A clock is a machine for keeping time. And for this there are four essentials, without any one of which there would be no clock. First, there must be a motive power to make it run; second, there must be a means of transmitting this power; third, there must be a regulating device to make the mechanism move steadily and slowly, and keep the motive power from running down too quickly; and, fourth, there must be some device to mark the time and make it known. In a typical modern clock the power comes from the pull of a weight or the pressure of a spring—although clocks may, of course, be operated by electricity or compressed air or some other means; also, the regulator is what is known as the "escapement" and the recording device consists of the hands, the dial, and the striking mechanism. Having stated this, let us return to the past and see if we can determine how these principles came to be applied. This is not altogether easy. Our forefathers were less particular than we over such trifling questions as names and spelling—even the learned Shakespeare, long afterward, used several different spellings of his own name. Thus, when we see in the records of the period the name of "clock" or "horologe" we cannot tell with certainty what type is meant, since "horologe" meant simply a device for keeping time; it might have been applied equally well to a clock, clepsydra, an hour-glass, or even a sun-dial. "It is quite possible," writes M. Gubelin Breitschmidt, the younger, an eminent horologist of Lucerne, Switzerland, "that a large number of the technical inventions of antiquity were lost during the migrations of the barbarians and under the chaotic conditions prevailing during the first thousand years of Christianity, but the most perfect surviving instrument for measuring time was the water-clock, known as the clepsydra, which was able to maintain its supremacy long after the appearance of the wholly mechanical clock, just as the beautiful manuscripts of the artist monks and laymen were favored by the cultured classes long after the invention of movable types for printing.