In a history of inventions, then, the historian cannot be precise in respect to dates and places. Fortunately this is not a cause for deep regret. It is not a great loss to truth that we cannot know precisely when the first book was printed, nor does it make much difference whether that book was printed in Holland or in Germany. In giving an account of an invention we may be content to treat the matter of time and place broadly, for the story is apt to carry us through a stretch of years that defies computation, a stretch that is immensely longer than the life of any nation. For our purpose these millenniums, these long stretches of time, may be thought of as being divided into three great periods, namely: the _primitive_, the _ancient_, and the _modern_ period. Even a division so broad as this is not satisfactory, for in the progress of their inventions all countries have not kept equal step with the march of time. In some things ancient Greece was modern, while in most things modern Alaska is primitive and modern China is ancient. Nevertheless it will be convenient at times in this book to speak of the _primitive_, the _ancient_ and the _modern_ periods, and it will be useful to regard the _primitive_ period as beginning with the coming of man on earth and extending to the year 5000 B. C.; the _ancient_ period may be thought of as beginning with the year 5000 B. C. and ending with the year 476 A. D., leaving for the _modern_ period the years that have passed since 476 A. D. In tracing the growth of an invention the periods indicated above can serve as a time-guide only for those parts of the world where the course of civilization has taken its way, for invention and civilization have traveled the same road. The region of the world's most advanced civilization includes the lands bordering on the Mediterranean Sea, Central and Northern Europe, the British Isles, North America, South America and Australia. It is within this region that we shall follow the development of whatever invention is under consideration. When speaking of the first forms of an invention, however, it will sometimes be necessary, when an illustration is desired, to draw upon the experience of people who are outside of the wall of civilization. The reason for going outside is plain. The first and simplest forms of the useful inventions have utterly perished in civilized countries, but they still exist among savage and barbarous peoples and it is among such peoples that the first forms must be studied. Thus in the story of the clock, we must go to a far-off peninsula of Southern Asia (p. 190) for an illustration of the beginning of our modern timepiece. Such a departure from the beaten track of civilization does not spoil the story, for as a rule, the rude forms of inventions found among the lowest races of to-day are precisely the same forms that were in use among the Egyptians and Greeks when they were in their lowest state.