Bank failures occur when there's not enough gold in the vault and then people start asking for the gold and the goldsmith banker doesn't have it to give out.

当金库没有足够的黄金时,银行就会倒闭,因为当人们开始索要黄金时,金匠银行家却拿不出来

耶鲁公开课 - 金融市场课程节选

They trust to the fact that the depositors won't all come on the same day, just like a goldsmith banker -he leaves only enough gold in the safe to cover the kind of withdrawals that are normally accepted.

他们是基于这样一种考量,即储户们不会在同一天上门来取钱,正如金匠银行家那样,他留存在保险箱中的黄金数量,刚好可以应付一般的提款要求

耶鲁公开课 - 金融市场课程节选