Dr. John Anderson tells about a cartoon that appeared in the NEW YORKER magazine. Approaching a small bridge plainly marked, "Load Limit " 8 tons" was a truck, also marked on its side, "8 tons." When the 8 ton truck was about in the middle of the bridge with the 8 ton limit, a bluebird lighted on the top girder. At that point the bridge gave way and crashed with the truck into the river below, to the obvious surprise of the bluebird.

The bridge was built as…