I shall be much obliged to MR. PAYNE, or to any other of your correspondents, if he will give me any information respecting the treatises on the Seventy Weeks and on the Millennium, or any other advice which may assist me in the completion of the fourteenth volume. I may add, that it is my intention, with the able assistance of my learned friend Dr. Reeves, of Ballymena, to print a complete index to Ussher's Works, which will be compiled by Dr. Reeves, and is now in active preparation. The references to the more important works, such as the _Primordia_, and _Annals_, will be so contrived as to be applicable to the old editions, as well as to Dr. Elrington's edition. This Index will form the seventeenth volume of the Works. JAMES H. TODD. Trinity Coll., Dublin, June 21. 1851. Replies to Minor Queries. _Mind your P's and Q's_ (Vol. iii., pp. 328. 357. 463. 523.).--I have always thought that the phrase "Mind your P's and Q's" was derived from the school-room or the printing-office. The forms of the small "p" and "q" in the Roman type, have always been puzzling to the child and the printer's apprentice. In the one, the down-ward stroke is on the left of the oval; in the other, on the right. Now, when the types are reversed, as they are when in the process of distribution they are returned by the compositor to his case, the mind of the young printer is puzzled to distinguish the "p" from the "q." In sorting _pie_, or a mixed heap of letters, where the "p" and the "q" are not in connexion with any other letters forming a word, I think it would be almost impossible for an inexperienced person to say which is which upon the instant. "Mind your _p_'s and _q_'s"--I write it thus, and not "Mind your P's and Q's"--has a higher philosophy than mind your _toupées_ and your _queues_, which are things essentially different, and impossible to be mistaken. It means, have regard to small differences; do not be deceived by apparent resemblances; learn to discriminate between things essentially distinct, but which look the same; be observant; be cautious. CHARLES KNIGHT. _Serius Seriadesque_ (Vol. iii., p. 494.).--Il Serio, a tributary to the Adda, which falls into the Po. Il Serio is, like the Po, remarkable for the quantity of foam floating upon it, and also for disappearing under ground, through part of its course. DE CAMERA.