... as such things may be said to have definite origins, after a visit, 30 years ago, to the Great Arch of Ctesiphon on the banks of the Tigris about 15 miles down-stream from Baghdad. This structure is 112 feet high and nearly 90 feet in span. So far as theAuthor can determine it is the &st column-free rectangular...

... be used without steel, but for the larger shells steel was required. It would be interesting to know where the limit of no steel was taken. The difficulty in popularizing and using a shell arch of Ctesiphon type was that, whilst it was a perfectly good structure for certain purposes-for storage...