The subject matter of Physical Bodies has undergone the least change. It is largely the first half of the previous edition of Physics for Everyone. Since the first book of the new edition contains phenomena that do not require a knowledge of the structure of matter, it was natural to call it Physical Bodies. Of course, another possibility was to use, as is usually done, the title Mechanics (i.e. the science of motion). But the theory of heat, which is covered in the second book, Molecules, also studies motion except that what is moving is the invisible molecules and atoms. So I think the title Physical Bodies is a better choice. Physical Bodies deals largely with the laws of motion and gravitational attraction. These laws will always re-
main the foundation of physics and for this reason of science as whole.