"Tic-tac! tic-tac! go the wheels of time. We cannot stop them; they will not stop themselves." Time passing is life passing and the measurement of time is the measurement of life itself. How important then that our chronometers, or time measures, be accurate and faithful! It is said that a slight error in a general's watch caused the overthrow of Napoleon at Waterloo and thus changed the history of the world. Because of its great importance the measurement of time has always been a subject of deep human interest and the story of the clock begins with the history of primeval man.

The larger periods of time are measured by the motion of the heavenly bodies. The year and the four seasons are marked off by the motion of the earth in its long journey around the sun; the months and the weeks are told by the changing moon; sunrise and sunset announce the coming and the going of day. The year and the seasons and the day were measured for primeval man by the great clock in the heavens, but how were smaller periods of time to be measured? How was the passing of fractional parts of a day, an hour or a minute or a second to be noted? An egg was to be boiled; how could the cook tell when it had been in the water long enough? A man out hunting wished to get back to his family before dark: how was he to tell when it was time to start homeward?

Plainly, the measurement of small portions of time was a very practical problem from the beginning. The first attempt to solve the problem consisted in observing shadows cast by the sun. The changing shadow of the human form was doubtless the first clock. As the shadow grew shorter the observer knew that noon was approaching; when he could reach out one foot and step on the shadow of his head he knew it was time for dinner; when his shadow began to lengthen he knew that evening was coming on. Observations of this kind led to the shadow clock or sun-dial (Fig. 1). You can make one for yourself. On a perfectly level surface exposed all day to the sun, place in an upright position (Fig. 1) a stick about three feet long, and trace on the surface the shadows as they appear at different times of the day. A little study will enable you to use the shadows for telling the time. Sun-dials have been used from the beginning of time and they have not yet passed out of use. They may still be seen in a few public places (Fig. 2), but they are retained rather as curiosities than as real timekeepers. For the sun-dial is not a good timekeeper for three reasons: (1) it will not tell the time at night; (2) it fails in the daytime when the sun is not shining; (3) it can never be used inside of a house.

The sun-dial can hardly be called an invention; it is rather an observation. There were, however, inventions for measuring time in the earliest period of man's history. Among the oldest of these was the fire-clock, which measured time by the burning away of a stick or a candle. The Pacific islanders still use a clock of this kind. "On the midrib of the long palm-leaf they skewer a number of the oily nuts of the candle-nut-tree and light the upper one." As the nuts burn off, one after another, they mark the passage of equal portions of time. Here is a clock that can be used at night as well as in the daytime, in the house as well as out of doors. Mr. Walter Hough tells us that Chinese messengers who have but a short period to sleep place a lighted piece of joss-stick between their toes when they go to bed. The burning stick serves both as a timepiece and as an alarm-clock. Fire-clocks of one kind or another have been used among primitive people in nearly all parts of the globe, and their use has continued far into civilized times. Alfred the Great (900 A.D.) is said to have measured time in the following way: "He procured as much wax as weighed seventy-two pennyweights, which he commanded to be made into six candles, each twelve inches in length with the divisions of inches distinctly marked upon it. These being lighted one after another, regularly burnt four hours each, at the rate of an inch for every twenty minutes. Thus the six candles lasted twenty-four hours."19

We all remember Irving's account of time-measurement in early New York: "The first settlers did not regulate their time by hours, but pipes, in the same manner as they measure distance in Holland at this very time; an admirably exact measurement, as the pipe in the mouth of a true-born Dutchman is never liable to those accidents and irregularities that are continually putting our clocks out of order." This, of course, is not serious, yet it is an account of a kind of fire-clock that has been widely used. Even to-day the Koreans reckon time by the number of pipes smoked.

If we could step on board a Malay proa we should see floating in a bucket of water a cocoanut shell having a small perforation through which the water by slow degrees finds its way into the interior. This orifice is so perforated that the shell will fill and sink in an hour, when the man on watch calls the time and sets it to float again. This sinking cocoanut shell, the first form of the water-clock, is the clock from which has been developed the timepiece of to-day. With it, therefore, the story of the clock really begins. In Northern India the cocoanut shell is replaced by a copper bowl (Fig. 3). At the moment the sinking occurs the attendant announces the hour by striking upon the bowl.

The second step in the development of the water-clock was made in China several thousand years ago. In the earlier Chinese clock the water, instead of finding its way into the vessel from the outside, was placed inside and allowed to trickle out through a hole in the bottom and fall into a vessel below. In the lower vessel was a float which rose with the water. To the float was attached an indicator which pointed out the hours as the water rose. By this arrangement, when the upper vessel was full, the water, by reason of greater pressure, ran out faster at first than at any other time. The indicator, therefore, at first rose faster than it ought, and after a while did not rise as fast as it ought to. After centuries of experience with the two-vessel arrangement, a third vessel was brought upon the scene. This was placed above the upper vessel, which now became the middle vessel. As fast as water flowed from the middle vessel it was replaced by a stream flowing from the one above it. The depth of the water in the middle vessel did not change, and the water flowed into the lowest vessel at a uniform rate. Finally a fourth vessel was brought into use. The Chinese water-clock shown in (Fig. 4) has been running in the city of Canton for nearly six hundred years. Every afternoon at five, since 1321, the lowest jar has been emptied into the uppermost one and the clock thus wound up for another day.

To follow the further development of the water-clock we must pass from China to Greece. In their early history the Greeks had nothing better than the sun-dial with which to measure time. About the middle of the fifth century B.C. there arose at Athens a need for a better timepiece. In the public assembly the orators were consuming too much time, and in the courts of law the speeches of the lawyers were too long. It was a common thing for a lawyer to harangue his audience for seven or eight hours. To save the city from being talked to death a time-check of some kind became necessary. The sun-dial would not answer, for the sun did not always shine, even in sunny Greece; so the idea of the water-clock was borrowed. A certain amount of water was placed in an amphora (urn), in the bottom of which was a small hole through which the water might slowly flow (Fig. 5). When the amphora was empty the speaker had to stop talking. The Greeks called the water-clock a clepsydra, which means "the water steals away." The orator whose time was limited by a certain amount of water would keep his eye on the clepsydra, just as a speaker in our time keeps his eye on the clock, and if he were interrupted he would shout to the attendant, "You there, stop the water," or would say to the one who interrupted him, "Remember, sir, you are in my water." The story goes that upon one occasion the speaker stopped every now and then to take a drink; the orator's speech, it seems, was as dry as his throat, and a bystander cried out: "Drink out of the clepsydra, and then you will give pleasure both to yourself and to your audience."

At first the Greeks used a simple form of the clepsydra, but they gradually adopted the improvements made by the Chinese, and finally added others. The great Plato is said to have turned his attention to commonplace things long enough to invent a clepsydra that would announce the hour by playing the flute. However this may have been, there was in use in the Greek world, about 300 B.C., a clepsydra something like the one shown in Fig. 6. This begins to look something like a clock. As the water drops into the cylinder E the float F rises and turns G, which carries the hour hand around. Inside of the funnel A is a cone B which can be raised or lowered by the bar D. In this way the dropping of the water is regulated. Water runs to the funnel through H, and when the funnel is full the superfluous water runs off through the pipe I, and thus the depth of the water in the funnel remains the same and the pressure does not change. Notice that when the hand in this old clock has indicated twelve hours it begins to count over again, just as it does on our clocks to-day. How easily it would have been to have continued the numbers on to twenty-four, as they do in Italy, and on the railroads in parts of Canada, to-day.

If we pass from Greece to Rome, our usual route when we are tracing a feature of our civilization, we find that the Romans were slow to introduce new methods of timekeeping. The first public sun-dial in Rome was constructed about 200 B.C., an event which the poet Plautus bewailed:

The water-clock was brought into Rome a little later than the sun-dial, and was used as a time-check upon speakers in the law courts, just as it had been in Athens. When the Romans first began to use the clepsydra it was already a very good clock. Whether it received any great improvements at their hands is not certain. Improvements must have been made somewhere, for early in the Middle Ages we find clepsydras in forms more highly developed than they were in ancient times. In the ninth century the Emperor Charlemagne received as gift from the King of Persia a most interesting timepiece which was worked by water. "The dial was composed of twelve small doors which represented the divisions of the hours; each door opened at the hour it was intended to represent, and out of it came the same number of little balls, which fell, one by one at equal distances of time, on a brass drum. It might be told by the eye what hour it was by the number of doors that were open; and by the ear by the number of balls that fell. When it was twelve o'clock, twelve horsemen in miniature issued forth at the same time, and, marching round the dial, shut all the doors." Less wonderful than the clock of the emperor, but more useful as an object of study, is the medieval clepsydra shown in Figure 7. This looks more than ever like the clock we are accustomed to see. It has weights as well as wheels. As the float A rises with the water it allows the weight C to descend and turns the spindle B on the end of which is the hand which marks the hours. Notice carefully that this is partly a water-clock and partly a weight-clock. The weight in its descent turns the spindle; the water regulates the rate at which the weight may descend.

The water-clock just described led easily and directly to the weight-clock. Clockmakers in the Middle Ages for centuries tried with more or less success to make clocks that would run by means of weights. In 1370, Henry De Vick, a German, succeeded in solving the problem. De Vick was brought to Paris to make a clock for the tower of the king's palace, and he made one that has become famous. In a somewhat improved form it can still be seen in Paris in the Palais de Justice. Let us remove the face of this celebrated timepiece and take a look at its works (Fig. 8). It had a striking part, and a timekeeping part, each distinct from the other. The figure shows only the timekeeping part. The weight (A), of 500 pounds, is wound up by a crank (the key) at P. O is the hour-hand. If A is allowed to descend, you can easily see how the whole system of wheels will be moved—and that very rapidly. But if something does not prevent, A will descend faster and faster, the hour-hand will run faster and faster and the clock will run down at once. If the clock is to run at a uniform rate and for any length of time, the power of the weight must escape gradually. In the clepsydra (Fig. 1) the descent of the weight was controlled by the size of the stream of flowing water. De Vick invented a substitute for the stream of flowing water. Fasten your attention upon the workings of the saw-toothed wheel II and the upright post K, which moves on the pivots l and k, and you may learn what he did. Fixed to the upper part of the post K is a beam or balance LL, at the ends of which are two small weights m and m, and projecting from the post in different directions are two pallets or lips i and h. Now, as the top of the wheel II turns toward you, one of its teeth catches the pallet i and turns the post K a part of the way round toward you. Just as the tooth escapes from i a tooth at the bottom of II (moving from you) catches the pallet h and checks the revolving post and turns it from you. Thus as II turns, it gives a to-and-fro motion to the post K and, consequently, a to-and-fro motion to the balance LL. II is called the escapement because the power of the descending weight gradually escapes from its teeth. In the clepsydra the trickling of water regulated the descent of the weight; in De Vick's clock the trickling of power or force from the escapement regulated the descent of the weight. The invention of this escapement is the greatest event in the history of the clock. The king was much pleased with De Vick's invention. He gave the clockmaker three shillings a day, and allowed him to sleep in the clock tower; a scanty reward indeed for one who had done so much for the world, for De Vick's invention led rapidly to the excellent timepieces of to-day, to both our watches and our clocks. After the appearance of the weight-clock, the water-clock gradually fell into disuse, and all the ingenuity of the clockmaker was bestowed upon weights and wheels and escapements and balances. A century of experimenting resulted in a clock without a weight (Fig. 9). In this timekeeper you recognize the beginnings of the modern watch. The uncoiling of a spring drove the machinery. Instead of the balancing beam with its weights as in De Vick's clock, a balance wheel is used. The escapement is the same as in the first weight-clock. The busy and delicately-hung little balance wheel in your watch is a growth from De Vick's clumsy balance beam. The spring-clock would run in any position. Because it could be carried about it led almost at once to the watch. Many places claim the distinction of having made the first watch, but it seems that the honor belongs to the city of NÜrenburg. "NÜrenburg eggs," as the first portable clocks were called, were made as early as 1470. The first watches were large, uncouth affairs, resembling small table clocks but by the end of the sixteenth century small watches with works of brass and cases of gold or silver were manufactured (Fig. 10).

The last important step in the development of the clock was taken when the pendulum was brought into use. The history of the pendulum will always include a story told by Galileo. This great astronomer, the story runs, while worshiping in the cathedral at Pisa one day, found the service dull, and began to observe the swinging of the lamps which were suspended from the ceiling. Using his pulse as a timekeeper he learned that where the chains were of the same length the lamp swayed to and fro in equal length of time, whether they traveled through a short space or a long space. This observation set the philosopher to experimenting with pendulums of different lengths. Among the many things he learned one of the most important was this: a pendulum thirty-nine inches in length will make one vibration in just one second of time. Now, if the pendulum could only be kept swinging and its vibrations counted it would serve as a clock. Galileo, of course, saw this, and he caused to be made a machine for keeping the pendulum in motion (Fig. 11), but he did not make a clock; he did not connect his pendulum with the works of a clock. This, however, was done about the middle of the seventeenth century, although it is somewhat difficult to tell who was the first to do it. The honor is claimed by an Englishman, a Frenchman, and a Dutchman. The truth is, clockmakers throughout Europe were trying at the same time to make the best of the discoveries of Galileo, and several of them about the same time constructed clocks with pendulums. The one who seems to have succeeded first was Christian Huygens, a Dutch astronomer, who, in 1656, constructed a clock, the motions of which were regulated by the swinging of a pendulum (Fig. 12). The weight was attached to a cord 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.

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.