cessive steps in the manufacturing process. Check cracks
resulting from the burning or from too rapid cooling are often
encountered, but unless these are deep, that is 3/16 inch or more,
they do not impair the wearing quality of the brick, nor indicate
structural weakness. Kiln marks are formed on some of the brick due to
the weight of the brick above in the kiln. These depressions are not
objectionable unless the brick are so distorted that they will not lie
evenly in the pavement.

Spacing lugs or raised letters are formed on one face of the brick to
insure sufficient space between the brick for the filler. These lugs
or letters are not less than 1/8 inch nor more than 1/4 inch high and
of such design that they will not obstruct the free flow of filler
into the joints between the brick.

Several varieties of paving brick are to be had, the difference being
principally in the design or size.

=Repressed Brick.=--In this type of brick the spacing lugs are formed
by pressing the green brick, after it has been cut to size, into a
mold on one face of which are recessed letters or other devices into
which the clay is pressed, thus forming the spacing lugs.

=Vertical Fiber Brick.=--These brick are designed to be laid with one
wire-cut face up and spacing is provided by two or more beads on the
side of the brick. Sometimes the vertical fiber brick has no spacing
lug, it being contended that the irregularities of the brick are such
as to provide all of the space required. In practice this does not
always work out, as the brick are so regular in shape that when laid
there is too little space between the brick to permit the introduction
of a suitable filler. The use of brick without spacing lugs is just
beginning and is not yet a generally accepted practice.

=Wire-cut-lug Brick.=--This is a type of non-repressed brick which has
spacing lugs provided by cutting one face in a special manner which
provided lugs for spacing. In this type the wire cut face is the one
between the brick as they are laid in the pavement.

=Tests for Quality.=--The standard test for quality of paving brick is
the rattler test. The brick rattler consists of a barrel of 14 sides
24 inches long, mounted so as to rotate at a speed between 29.5 and
30.5 revolutions per minute. The duration of a test is 1800
revolutions. Ten brick constitute a charge and these are placed in the
rattler along with 300 lbs. of cast iron spheres. The spheres are of
two s