This book sets out to demonstrate the purpose and critical approach that should be made to all experimental work in physics. It does not describe a systematic course in practical work. The present edition retains the basic outlook of earlier editions, but modifications have been made in response to important changes in computational and experimental methods in the past decade. The text is in three parts. The first deals with the statistical treatment of data, and here the text has been extensively revised to take account of the now widespread use of electronic calculators. The second deals with experimental methods, giving details of particular experiments that demonstrate the art and craft of the experimenter. The third part deals with such essential matters as keeping efficient records, accuracy in arithmetic, and writing good, scientific English.