But the closest friendship that Franklin formed in England was with Mary, or Polly, Stevenson. To her, perhaps, the most delightful of all his familiar letters were written--letters so full of love and watchful interest as to suggest a father rather than a friend. It is not too much to say that they are distinguished by a purity and tenderness of feeling almost perfect, and by a combination of delicate humor and instructive wisdom to which it would be hard to find a parallel. The first of them bears date May 4, 1759, and the last bears date May 30, 1786. That the letters, some forty-six in number, are not more numerous even than they are is due to the fact that, during the period of their intercourse, the two friends were often under the same roof, or, when they were not, saw each other frequently. In his first letter, addressed to "My Dear Child," Franklin tells Polly, who was then about twenty years of age, that he had hoped for the pleasure of seeing her the day before at the Oratorio in the Foundling Hospital, but that, though he looked with all the eyes he had, not excepting even those he carried in his pocket, he could not find her. He had, however, he said, fixed that day se'nnight for a little journey into Essex, and would take Mrs. Stevenson with him as far as the home of Mrs. Tickell, Polly's aunt, at Wanstead, where Polly then was, and would call for Mrs. Stevenson there on his return. "Will," he says in a postscript, "did not see you in the Park." Will, of course, was his son. In the succeeding year, he writes to Polly that he embraces most gladly his dear friend's proposal of a subject for their future correspondence, though he fears that his necessary business and journeys, with the natural indolence of an old man, will make him too unpunctual a correspondent. But why will you [he asks], by the Cultivation of your Mind, make yourself still more amiable, and a more desirable Companion for a Man of Understanding, when you are determin'd, as I hear, to live single? If we enter, as you propose, into _moral_ as well as natural Philosophy, I fancy, when I have fully establish'd my Authority as a Tutor, I shall take upon me to lecture you a little on that Chapter of Duty. He then maps out a course of reading for her, to be conducted in such a manner as to furnish them with material for their letters. "Believe me ever, my dear good Girl," he concludes, "your affectionate Friend and Servant."