The steel columns reach from the foundation to the top, riveted together
by plates and may be extended to an indefinite height. In fact there
is no engineering limit to the height.

The outside walls of the sky-scraper vary in thickness with the height
of the building and also vary in accordance with the particular kind
of construction, whether cage or skeleton. If of the cage variety, the
walls, as has been said, act as curtains and consequently they are
thinner than in the skeleton type of construction. In the latter case
the walls have to resist the wind pressure unsupported by the steel
frame and therefore they must be of a sufficient width. Brick and
terra-cotta blocks are used for construction generally.

Terra-cotta blocks are also much used in the flooring, and for this
purpose have several advantages over other materials; they are
absolutely fire-proof, they weigh less per cubic foot than any other
kind of fire-proof flooring and they are almost sound-proof. They do
equally well for flat and arched floors.

It is of the utmost importance that the sky-scraper be absolutely
fire-proof from bottom to top. These great buzzing hives of industry
house at one time several thousand human beings and a panic would
entail a fearful calamity, and, moreover, their height places the upper
stories beyond reach of a water-tower and the pumping engines of the
street.

The sky-scrapers of to-day are as fireproof as human ingenuity and
skill can make them, and this is saying much; in fact, it means that
they cannot burn. Of course fires can break out in rooms and apartments
in the manufacturing of chemicals or testing experiments, etc., but