execution of this project. Had I been at Geneva, I should have made it my duty and pleasure to take a journey to the mountains in the neighbourhood, to ascertain with precision the dimensions of the mountains and valleys, which I thought best suited for the execution of this design. I do not believe, however, that, among those with which I am acquainted, there is any place exactly suited to give certain information on the subject, to which their researches are directed. In the Jura, there is no summit sufficiently high, since the Dole, the mountain which rises highest above the level of our lake, does not reach seven hundred toises above this level. Then it must be considered that the Jura, as well as the Alps, form continued chains of mountains, all *The Royal Society had recently engaged in the project of ascertaining the lateral attraction of mountains, with the view of determining the mean density of the earth upon the Newtonian theory of gravitation. On this subject, it would seem, Dr. Franklin had written to request the aid of M. de Saussure, who had bestowed much time and attention in observing the geological structure and formation of the mountains of the Alps. The object of the Society was finally attained by Dr. Maskelyne, the Astronomer Royal, who made a series of observations on the Schihallion Mountain, in Perthshire, which were considered conclusive. He received the Copley Medal from the Royal Society, for this successful enterprise, on the 30th of November, 1775. See SIR JOHN PRINGLE'S Six Discourses, p. 93. — EDITOR. |