From Mary Stevenson
Draft: American Philosophical Society
Wanstead, Janr. 13th. 1761
It is long since I troubled you with a Letter, which I am afraid you will impute to Indolence rather than Modesty. The Subject you are so condescending to enter upon with me would afford frequent matter of inquiry for me and give you too much trouble did I not suppress my inclination.
The continual Engagements I am in prevent my spending as much time as I should chuse in the pursuit of Knowledge. I would not be thought to speak from an Affectation of Wisdom, or a Discontent at my Situation; but I cannot help wishing I might employ some of that time in reading, which is devoted to the Card Table.
When I have had opportunity for Study I have attended chiefly to the Discourse on the Nature and Properties of Air.2 Nothing ever more excited my Wonder and Admiration than the Effects of this Invisible Body [torn]3 useful to us. I did not think I should [torn] comprehended the Evaporation of Water; [torn] observes, it appears at first View impossible and contrary to Reason, as Water is of much greater specifick Gravity than Air; but the means by which it is render’d lighter, are very fully explain’d. He4 says, “The burning Matter which the Sun darts upon the Earth, as it finds easy Admittance through the Surface of all fluid Bodies, insinuates itself into the Folds and Interstices of the Air, which gives Fluidity to the Water, thereby rarefying the Air, and consequently those Globules of Water in which it is inclos’d. These fiery Particles being reflected back from the surface of the Earth and Water carry with them those little watery Bubbles into which they insinuate themselves, and together with them several Corpuscles of a different Nature that happen to be intermix’d with them. The Fire and rarefied Air occupy the Inside of the Bubbles while the Water and other gross Particles, which are repell’d on every side from the Centre go to form the Shell. These little Globules thus form’d, being lighter than so many equal Bulks of compress’d Air at the Bottom of the Atmosphere, must necessarily ascend into those Regions where the Air is of the same specifick Gravity with themselves.” This is a very clear Account; but take away the Fire, and the whole Hypothesis is destroy’d. How then does Water evaporate where there is no Fire? My Author is silent here, and there[for I refer?] to my indulgent Preceptor, whose Instructions [torn], and pleasing.
My Mind is not always idle when I have not a Book before me. The Book of Nature is ever open, and I frequently observe Things that excite my Curiosity, and employ my Thoughts to discover their Causes. I have often remark’d as I sat at the Tea Table, that when a Cup has been turn’d down, and there has been some Tea in the Saucer, the Cup would be lifted up, and Bubbles would rise in the Tea. I think I can discover the Cause of this to be the Heat rarefying the Air within the Cup, which endeavouring to expand itself lifts up the Cup, and forces its way out at the Bottom where the Tea rises in Bubbles. To confirm the Truth of this Suggestion I try’d the Experiment with cold Water, and then the Effect ceas’d.
How happy am I in a Friend who has Judgment to correct my Errors, and Candour to excuse them. To [him] therefore I dare deliver my Thoughts without reserve, and confess I am ambitious he should approve them, tho ready at all times to submit to his Reproofs. As you love me Sir endeavour to make me worthy of that Love; and believe me I am ever Dear Sir Your most grateful and devot[ed remainder and signature missing].
Endorsed:5 Copies of my mothers letters to Dr Franklin
2. Polly had been reading Pluche’s Spectacle de la Nature, a set of which, in the eighth edition of Samuel Humphreys’ edition, BF had started giving her in May 1760; see above, p. 102 n. In this edition the passage she quoted here is in the third volume, p. 169, but because that volume was not published until 1762, she must have been using an earlier edition of it.
3. There are several tears in the MS, including one at this point by which two or three words are lost from each of three lines.
5. Probably by Mary Stevenson’s daughter, Elizabeth Hewson Caldwell, to whom she bequeathed this group of letters.