The first part of this includes many old and unusual theories that go back even to ancient times. These should interest us from a historical point of view and also shed some light on the current theories. In many cases, a new current theory is just an old theory brought out again, revised and updated. This article does not favor one theory or another. It's purpose is to list as many theories as possible and give some basic information about them. It is not a detailed or exhaustive report but a short concise summary. Some of these theories may sound trite, but at one time were accepted by many people.