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Why is a clock devided in 12 hrs and not in 24?
Originally, day and night were each divided into 12 hours; the length of the hour would depend on the time of year. This came about because of the widespread use of sundials to tell time; the faster the sun "moves" from horizon to horizon, the shorter the hours are. Water clocks, the only timepieces which could keep track of equal subunits of time at night without constant attention, were too inaccurate to serve as a standard. The practice of numbering twelve hours between sunrise and sunset still survives, though often in modified form, in Christian monastic practice. The first morning devotions are traditionally called prime ("one") and are at sunrise or around 6 AM; tierce ("three") is at 9 AM; sext ("six") is at noon and none ("nine") is at 3 PM. Vespers, of course, take their name not from a number but from the Latin for "evening." Even after mechanical clocks were invented, the sundial continued as the standard timepiece, which created some interesting engineering problems for clockmakers! But since clocks weren't all that accurate in the first place, this didn't matter much. Presumably, as clocks became more accurate and more common it was found convenient to divide the period from noon to noon, or midnight to midnight, or evening to evening, into 24 equal units. But (most) clocks still keep a 12-hour schedule, and even in Europe, where train schedules are given as (for example) 18:32 rather than 6:32 PM, or in the military, which uses a 24-hour system, conventional usage is still on a twelve-hour system. References:
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