My first impressions on considering the magnitude and pretensions of the investigation which I now present to the Institute were that it would perhaps be more suitable for a communication to the Journal, or that at any rate it would scarcely provide sufficient matter to occupy the attention of the meeting for a whole evening. This impression was strengthened when I considered the importance and wide scope of the researches which have been presented as food for our reflection at many of the meetings held in recent years. But, on further consideration, it appeared the subject I had chosen was one likely to give rise to useful discussion. Also I feel that, from a good number of the members of the Institute, at any rate, on whose time and attention the cares of business are continually making greater and greater demands, I may hope for forgiveness if I place before them a somewhat lighter paper than usual.