"Jack-knife bridge," Buffalo, New York, 1905

What fascinates me about this lift-bridge is the mathematics of the counterweights.  Notice how they roll down a parabolic track, so at the start of the lift, when the greatest force is required, they are dropping almost vertically; later, as the spans are partway up and less force is needed, the track starts to level out.  Excellent.

The image I've embedded will enlarge twice with two clicks, but those interested in this would be better off viewing the huge image at the Shorpy source page.