Here’s an excerpt from Walpole’s letter to Mann:
This discovery I made by a talisman, which Mr. Chute calls the sortes Walolianae, by which I find everything I want, a pointe nomm ́ee [at the very moment], wherever I dip for it. This discovery, indeed, is almost of that kind which I call Serendipity, a very expressive word, which, as I have nothing better to tell you, I shall endeavour to explain to you: you will understand it better by the derivation than by the definition. I once read a silly fairy tale, called the three Princes of Serendip: as their Highnesses travelled, they were always making discoveries, by accidents and sagacity, of things which they were not in quest of: for instance, one of them discovered that a mule blind of the right eye had travelled the same road lately, because the grass was eaten only on the left side, where it was worse than on the right now do you understand Serendipity? One of the most remarkable instances of this accidental sagacity (for you must observe that no discovery of a thing you are looking for comes under this description) was of my Lord Shaftsbury, who happening to dine at Lord Chancellor Clarendon’s, found out the marriage of the Duke of York and Mrs. Hyde, by the respect with which her mother treated her at table.“Serendipity” means the making of happy and unexpected discoveries by accident.