In a review of progress such as this, it is obviously impossible, when a certain stage of development has been reached, owing to the very multiplicity of experimenters, to continue dealing in anything approaching detail with all the different types of machines; and it is proposed, therefore, from this point to deal only with tendencies, and to mention individuals merely as examples of a class of thought rather than as personalities, as it is often difficult fairly to allocate the responsibility for any particular innovation.