COMMON THINGS CLOCKS AND WATCHES. 



or chronometers were therefore among the earliest mechanical and 

 physical inventions. 



2. Although nature has supplied visible signs to measure and 

 mark the larger chronometric units, such as days, months, and 

 years, she has not furnished any corresponding measures of the lesser 

 units of hours, minutes* and seconds. There are no visible marks 

 on the firmament by passing from one to another of which the 

 sun can note the hours, still less are there any signs for minutes 

 or seconds. These subdivisions are therefore merely artificial and 

 conventional, and to measure and mark them, artificial motions 

 must be contrived. 



3. Rough approximations were first made to the chief divisions 

 of the day, by observing the apparent motion of the sun from 

 rising to setting. Thus the direction of the meridian, or of the 

 south, being once known, and marked by some fixed and visible 

 object, the time of noon was known by observing when the sun 

 had this direction. The hours before and after noon were roughly 

 estimated by the position of the sun between noon and the times 

 of its rising and setting. Greater precision was given to this 

 method, by erecting a wand or gnomon, the shadow of which 

 would fall upon a level surface, in a direction always opposite to 

 that of the sun. Thus, after sunrise, the shadow would be inclined 

 towards the west, the sun being then towards the east. From the 

 moment of sunrise until noon, the shadow would move continually 

 nearer and nearer to the direction of the north, and at noon it 

 would have exactly that direction. From noon to sunset the 

 shadow would be more and more inclined towards the east. 



It is evident, however, that such a dial would not afford uniform 

 indications at all seasons of the year, so that the hour-lines of the 

 shadow determined in spring, for example, would not show the 

 same hours in winter as in summer. Without much astronomical 

 knowledge, it is easy to be convinced of this. At the equinoxes, 

 the sun rises and sets at six o'clock, and at the east and west 

 points precisely ; and, therefore, at these seasons, the six o'clock 

 hour-lines of such a dial would be for the morning due west, and 

 for the evening due east. But on the first day of summer (21st 

 June), the sun rises and sets at points of the horizon very much 

 north of the east and west points, and at six o'clock in the forenoon 

 and afternoon its bearing is north of the east and west points. 



4. A dial so constructed at any given place would be useless 

 as a time indicator. To render it useful, it would be necessary 

 that the shadow of the style should fall in the same directions at 

 the same hours at all seasons of the year. Now, to attain this 

 object, the style must be not vertical, but must be directed to the 

 celestial pole. It is easy to comprehend that in that case a plane 



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