Almost two years ago I had written an article on the subject of this book: its brevity, however, did not allow of adequate treatment at all, and it was not printed. The Rev. Charles A. Hall, sug gested that it should be written in extended form, and the present little volume is the fruit of that suggestion. I trust the drawings l have produced for this little work may be found helpful, and convey to the reader a better. Realisation than mere descriptions can usually do.