PREFACE.

THERE are several reasons which have induced the Author of the following sheets, to give the Public some-account of their origin and progress, previously to their coming under its examination. They are a series of Essays closely connected with one another, and written on a subject, in the examination of which, he has at intervals employed himself for a considerable part of his life. Considered separately, each may justly be termed a whole, and complete in itself; taken together, they are constituent parts of one Work. The Author entered on this inquiry as early as the year 1750; and it was then that the two first Chapters