When studying the history of an invention there are two facts or principles which should ever be borne in mind. The first principle is this: _Necessity is the mother of invention._ This principle was touched upon when it was said that an invention appears as a response to a want. When the world wants an invention it usually gets it and makes the most of it, but it will have nothing to do with an invention it does not want. The steam-engine was invented two thousand years ago (p. 55) but the world then had no work for steam to do, so the invention attracted little attention and came to naught. About two hundred years ago, however, man did want the services of steam and inventors were not long in supplying the engine that was needed. About a hundred years ago the broad prairie lands of the United States began to be tilled but it was soon found that the vast areas could not be plowed and that the immense crops could not be harvested by the old methods. So improvements upon the plow and the reaper began to be made and in time the steam gang-plow and the complete harvester were invented. When the locomotive first came into use a simple handbrake was used to stop the slow-going trains, but as the size and the speed of trains increased the handbrake became more and more unsatisfactory. Sometimes a train would run as much as a half mile beyond a station before it could be stopped and then when "backed" it would again pass beyond the station. The problem of stopping the train promptly became fully as important as starting it. The problem was solved by the invention of the air-brake. And thus it has been with all the inventions which surround us: necessity has been the mother of them all. The other principle is that a mechanical invention is a _growth_, or, to state the truth in another way, an invention nearly always is simply an improvement upon a previous invention. The loom, for example, was not invented by a particular person at a particular time; it did not spring into existence in a day with all its parts perfected; it _grew_, century by century, piece by piece. In the stories which will follow the steps in the growth of an invention are shown in the illustrations. These pictures are not for amusement but for study. As you read, examine them carefully and they will teach you quite as much about the growth of the invention as you can be taught by words. FOOTNOTE: [1] Where readers are quite young the Foreword had better be postponed until the stories themselves are read. STORIES OF USEFUL INVENTIONS THE MATCH