text:  A mathematical proposition a passage of poetry a page of history are all admirable things in their way and each may be part of a work destined to durable celebrity; but what should we say to a composition which should present us page about with a theorem of Euclid a scene from Shakspeare and a section from Gibbon? Unity of effect identity of train of thought similarity of ideas are as necessary in a book of travels as in an epic poem a tragedy or a painting