Many cities and towns in Northern Germany are also brick built, and furnish good examples of the successful treatment of the material.  In some of these German buildings, indeed, very difficult pieces of construction, such as we are in the habit of thinking can only be executed in stone, are successfully attempted in brick.  For example, they execute large tracery windows in this material.  Great brick gables, often with the stepped outline known as crows’ feet, are an excellent architectural feature of these German brick-built towns.  In parts of France, also, ornamental brickwork was from time to time made use of, but not extensively.  It is not necessary to go very minutely into the manufacture of bricks; but perhaps I ought to say a word or two on the subject.  Good brick earth is not simple clay, but a compound substance; and what is essential is that it should burn hard or, in other words, partly vitrify under the action of heat.  The brick earth is usually dug up in the autumn, left for the frosts of winter to break it up, and worked up in the early spring.

The moulding is to a very large extent done by hand, sometimes in a wet mould, sometimes in a dry sanded mould, and the bricks are first air-dried, often under some slight shelter, as the rain or frost damages them when fresh made; and then, when this process has made them solid enough to handle, they are burned, and sorted into qualities.  The ordinary or stock brick of London and the neighborhood presents a peculiarity the origin of which is not known, and which is not met with, so far as I know, in other parts.  Very fine coal or cinders is mixed with the brick earth, and when the bricks are fired these minute particles of fuel scattered through the material all of them burn, and serve to bake the heart of the brick.  Stock bricks are burnt in a clamp made of the raw bricks themselves with layers of fuel, and erected on earth slightly scooped out near the middle, so that as the bricks shrink they drop together, and do not fall over sideways.

Most other varieties of bricks are kiln burnt.  A very large number of inventions for making bricks by machinery have been patented.  If you have occasion to look through the specifications of these patents, you will find four or five main ideas appearing and reappearing, and only here and there an invention which is to some extent different from the others.  A great majority of these inventions include machinery for preparing the clay or brick earth, so that it may be dug up and filled into a receptacle and worked up, screened from pebbles, and made fit for use in a short time, so as not to have to wait a whole winter.  This is done in some sort of pug mill.  A pug mill is a machine consisting of a large cylinder with a central shaft passing through it from top to bottom.  Knives or blades are arranged spirally on the shaft, and other blades project into the interior of the cylinder from the walls of it.  The material,