Waves and ripples in water, air, and æther : $b Being a course of Christmas lectures delivered at the Royal Institution of Great Britain

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[Illustration: A CHRISTMAS LECTURE AT THE ROYAL INSTITUTION: “WAVES AND
RIPPLES IN THE AIR.”

                        FIG. 46 (_see p. 109_).

 _Drawn by F. C. Dickinson._]                   [_From the “Graphic.”_
]




                           WAVES AND RIPPLES

                                  IN

                         WATER, AIR, AND ÆTHER


                                 BEING

               A COURSE OF CHRISTMAS LECTURES DELIVERED
                      AT THE ROYAL INSTITUTION OF
                             GREAT BRITAIN


                                  BY

                  J. A. FLEMING, M.A., D.Sc., F.R.S.
                   M. INST. E.E., M.R.I., ETC., ETC.
   PROFESSOR OF ELECTRICAL ENGINEERING IN UNIVERSITY COLLEGE, LONDON


                        _SECOND ISSUE, REVISED_


                                LONDON
               SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE
                      NORTHUMBERLAND AVENUE, W.C.
                    43, QUEEN VICTORIA STREET, E.C.
                      BRIGHTON: 129, NORTH STREET
                        NEW YORK: E. S. GORHAM
                                 1912




  [PUBLISHED UNDER THE DIRECTION OF THE GENERAL LITERATURE COMMITTEE]


             PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
                          LONDON AND BECCLES.




                               PREFACE.


The Christmas Lectures at the Royal Institution are, by a time-honoured
custom, invariably addressed to a “juvenile audience.” This term,
however, has always been held to be an elastic one, and to include
those who are young in spirit as well as those who are young in
years. The conditions, therefore, necessarily impose on the Lecturer
the duty of treating some subject in such a manner that, whilst not
beyond the reach of youthful minds, it may yet possess some elements
of interest for those of maturer years. A subject which admits of
abundant experimental illustrations is accordingly, on these occasions,
a popular one, particularly if it has a bearing upon topics then
attracting public attention. The progress of practical invention or
discovery often removes at one stroke some fact or principle out of
the region of purely scientific investigation, and places it within
the purview of the popular mind. A demand then arises for explanations
which shall dovetail it on to the ordinary experiences of life. The
practical use of æther waves in wireless telegraphy has thus made the
subject of waves in general an interesting one. Hence, when permitted
the privilege, for a second time, of addressing Christmas audiences
in the Royal Institution, the author ventured to indulge the hope
that an experimental treatment of the subject of Waves and Ripples in
various media would not be wanting in interest. Although such lectures,
when reproduced in print, are destitute of the attractions furnished
by successful experiments, yet, in response to the wish of many
correspondents, they have been committed to writing, in the hope that
the explanations given may still be useful to a circle of readers. The
author trusts that the attempt to make the operations of visible waves
a key to a comprehension of some of the effects produced by waves of an
invisible kind may not be altogether without success, and that those
who find some of the imperfect expositions in this little book in any
degree helpful may thereby be impelled to study the facts more closely
from that “open page of Nature” which lies ever unfolded for the
instruction of those who have the patience and power to read it aright.

                                                            J. A. F.

  UNIVERSITY COLLEGE,
    LONDON, 1902.




                               CONTENTS.

                                  —⋄—

                              CHAPTER I.

                    WATER WAVES AND WATER RIPPLES.

                                                                    PAGE

  A visit to the seaside—What is a wave?—Wave-motion on
  water—Definition of a wave—Sea waves—Various forms of
  wave-motion—Wave length, velocity, and frequency—Atlantic
  waves—Rules for speed of sea waves—Illustrations of
  wave-motion—A stone falling on water—Production of a
  wave-train—Wave-energy—Conditions for the production
  of wave-motion—D