Larson, writing about the 1890s Chicago World's Fair, uses Daniel Burnham and Henry Holmes to symbolize the two directions in which technology would go in the twentieth century.
Daniel Burnham, the architect who, with several others, masterminded the magnificent Chicago World's Fair, which showcased how far the world had come technologically in a hundred years, foreshadows the glorious achievements of twentieth century technology, such as moon shots and polio vaccines. He represents all the creative potential for good in modern invention.
Henry Holmes, coexisting side by side with Burnham in the same city, represents man's capacity to use modern technology and invention for evil. A sociopath and serial killer, Burnham, with his ingenious gas chambers and ovens meant to hide the evidence of his misdeeds, chillingly foreshadows the evil technology would be used for in the twentieth century.
By juxtaposing the stories of two men who represent extremes in the use and misuse of invention, Larson highlights that technology is a complicated force that can't be simply applauded or condemned, but must be handled with care.