consists of a series of wedge shaped blocks, known as voussoirs,
arranged in a curve, and so locking one another together that unless the
abutments from which the arch springs give way, it will not only carry
itself, but sustain a heavy load. It is a constant practice to cut
bricks to this shape and build them into an arch, and these are
sometimes cut and rubbed; sometimes, when the work is rougher, they are
axed. But in order to save the labor of cutting, arches are sometimes
turned with the bricks left square, and the joints wedge shaped. In this
case the rings should be only half a brick each, so that the wedge need
not be so very much wider at back than at face, and they are set in
cement, as that material adheres so closely and sets so hard. Arches of
two or more half-brick rings in cement are good construction, and are
also used for culvert work.

A less satisfactory sort of arch is what is called the flat arch. Here,
instead of being cambered as it ought to be, the soffit is straight; but
the brickwork being deep, there is room enough for a true arch that does
the work, and for useless material to hang from it. These arches are
generally rubbed or axed, and are very common at the openings of
ordinary windows. But no one who has studied construction can look at
them without a kind of wish for at least a slight rise, were it only two
inches. Sometimes when these straight arches are to be plastered over
they are constructed in a very clumsy manner, which is anything but
sound, and from time to time they give way. The weight of brickwork, of
course, varies with the weight of the individual bricks. But stock
brickwork in mortar weighs just about one hundred weight per cubic foot,
or 20 cubic feet to the ton. In cement it is heavier, about 120 lb. to
the cubic foot.

The strength of brickwork depends of course on the strength of the