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, after being screened, is fed into this at the top, and properly moistened. The shaft is caused to rotate, and the blades divide and subdivide the material, forcing it always downward, so that it at last escapes at the bottom of the pug mill in a continuous stream of moist, well worked up clay, issuing with some force. In one type of machine this clay stream is forced through a square orifice, from which it comes out of the section of a brick, and by a knife or wire or some other means it is cut into lengths.