Benjamin Franklin Papers

From Benjamin Franklin to Humphry Marshall, 14 February 1773

To Humphry Marshall

ALS: Yale University Library; letterbook draft: Library of Congress

London, Feb. 14. 1773


A considerable Time after its Arrival I received the Box of Seeds you were so good as to send me the Beginning of last Year, with your Observations on the Spots of the Sun.5 The Seeds I distributed among some of my Friends who are curious; please to accept my thankful Acknowledgements for them: The Observations I communicated to our Astronomers of the Royal Society, who are much pleas’d 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 return’d 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 Clouds 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 are rekindled. It is however remarkable that tho’ large Spots are seen gradually to become small ones, no one has observ’d a small Spot gradually to become a large one; at least I do not remember to have met with such an Observation. If this be so, it should seem that they are suddenly form’d of the 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.6 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 a burning Coal, while all within remains Wood. Then he supposes, that 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 length 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 they are such Hollows in the Sun’s Face as he supposes, and the bright Border round the Edges be the fluid burning Matter flowing down the Banks in to 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, tho’ the Eye, which now views it aslant, can see full the farthest Bank, yet that which is nearest is hidden and not to be distinguish’d; and when the same Spot comes to emerge again on the other Side of the Sun, the Bank which before was visible is now conceal’d, and that conceal’d 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 return’d to me, and I have consider’d them, I shall lodge them among the Papers of the Society, and let you know their Sentiments.

As to procuring you a Correspondence with some ingenious Gentelman here, who is curious, which you desire, I find many who like to have a few Seeds given them, but do not desire large Quantities, most considerable Gardens being now supply’d like Dr. Fothergill’s, with what they chuse to have; and there being Nursery-men now here, who furnish what Particulars are wanted, without the Trouble of a foreign Correspondence and the Vexations at the Customhouse. You will therefore oblige me by letting me know if in any other way I can be serviceable to you. With great Respect and Esteem, I am, Sir, Your most obedient humble Servant

B Franklin

Mr Humphry Marshall

Addressed: To / Mr Humphry Marshall / of Bradford / Chester County / Pensylvania / per Capt All

[Note numbering follows the Franklin Papers source.]

5The box of seeds, with the observations, was missing for some time; see above, XVIII, 255–6; XIX, 88, 92. The customs officers were to blame, it turned out, for the second paragraph of this letter in BF’s draft began with two sentences that he deleted: “I have received but within these few Days your last Box of Seeds, which has been detain’d unreasonably at the Custom House. I am exceedingly oblig’d by the Present.” The observations on sunspots were presumably included, but have been lost; for Marshall’s illustrations of them see his letter below, May 3.

6The first theory, that sunspots were smoke, was initially promulgated by William Derham in the Phil. Trans., XXVII (1710), 270–90. BF subscribed to a variant of this theory (above, IV, 300–1), and may be referring here either to his own views or to those of the Derham school. His belief that spots first appeared at full size, however, was a remarkable error: it had been known for more than a century that they could increase as well as decrease. Alexander Wilson was developing a theory of their nature that became before long the predominant one. (Charles Hutton, A Mathematical and Philosophical Dictionary … [2 vols., London, 1795], II, 62–3.) It grew out of observations begun in 1769, and was clearly arousing attention before he made it public in a paper read before the Royal Society in late April, 1773: “Observations on the Solar Spots,” Phil. Trans., LXIV (1774), 1–30. For Wilson, who achieved distinction as a type-founder as well as astronomer, see above, XVIII, 67 n.

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