four o'clock
`` My suspicion having been thus stimulated , and having been directed in a certain way , I began , from that moment to notice closely the hours during which the engine labored . At night it was always quiet , except on that one brief occasion . Sometimes it began early in the morning and stopped about noon . At other times the work was done entirely in the afternoon , beginning sometimes as late as three or four o'clock , and ceasing invariably at sundown . Then again it would start at sunrise and continue the whole day through . `` For a long time I was unable to account for these eccentricities , and the problem was not rendered much clearer , although a startling suggestiveness was added to it , when , at length , I noticed that the periods of activity of the engine had a definite relation to the age of the moon . Then I discovered , with the aid of an almanac , that I could predict the hours when the engine would be busy .