The following paper was originally ritten simply as a reply to Mr. Macfadyen's, which was redd before the Institute in May last. It has since been carefully revised and to a great extent recast, in consequence of suggestions receivd, to the effect that the subject was of efficient importance to be delt with in an independent paper and submitted afresh to the consideration of the Institute. In consequence of this change, a considerable amount of personal criticism, which might hav been suitable enuf for a personal reply to another paper, has been either struck out altogether, or much modified. It will therefore be understood that, in what follows, I hav somtimes had Mr. Macfadyen's arguments in view when I hav not mentiond his name