Sentence:  It seems to be a kind of understanding or unwritten compact between the orator and the audience, that he be allowed to talk without interruption as much as he pleases, so long as they are not called upon to listen to one word that he utters. Accordingly, during the delivery of one of these <WSD>triduan</WSD> discourses, the Senate of the United States wears the appearance of an orderly well-regulated reading-room; the members being comfortably seated in their arm-chairs, some looking over and answering private letters, some exchanging a few words in a low whisper with each other, or with friends in the strangers' gallery, others reading a newspaper, and all evincing the most philosophic indifference to the tedious harangue and the exhaustless lungs of the orator. 
Meaning of the word marked with <WSD> tags:  Lasting three days.