The one footnote has been moved to the end of its section. Punctuation has been made consistent. Mathematical notation has been standardized to current conventions. For example, the notation 1-2 for fractions has been changed to 1/2. Variations in spelling and hyphenation were retained as they appear in the original publication, except that obvious typographical errors have been corrected. p. 14: There is no illustration in the original book (This illustration represents) The following change was made: p. 7: 2 changed to 3 (No. 3, it)