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`The Ministry of Communications has been issuing science postage stamps in serial form since 1986 featuring our ancestors` scientific inventions and creativeness so that we of today will inherit and develop their scientific-mindedness. The scientific quest of our forefathers for a time-measuring device is the theme of this third set of science postage stamps, which consists of the following two kinds. Water clock(chagyokru,National Treasure No. 229) Invented in 1434 by Chang Yongshil and Kim Bin, this water clock devised to signal time automatically operated with remarkable precision, contributing greatly to measuring time. Sundial(angbuilgu) This sundial, part of a series of astronomical observation equipment completed during King Sejon`s reign(1418-1450) had a variety of functions and sophistication incomparable with any previous device. It was installed on a big bridge crossed by a large number of people, thus becoming the first public clock of our country.`