From the fifth edition onwards, an additional note was added to the 
preface:

 FIFTH EDITION

 In this Edition, the Author has revised the original matter and added
 a large number of new devices, incorporating these, with the Appendix
 to the previous edition, in the body of the work. The numbers in this
 edition are therefore not the same as in previous issues, and in some
 instances the titles of the sections have been also amended.