ln, which has fixed side walls and either an open or
closed top. Then, too, there is a "continuous" kiln. This has a number
of chambers, and the heat from each one passes into the next; so that
bricks in one chamber may be just warming up while in another they are
ready to be taken out.

When the bricks come out of the kiln, some of them are good and some
are not. Those that were on the outside are not burned enough; those
next it are not well baked, but can be used for the middle of thick
walls. The next ones are of good quality; but those directly over the
fires are so hard and brittle that they are of little use except for
pavements.

Paving-bricks, however, are not to be despised. They are not as smooth
and well finished as pressed brick, but they are exceedingly useful.
They need as much care in making as any others, and they must be
burned in a much hotter fire to make them dense and hard. The tests
for paving-bricks are quite different from those for ordinary
building-brick. If first-class paving-bricks weighing fifty pounds are
soaked in water for twenty hours, they take up so little water that
they will not weigh more than fifty-one or fifty-one and a half pounds
when taken out. To find out how hard they are, the bricks are weighed
and shaken about with foundry shot for a number of hours. Then they
are weighed again to see how much of their material has been rubbed
off. A third test is to put one brick on edge into a crushing machine
to see how much pressure it will stand. Paving-brick is cheaper than
granite blocks, and if it has a good foundation of concrete covered
with sand, it will last about three fourths as long. Brick is less
noisy than stone and is easier to clean.

Not so very long ago, when particularly handsome bricks were needed
for the outside of walls and other places where they would be
conspicuous, they were "re-pressed"; that is, they were made by hand
or in a "soft-mud" machine, and then, after drying for a while, were
put into a re-pressing machine to give them a smooth finish. These
machines are still used, but they are hardly necessary, for the
"dry-clay" brick machine will turn out a smooth brick in one
operation.

Another substance which is made of almost the same materials as brick
is terra cotta. To make this, fire brick, bits of pottery, partly
burned clay, and fine white sand are ground to a powder and mixed very
thoroughly. This mixture is moulded, dried, and burned. Until
rec