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CCCCLXXX: TO HUMPHREY MARSHALL ON THE SPOTS IN THE SUN—DR. WILSON’S NEW HYPOTHESIS - Benjamin Franklin, The Works of Benjamin Franklin, Vol. VI Letters and Misc. Writings 1772-1775 [1904]

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The Works of Benjamin Franklin, including the Private as well as the Official and Scientific Correspondence, together with the Unmutilated and Correct Version of the Autobiography, compiled and edited by John Bigelow (New York: G.P. Putnam’s Sons, 1904). The Federal Edition in 12 volumes. Vol. VI (Letters and Misc. Writings 1772-1775).

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CCCCLXXX

TO HUMPHREY MARSHALL ON THE SPOTS IN THE SUN—DR. WILSON’S NEW HYPOTHESIS

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 in 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 in 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,

B. Franklin.