rt 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
weakest material--i.e., the mortar--though when it is in cement the
strength of brickwork to withstand a weight probably approaches that of
the individual bricks. Some experiments quoted in Rivington's Notes give
the following as the crushing weight per foot--that is to say, weight at
which crushing began--of piers having a height of less than twelve times
their diameter:

Tons per
foot.
Best stocks, set in Portland cement and
sand 1 to 1, and three months old. 40
Ordinary good stocks, three months old. 30
Hard stocks, Roman cement and sand 1 to 1,
three months old. 28
Hard stocks, lias lime, and sand 1 to 2,
and six months old. 24
Hard stocks, gray chalk lime, and sand,
six months old. 12

The rule given in popular handbook, that brickwork in mortar should not
have to carry more than three tons per superficial foot, and in cement
more than five tons, is probably sound, as in no building ought the load
to approach the crushing point, and, indeed, there are many sorts of
foundations on which such a load as five tons per foot would be too
great to be advisable.

It is a rather interesting inquiry, whenever we are dealing with a
building material, if we ask what can we best do with it, and for what
is it ill fitted.