ards, refrigerator
linings, and railroad ties it has been found available, and for poles
or posts of all sizes it has already proved itself a success. It has
even been suggested as an excellent material for boats, if reinforced;
and minute directions are given by one writer for making a concrete
rowboat. To do this, the wooden boat to be copied is hung up just
above the ground, and clay built around it, leaving a space between
boat and clay as thick as the concrete boat is to be. The wooden boat
is covered with paper and greased, then the concrete is poured into
the space between the boat and the clay mould; and when it hardens and
the wooden boat is removed, there is a boat of stone--or so the
directions declare; but I think most people would prefer one of wood.
However it may be with rowboats, concrete is taking an important place
in the construction of battleships, a backing for armor being made of
it instead of teakwood. The Arizona is built in this way.

Concrete that is carelessly made is very poor stuff, and dangerous to
use, for it is not at all reliable and may give out at any time; but
concrete that is made of the best materials and properly put together
is an exceedingly valuable article.

IV

BRICKS, THEIR FAULTS AND THEIR VIRTUES

The simplest way to make a brick is to fill a mould with soft clay,
then take it out and let it stiffen, and then put it in the sun to
dry. This is the way in which the "adobe" bricks of Central America
are made. They answer very well in countries where there is little
rain; but one or two heavy downpours would be likely to melt a house
built of such material.

Clay is a kind of earth containing mostly alumina and silica or sand,
that can be mixed with water, moulded into any shape, retain that
shape after it is dry, and become hard by being burned. If you want to
make a china cup, you must have a fine sort of clay called "kaolin,"
which is pure white when it is fired and is not very common; but if
you want to make bricks, it will not be at all difficult to find a
suitable clay bank. And yet the clay, even for bricks, must be of the
right kind. If it contains too much silica (sand), the brick will not
mould well; if too much alumina it will be weak; if too much iron, it
will lose its shape in burning; if too much lime, it will be
flesh-colored when it is burned.

If you want to find out whether a building-brick is of good quality,
there are some tests that a boy or