These notes and letters were first published by Simon Wilkin in his Edition of Sir Thomas Browne's Works in 1835, but they were not treated from a naturalist's point of view, and in some places were not correctly transcribed, added to which, in the vast mass of matter contained in Wilkin's four large volumes (or in the closely printed three volumes of Bohn's Edition), these interesting passages are in danger of being overlooked or are inconvenient for reference. Two letters, moreover, were needed to make the correspondence with Merrett complete, and these I have been enabled to supply. I hope also that my explanatory notes, which I trust will not be deemed too voluminous, will be found more useful than the necessarily brief notes furnished by Wilkin and his collaborators. Furthermore, I think that the retention of the original spelling and punctuation may lend a charm to the quaintness of the language which is in a measure destroyed by any attempt at modernising. There is much that is interesting bearing upon Natural Science scattered throughout Browne's writings, especially in his _Pseudodoxia Epidemica_, or inquiries into Vulgar and Common Errors, first published in 1646, and the reader cannot fail to be impressed not only with the extent of his classical knowledge but also with the shrewdness with which he pursued his original investigations; but here it is only proposed to deal with certain manuscript notes and a series of rough notes for, or copies of, letters addressed to Dr. Christopher Merrett, the author of the _Pinax Rerum Naturalium Britannicarum_. These, as remarked by their editor, with regard to some other manuscripts published[A] in 1684, under the title of "Certain Miscellany Tracts," were doubtless "rather the _diversions_ than the _Labours_ of his Pen; and ... He did, as it were, drop down his Thoughts of a sudden, in those spaces of vacancy which he snatch'd from those very many occasions which gave him hourly interruption;" but I cannot in this instance agree with the conclusion arrived at by the same writer that it "seemeth probable that He designed them for publick use," for they appear to be the rough drafts or memoranda used in the production of the finished letters (which are unfortunately not forthcoming), and were never intended for publication in their present crude form, thus rendering pardonable such annotations as I have ventured to add. But before proceeding further it is necessary to consider briefly the time and circumstances under which they were written, and the state of what passed for Natural Science at that period.