It may be necessary to cut out a small trench in front of the constructional hearth to allow the air pipe to be fitted.
In some European and other countries, however, the safe working stresses of timber used for constructional purposes are defined.
One of the most difficult problems with which the metal-worker who handles constructional forms has to deal is the maintenance of a due relation between absolute strength and a useful degree of elasticity.
Nazario e Celso), a small structure in the form of a Latin cross with a dome (in which, as in the baptistery of Neon, the old cathedral, &c., the constructional use of amphorae is noteworthy), with a plain brick exterior, and rich mosaics on a dark blue ground within.
The result is that the tracery itself has to support the structure above it - is, in fact, constructional - whereas in most other countries the tracery is merely, as it were, a pierced screen filling in a constructional arch.