Round Arch and Vault
Although the round arch was used by the ancient peoples of Mesopotamia several centuries before our common era (see 14.9), it was most fully developed by the Romans, who perfected the form in the 2nd century b.c.e. To get a sense of how the arch works, we might go back to the analogy of the body. Imagine that, instead of lying flat on your back, you are bent over forward into a curve, and again you will be lifted into the air. One person will support your hands, another your feet. As long as your body follows the proper arc—that is, your two supporters stand the correct distance apart—you can maintain the pose for some time. If they stand too close together, you start to topple first one way and then the other; if they move too far apart, you have insufficient support in the middle and plunge to the floor. An arch incorporates more complex forces of tension (pulling apart) and compression (pushing together), but the general idea is the same.