Originally published in 1900, this book contains the text of the Adams Prize-winning essay for 1899, which attempts to construct a theory of the electromagnetic field 'consistent with the modern interpretation of chemical, optical and magnetic phenomena in terms of electrically charged particles'. Walker became an expert on the science of climate and was knighted in 1924, and this early essay is a good example of his early interest in electromagnetism. This book will be of value to anyone with an interest in electromagnetism or the early work of Sir Gilbert Walker.