In preparing these Essays, my chief object has been to present scientific truths in a light and readable form—clearly and simply, but with an exact adherence to the facts as I see them. I have followed—here and always—the rule of trying to explain my meaning precisely as I should wish others to explain, to myself, matters with which I was unfamiliar. Hence I have avoided that excessive simplicity which some seem to consider absolutely essential in scientific essays intended for general perusal, but which is often even more perplexing than a too technical style. The chief rule I have followed, in order to make my descriptions clear, has been to endeavour to make each sentence bear one meaning, and one only. Speaking as a reader, and especially as a reader of scientific books, I venture to express an earnest wish that this simple rule were never infringed, even to meet the requirements of style.It will hardly be necessary to mention that several of the shorter Essays are rather intended to amuse than to instruct.