midnight
`` Oh , then , very well ! '' she said . `` If you know who I am , who are _ you _ , and why do you talk in this absurd way with me , a stranger ? '' `` And why , Madam , do you take me up , a stranger , in this absurd way , at midnight , on the streets of Washington ? -- I , who am engaged on business for my chief ? '' She tapped again with her foot on the carriage floor . `` Tell me who you are ! ''