次第に: gradually

春はるになり、次第しだいに暖あたたかくなってきた。

Spring has come and it has gradually gotten warmer.

銅剣どうけんや銅矛どうほこは、もとは武器ぶきでしたが、日本にほんではしだいに大おおきくなって、まつりの道具どうぐになりました。

Bronze swords and spears were originally weapons, but in Japan they gradually became larger and turned into tools for ceremonies.