Benjamin Franklin Papers
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From Benjamin Franklin to Mary Stevenson, 11 June 1760

To Mary Stevenson

ALS: Library of Congress9

Cravenstreet, June 11. 1760

Dear Polly,

’Tis a very sensible Question you ask, how the Air can affect the Barometer, when its Opening appears covered with Wood?1 If indeed it was so closely covered as to admit of no Communication of the outward Air to the Surface of the Mercury, the Change of Weight in the Air could not possibly affect it. But the least Crevice is sufficient for the Purpose; a Pinhole will do the Business. And if you could look behind the Frame to which your Barometer is fixed, you would certainly find some small Opening.

There are indeed some Barometers in which the Body of Mercury at the lower End is contain’d in a close Leather Bag, and so the Air cannot come into immediate Contact with the Mercury: Yet the same Effect is produc’d. For the Leather being flexible, when the Bag is press’d by any additional Weight of Air, it contracts, and the Mercury is forc’d up into the Tube; when the Air becomes lighter, and its Pressure less, the Weight of the Mercury prevails, and it descends again into the Bag.

Your Observation on what you have lately read concerning Insects, is very just and solid. Superficial Minds are apt to despise those who make that Part of Creation their Study, as mere Triflers; but certainly the World has been much oblig’d to them. Under the Care and Management of Man, the Labours of the little Silkworm afford Employment and Subsistence to Thousands of Families, and become an immense Article of Commerce. The Bee, too, yields us its delicious Honey, and its Wax useful to a multitude of Purposes. Another Insect, it is said, produces the Cochineal, from whence we have our rich Scarlet Dye.2 The Usefulness of the Cantharides, or Spanish Flies, in Medicine, is known to all, and Thousands owe their Lives to that Knowledge.3 By human Industry and Observation, other Properties of other Insects may possibly be hereafter discovered, and of equal Utility. A thorough Acquaintance with the Nature of these little Creatures, may also enable Mankind to prevent the Increase of such as are noxious or secure us against the Mischiefs they occasion. These Things doubtless your Books make mention of: I can only add a particular late Instance which I had from a Swedish Gentleman of good Credit.4 In the green Timber intended for Ship-building at the King’s Yards in that Country, a kind of Worms were found, which every Year became more numerous and more pernicious, so that the Ships were greatly damag’d before they came into Use. The King sent Linnaeus,5 the great Naturalist, from Stockholm, to enquire into the Affair, and see if the Mischief was capable of any Remedy. He found on Examination, that the Worm was produc’d from a small Egg deposited in the little Roughnesses on the Surface of the Wood, by a particular kind of Fly or Beetle; from whence the Worm, as soon as it was hatch’d, began to eat into the Substance of the Wood, and after some time came out again a Fly of the Parent kind, and so the Species increas’d. The Season in which this Fly laid its Eggs, Linnaeus knew to be about a Fortnight (I think) in the Month of May, and at no other time of the Year. He therefore advis’d, that some Days before that Season, all the green Timber should be thrown into the Water, and kept under Water till the Season was over. Which being done by the King’s Order, the Flies missing their usual Nests, could not increase; and the Species was either destroy’d or went elsewhere; and the Wood was effectually preserved, for after the first Year, it became too dry and hard for their purpose.

There is, however, a prudent Moderation to be used in Studies of this kind. The Knowledge of Nature may be ornamental, and it may be useful, but if to attain an Eminence in that, we neglect the Knowledge and Practice of essential Duties, we deserve Reprehension. For there is no Rank in Natural Knowledge of equal Dignity and Importance with that of being a good Parent, a good Child, a good Husband, or Wife, a good Neighbour or Friend, a good Subject or Citizen, that is, in short, a good Christian. Nicholas Gimcrack, therefore, who neglected the Care of his Family, to pursue Butterflies, was a just Object of Ridicule, and we must give him up as fair Game to the Satyrist.6

Adieu, my dear Friend, and believe me ever Yours affectionately

B Franklin

Your good Mother is well, and gives her Love and Blessing to you. My Compliments to your Aunts, Miss Pitt, &c.

Miss Stevenson

[Note numbering follows the Franklin Papers source.]

9Printed, except for the postscript, as Letter XLIX in Exper. and Obser. (1769 edit.), pp. 445–8; (1774 edit.), pp. 456–8.

1For many of the matters discussed in this letter, see the document immediately above.

2Cochineal is extracted from the dried bodies of females of a species of insect (Dactylopius coccus, Coccus cacti) native to Mexico and Central America.

3The cantharis or Spanish fly (Lytta vescatoria), also known as the blister beetle, is a brilliant green beetle, common in southern and eastern Europe. For medicinal purposes it is dried and powdered; applied externally, it reddens or blisters the skin and serves as a counter-irritant; taken internally, it was formerly used as an irritant or stimulant to the genito-urinary organs and was regarded as an aphrodisiac. It is still sometimes used for the last-named purpose for cattle.

4Possibly Pehr Bjerchén (1731–1774), a student of Linnaeus and an eminent physician of Stockholm, who became an intimate of Peter Collinson and other scientific friends of BF during a prolonged stay in London, 1758–59. Another source of the Swedish report might have been Linnaeus’ “much-loved pupil,” Daniel Charles Solander (1736–1782), F.R.S., botanist on Captain Cook’s voyages in the Endeavour and keeper of the Natural History Department of the British Museum, but Solander does not seem to have arrived in London from Sweden until July 1760. Sir James Edward Smith, ed., A Selection of the Correspondence of Linnaeus and Other Naturalists (2 vols., London, 1821), I, 51–2, 97, 125–6, 131–2; Benjamin D. Jackson, Linnaeus (London, 1923), p. 382; Norah Gourlie, The Prince of Botanists Carl Linnaeus (London, 1953), pp. 245–6; DNB.

5Carl Linnaeus (1707–1778), the great Swedish scientist and father of modern systematic botany.

6Joseph Addison in The Tatler, nos. 216 and 221, created Sir Nicholas Gimcrack, a “virtuoso,” who squandered his estate on natural curiosities, died of exhaustion after chasing a butterfly five miles, and bequeathed such things as crocodile eggs, a hummingbird’s nest, “English weeds pasted on royal paper,” and “rat’s testicles” to his friends and family.

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