As one or two questions, asked of me since the publication of this work, have indicated points requiring elucidation, I have added a few short notes in the first Appendix. It is not, I think, desirable otherwise to modify the form or add to the matter of a book as it passes through successive editions; I have, therefore, only mended the wording of some obscure sentences; with which exception the text remains, and will remain, in its original form, which I had carefully considered. Should the public find the book useful, and call for further editions of it, such additional notes as may be necessary will be always placed in the first Appendix, where they can be at once referred to, in any library, by the possessors of the earlier editions; and I will take care they shall not be numerous.