In Dr. Derek Price's most interesting article ‘Two Mariner's Astrolabes’ he suggests that the heyday of this instrument was from 1530 to 1630. Some additional light is thrown on the closing date of the period of use of the astrolabe by Seller's Practical Navigation, a book which ran into many editions, the earlier ones being now somewhat rare. Successive editions of a book are usually of little value in giving a terminal date for the era of an instrument, for descriptions of it are only too often left in, unweeded long after the instrument is obsolete. Here, however, we have a description of the astrolabe inserted in the first edition of 1669 while it is omitted from the second edition of 1672. This later edition shows signs of careful rewriting and the fact that the author has expunged mention of some instruments, while retaining descriptions of others which never had much vogue, would seem to indicate that he thought that those which he had deleted would no longer be of value to the seaman.