My foremost design in writing this Preface is to address a word of exhortation to the Society for Promoting Christian Knowledge. In the essay which follows, the reader will often find Bishop Wilson quoted. To me and to the members of the Society for Promoting Christian Knowledge his name and writings are still, no doubt, familiar; but the world is fast going away from old-fashioned people of his sort, and I learnt with consternation lately from a brilliant and distinguished votary of the natural sciences, that he had never so much as heard of Bishop Wilson, and that he imagined me to have invented him. At a moment when the Courts of Law have just taken off the embargo from the recreative religion furnished on Sundays by my gifted acquaintance and others, and when St. Martin's Hall [iv] and the Alhambra will soon be beginning again to resound with their pulpit-eloquence, it distresses one to think that the new lights should not only have, in general, a very low opinion of the preachers of the old religion, but that they should have it without knowing the best that these preachers can do. And that they are in this case is owing in part, certainly, to the negligence of the Christian Knowledge Society. In old times they used to print and spread abroad Bishop Wilson's Maxims of Piety and Christianity; the copy of this work which I use is one of their publications, bearing their imprint, and bound in the well-known brown calf which they made familiar to our childhood; but the date of my copy is 1812. I know of no copy besides, and I believe the work is no longer one of those printed and circulated by the Society. Hence the error, flattering, I own, to me personally, yet in itself to be regretted, of the distinguished physicist already mentioned.