Sejong the Great (May 6, 1397 – May 18 1450) was the fourth king of the Joseon Dynasty. He is one of only two Korean kings called "great" (the other is Gwanggaeto the Great of Goguryeo). He picked out young scholars and tried to fix the prejudice of classes by helping intelligent but low-class prodigies. In 1442, he had Jang Young-Sil, a scientist in Joseon, invent the 'Chuk-Oo-Gi (測雨器)'. This was a kind of tool to help measure the rain and how much it fell. Also, he expanded the territory of Joseon. 