In the mid-eighteenth century, the famed mathematician Leonhard Euler developed a spe- cial "path construction" argument to prove that it was impossible to make such a trip. Later, in the early nineteenth century thesame problem was solved by representing the situation as a net- work in which each ofthe four sections (A, B, C, and D) is a node and each bridge is an arc join- ing applicable nodes, as shown in Figure 6.4.