Weighing In Students may choose to find their own weight comparisons. In an arch, the top stones distribute their weight to the blocks on either side and will not fall unless they can push the stones beneath them sideways. Stable arches, therefore, require that side stones be firmly set in place. The riverbanks of Chinese rainbow bridges provided this sideways support. The multiple arches in Roman aqueducts and the double arches of the Roman baths had similar support.
Arches are unstable during construction until the two sides meet in the middle.
To experience the instability, have students stand back to back with a classmate with their shoulders touching. Have them slowly step away from each other, but keep their shoulders in contact. The two students maintain stability because the weight of their bodies is distributed down and sideways through each other's legs. If either were to move away suddenly, both would crash to the floor. Raising the arch requires some dexterity.