Ask a dozen people to name a genius and the odds are that "Einstein" will spring to their lips. Ask them the meaning of "relativity" and few of them will be able to tell you what it is. The basic principles of relativity have not changed since Russell first published his guide for the general reader. In the text Russell steers the reader who has no knowledge of maths or physics through the subtleties of Einstein's thinking. In easy, assimilable steps, he explains the theories of special and general relativity and describes their practical application to, amongst much else, discoveries about gravitation and the invention of the hydrogen bomb.