McCormick Bridgehouse & Chicago River Museum

This guide to the McCormick Bridgehouse & Chicago River Museum in Chicago explains the engineering trick behind the trunnion bascule bridges that cross the river: a hinge and counterweight balanced so precisely that a motor no bigger than a delivery van's can lift thousands of tons of steel. It follows engineers John Ericson, Edward Wilmann and Thomas Pihlfeldt as they worked out the design at Cortland Street and beyond. Feel for the low hum underfoot as the gears turn in the room below.