以前的闹钟是怎样设置的？

人们会先计算，单位时间内能燃烧多长的蜡烛，然后在相应刻度放钉子，到点蜡烛烧完钉子就会掉下敲打金属盘。

When people did not have clocks, they used 「candle clocks」. Candles that would burn for a set amount of hours, lf you needed an alarm, you would push a nail at the desired time length in the candle and once it melted to the point, the nail would fall and hit the metal holder, alerting you.