because, whether the washing is done or not, it smartens up the appearance of the work.  The misfortune is that this pointing, instead of being the edge of the same mortar that goes right through, is only the edge of a narrow strip, and does not hold on to the old undisturbed mortar, and so is far less sound, and far more liable to decay.  There is a system of improving the appearance of old, decayed work by raking out and filling up the joint, and then making a narrow mortar joint in the middle of this filling in, and projecting from the face.  This is called tuck pointing.  It is very specious, but it is not sound work.

Brick arches are constantly being turned, and of many sorts.  An arch 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 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: