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The Perfect Dome

To the Editors:

Henry Petroski’s article “Arches and Domes” (March–April) reminded me that the earliest masonry domes were probably snow houses or igloos, rather than highly decorated houses of worship. The igloo is not a hemisphere, which would collapse as the snow compresses and the sides bulge out. Instead, igloos follow a more complex curve called a catenary. Catenary, which derives from the Latin word for chain, describes the curve taken by a hanging chain that is held at both ends. As it compresses, an igloo retains its catenary shape and simply becomes shorter. When it gets too short, it is time to build a new igloo.

Click to Enlarge ImageThe igloo in the photo (at right) was built in 1970 as a demonstration by Inuit of Pond Inlet, Nunavut, Canada. Once it was complete, I tested its strength with a device that measures the snow’s cohesion and internal friction. Based on the results, I calculated that an igloo could have a maximum diameter of 10 meters. An igloo of approximately that dimension, built for partying, has indeed been reported in the literature.

An igloo’s catenary shape contrasts with that of a Gothic arch. The Gothic arch has a circular cross section with the top cut out and the sides pushed together. That was not a good move in terms of stability, because it is the part that is cut out that would be stable without any extras like gargoyles or flying buttresses to transfer stresses to adjacent rows of columns. If cathedral designers had studied a length of chain held by its ends, they might have stumbled upon a better way to shape an arch. A few well-known structures, such as the Gateway Arch in St. Louis and the domes of St. Paul’s Cathedral in London and the U.S. Capitol, do approximate catenaries. But it is the Inuit who capitalized on the catenary without embellishments.

Richard L. Handy
Iowa State University

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