unpublished experiments by the author, and some criticism of the conclusions formulated by earlier investigators.
A few terminological innovations have been made at one time and another, as necessity has arisen. New words, or words bearing a special or restricted meaning, are given in the glossary following Chapter X., in addition to the usual footnote references.
Numerical work has been done by the aid of an ordinary 25 c.m. slide rule, with a liability to error of about 15th of 1 per cent., an amount which is quite unimportant.
The author desires to express his thanks to Mr. P. L. Gray in connection with the preparation of the present volume for the Press, in particular for his most welcome assistance in the examination and correction of the proof sheets.
BIRMINGHAM,
October, 1907.
