midnight
None of the invoices which were missing had been entered in this book . It appeared , therefore , that the firm had thirty thousand dollars worth of stock on hand more than was exhibited by the lumber book . I did not understand it , and I came to the conclusion that I did not know half so much about book-keeping as I had flattered myself I did . Still my accounts all `` proved , '' and though I worked over this problem every evening till midnight , I could not arrive at any different result . I was amazed , and even vexed . I did not like to say anything to Mr. Whippleton , because I wished to surprise him with my knowledge of accounts on the first of the month . It would astonish him to learn that the firm had lost over ten thousand dollars in five months , several of them the best in the year for business .