About the year 1722, Sir Isaac was desirous of publishing a third edition of his Principia, and the premature death of Mr. Cotes having deprived him of his valuable aid, he had the good fortune to become acquainted with Dr. Henry Pemberton, a young and accomplished physician, who had cultivated mathematical learning with considerable success. M. Poleni, an eminent professor in the University of Padua, having endeavoured, on the authority of a new experiment, to overturn the common opinion respecting the force of bodies in motion, and to establish that of Leibnitz in its place, Dr. Pemberton transmitted to Dr. Mead a demonstration of its inaccuracy. Dr. Mead communicated this paper to Sir Isaac, who not only highly approved of it, but added a demonstration of his own, drawn from another consideration of the subject; and this was printed without his name, as a postscript to Pemberton’s paper, when it appeared in the Transactions.[121] In a short time after the commencement of their acquaintance, Sir Isaac engaged Dr. Pemberton to superintend the new edition of the Principia. In discharging this duty, Dr. Pemberton had occasion to make many remarks on this work, which Sir Isaac always received with the utmost goodness, and the new edition appeared with numerous alterations in 1726. On the occasions upon which he had personal intercourse with Sir Isaac, and which were necessarily numerous, he endeavoured to learn his opinions on various mathematical subjects, and to obtain some historical information respecting his inventions and discoveries. Sir Isaac entered freely into all these topics and during the conversations which took place, and while they were reading together Dr. Pemberton’s popular account of Sir Isaac’s discoveries, he obtained the most perfect evidence that, though his memory was much decayed, yet he was fully able to understand his own writings. During the last twenty years of his life, which he spent in London, the charge of his domestic concerns devolved upon his beautiful and accomplished niece, Mrs. Catharine Barton, the wife of Colonel Barton, for whom, as we have already seen, the Earl of Halifax had conceived the warmest affection. This lady, who had been educated at her uncle’s expense, married Mr. Conduit, and continued to reside with her husband in Sir Isaac’s house till the time of his death.