He might have let the matter rest there but he did not. The occasion was too opportune a one to impress upon Polly the importance of not jumping at conclusions too quickly for him to refrain from borrowing an apt story from Selden about a young woman who, finding herself in the presence of some gentlemen, when they were examining what they called a Chinese shoe, and carrying on a dispute about it, put in her word, and said modestly, "Gentlemen, are you sure it is a Shoe? Should not that be settled first?"
Then he passes to a highly edifying explanation of tidal movements in rivers, so simple that even a child, to say nothing of a bright-witted girl, could experience no difficulty in understanding it, and ends with the question:
After writing 6 Folio Pages of Philosophy to a young Girl, is it necessary to finish such a Letter with a Compliment? Is not such a Letter of itself a Compliment? Does it not say, she has a Mind thirsty after Knowledge, and capable of receiving it; and that the most agreeable Things one can write to her are those that tend to the Improvement of her Understanding?
With his next letter, he enclosed a paper containing his views on several points relating to the air and the evaporation of water, and informed Polly that he would shortly accompany her good mother again to Wanstead, when they could take a walk to some of Lord Tilney's ponds, and make a few experiments there that would explain the nature of tides more fully.
"Adieu, my dear little Philosopher," he exclaims in another letter, after suggesting that thirsty unfortunates at sea might be greatly relieved by sitting in sea water, and declaring that wet clothes do not create colds, whatever damp may do. No one catches cold by bathing, he said, and no clothes can be wetter than water itself.
In another letter, he makes some most readable observations upon the evaporation of rivers and the relations of colors to heat. The ignorant, he declared, suppose in some cases that a river loses itself by running underground, whereas in truth it has run up into the air. And, with reference to the interdependence of heat and color, he pursued this fresh train of ideas:
What signifies Philosophy that does not apply to some Use? May we not learn from hence, that black Clothes are not so fit to wear in a hot Sunny Climate or Season, as white ones; because in such Cloaths the Body is more heated by the Sun when we walk abroad, and are at the same time heated by the Exercise, which double Heat is apt to bring on putrid dangerous Fevers? That Soldiers and Seamen, who must march and labour in the Sun, should, in the East or West Indies have an Uniform of white? That Summer Hats, for Men or Women, should be white, as repelling that Heat which gives Headaches to many, and to some the fatal Stroke that the French call the Coup de Soleil? That the Ladies' Summer Hats, however, should be lined with Black, as not reverberating on their Faces those Rays which are reflected upwards from the Earth or Water? That the putting a white Cap of Paper or Linnen within the Crown of a black Hat, as some do, will not keep out the Heat, tho' it would if placed without? That Fruit-Walls being black'd may receive so much Heat from the Sun in the Daytime, as to continue warm in some degree thro' the Night, and thereby preserve the Fruit from Frosts, or forward its Growth?—with sundry other particulars of less or greater Importance, that will occur from time to time to attentive Minds?
Sometimes he exchanges language like this for such bantering questions as these: "Have you finish'd your Course of Philosophy? No more Doubts to be resolv'd? No more Questions to ask? If so, you may now be at full Leisure to improve yourself in Cards."
Another letter, dated June 7, 1762, was written in contemplation of the fact that he was about to leave the Old World for the New.