joint expand and contract three-eighths of an inch, and even when a carriage passes on the roadway you can see it move a little. These slip joints are necessary chiefly because of the heat and cold. In summer the cables are fifteen inches longer between the towers than in the winter. The bridge structure is cut in two in the middle, and an arm is fastened to one of these ends. It slips into an opening in the other end, and moves back and forth as any expansion or contraction occurs. I noticed one day last winter, when the greatest crush in the history of the bridge occurred, and when it was estimated that 2500 tons of human beings were distributed along the bridge at one time, that this slip joint in the middle was drawn out at least fifteen inches because of the unusual weight. As each cable, however, is intended to sustain a weight of 12,000 tons, this great crush was a small matter. Still, the constant motion of the bridge that seems so solid and inflexible is well worth studying.
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