Although the first edition of the Principia had been for some time sold off, and copies of it had become extremely rare, yet Sir Isaac’s attention was so much occupied with his professional avocations that he could not find leisure for preparing a new edition. Dr. Bentley, who had repeatedly urged him to this task, at last succeeded, by engaging Roger Cotes, Plumian Professor of Astronomy at Cambridge, to superintend its publication at the university press. In June, 1709, Sir Isaac committed this important trust to his young friend; and about the middle of July he promised to send him in the course of a fortnight his own revised copy of the work. Business, however, seems to have intervened, and Mr. Cotes was obliged to remind Sir Isaac of his promise, which he did in the following letter:— _Cambridge, Aug. 18th, 1709._ “SIR, “The earnest desire I have to see a new edition of your _Principia_ makes me somewhat impatient till we receive your copy of it, which you were pleased to promise me about the middle of last month you would send down in about a fortnight’s time. I hope you will pardon me for this uneasiness, from which I cannot free myself, and for giving you this trouble to let you know it. I have been so much obliged by yourself and by your book, that (I desire you to believe me) I think myself bound in gratitude to take all the care I possibly can that it shall be correct.—Your obliged servant, “ROGER COTES. “_For Sir Isaac Newton, at his house in Jermyn-street, near St. James’s Church, Westminster._” This was the first letter of that celebrated correspondence, consisting of nearly three hundred letters, in which Sir Isaac and Mr. Cotes discussed the various improvements which were thought necessary in a new edition of the Principia. This valuable collection of letters is preserved in the library of Trinity College; and we cannot refrain from repeating the wish expressed by Dr. Monk, “that one of the many accomplished Newtonians who are resident in that society would favour the world by publishing the whole collection.” When the work was at last printed, Mr. Cotes expressed a wish that Dr. Bentley should write the preface to it, but it was the opinion both of Sir Isaac and Dr. Bentley that the preface should come from the pen of Mr. Cotes himself. This he accordingly undertook; but previous to its execution he addressed the following letter to Dr. Bentley, in order to learn from Sir Isaac the particular view with which it should be written. _March 10th, 1712–13._ “SIR,