passing over a pulley and gave motion to all the wheels, as in De Vick's clock. Like De Vick's clock also Huygens's clock had its escapement wheel acting upon two pallets. In the Dutchman's clock, however, the escapement, instead of turning a balance beam to and fro, acted upon the pendulum, giving it enough motion to keep it from stopping. [Illustration: FIG. 12.--THE FIRST PENDULUM CLOCK. (1656.)] We need not carry our story further than the invention of Huygens. Timepieces are cheaper and better made and more accurate than they were two hundred years ago, but no really important discovery has been made since the pendulum was introduced. FOOTNOTE: [19] Wood, "Curiosities of Clocks and Watches." THE BOOK What is a book? It is an invention by means of which _thought_ is recorded, and carried about in the world, and handed down from one age to another. Almost as soon as men began to think they began to make books and they will probably continue to make them as long as they continue to think. The story of the Book, therefore, takes us back to the very beginning of human existence. At first thought was recorded and preserved by _tradition_. An account of a nation's deeds, its laws, the precepts of its religion were stamped, printed, on the memory of persons specially trained to memorize these things and hand them down by word of mouth from generation to generation (Fig. 1). These persons were usually priests, who underwent long years of daily and hourly training in memorizing what was to be handed down. The Sanskrit Vedas, the sacred scripture of the Hindoos, were for many centuries transmitted by tradition, and it is said it took forty years to memorize them. It is a wonder it did not take longer, for the Vedas make a volume as large as our Bible. It is believed that primitive people everywhere first adopted the method of tradition to record and preserve the thought which they did not wish to perish. We may say, then, that the first book was written on the tablet of the human memory. [Illustration: FIG. 1.--TRADITION. A Mural Decoration in the Library of Congress.]