Twelve o'clock
Barbicane and his colleagues , installed on a neighbouring eminence , assisted at the operation . Before them a cannon was planted ready to be fired as a sign from the engineer . A few minutes before twelve the first drops of metal began to run ; the reservoirs were gradually filled , and when the iron was all in a liquid state it was left quiet for some instants in order to facilitate the separation of foreign substances . Twelve o'clock struck . The cannon was suddenly fired , and shot its flame into the air . Twelve hundred tapping-holes were opened simultaneously , and twelve hundred fiery serpents crept along twelve hundred troughs towards the central well , rolling in rings of fire . There they plunged with terrific noise down a depth of 900 feet .