One of the results of the World War has been a widespread desire to see the forces of science which proved so mighty in destruction employed generally and systematically for the promotion of human welfare. World Book Company, whose motto is The Application of the World's Knowledge to the World's Needs, has been much in sympathy with the movement to make science an integral part of our elementary education, so that all our people from the highest to the lowest will be able to use it for themselves and to appreciate the possibilities of ameliorating the conditions of human life by its application to the problems that confront us. We count it our good fortune, therefore, that we are able at this time to offer _Common Science_ to the schools. It is one of the new type of texts that are built on educational research and not by guess, and we believe it to be a substantial contribution to the teaching of the subject NWSS:WCS-2 Copyright, 1920, by World Book Company Copyright in Great Britain _All rights reserved_ PREFACE A collection of about 2000 questions asked by children forms the foundation on which this book is built. Rather than decide what it is that children ought to know, or what knowledge could best be fitted into some educational theory, an attempt was made to find out what children want to know. The obvious way to discover this was to let them ask questions. The questions collected were asked by several hundred children in the upper elementary grades, over a period of a year and a half. They were then sorted and classified according to the scientific principles needed in order to answer them. These principles constitute the skeleton of this course. The questions gave a very fair indication of the parts of science in which children are most interested. Physics, in simple, qualitative form,--not mathematical physics, of course,--comes first; astronomy next; chemistry, geology, and certain forms of physical geography (weather, volcanoes, earthquakes, etc.) come third; biology, with physiology and hygiene, is a close fourth; and nature study, in the ordinary school sense of the term, comes in hardly at all. The chapter headings of this book might indicate that the course has to do with physics and chemistry only. This is because general physical and chemical principles form a unifying and inclusive matrix for the mass of applications. But the examples and descriptions throughout the book include physical geography and the life sciences. Descriptive astronomy and geology have, however, been omitted. These two subjects can be best grasped in a reading course and field trips, and they have been incorporated in separate books.