the illustrations: American Radiator Company; Commercial Museum,
Philadelphia; General Electric Company; Hershey Chocolate Company;
_Scientific American_; The Goulds Manufacturing Company; Victor
Talking Machine Company. Acknowledgment is also due to Professor Alvin
Davison for figures 19, 23, 29, 142, and 161.

Mr. W.D. Lewis, Principal of the William Penn High School, has read
the manuscript and has given me the benefit of his experience and
interest. Miss. Helen Hill, librarian of the same school, has been of
invaluable service as regards suggestions and proof reading. Miss.
Droege, of the Baldwin School, Bryn Mawr, has also been of very great
service. Practically all of my assistants have given of their time and
skill to the preparation of the work, but the list is too long for
individual mention.

BERTHA M. CLARK.

WILLIAM PENN HIGH SCHOOL.




CONTENTS


CHAPTER

I. HEAT

II. TEMPERATURE AND HEAT