down a chute. The paper soon becomes soft and breaks, and lets the concrete out. Sometimes concrete blocks are moulded on land and lowered by a derrick, while a diver stands ready to see that they go into their proper places.

Concrete is used for houses, churches, factories, walls, sidewalks, steps, foundations, sewers, chimneys, piers, cellar bottoms, cisterns, tunnels, and even bridges. In the country, it is used for silos, barn floors, ice houses, bins for vegetables, box stalls for horses, doghouses, henhouses, fence posts, and drinking-troughs. It is of very great value in filling cavities in decaying trees. All the decayed wood must be cut out, and some long nails driven from within the cavity part-way toward the outside, so as to help hold the concrete. Then it is poured in and allowed to harden. If the cavity is so large that there is danger of the trunk's breaking, an iron pipe may be set in to strengthen it. If this is encased in concrete, it will not rust. A horizontal limb with a large cavity may be strengthened by bending a piece of piping and running one part of it into the limb and the other into the trunk, then filling the whole cavity with concrete. If the bark is trimmed in such a way as to slant in toward the cavity, it will sometimes grow entirely over it.

Concrete is also used for stucco work, that is, for plastering the outside of buildings. If the building to be stuccoed is of brick or stone, the only preparation needed is to clean it and wet it; then put on the plaster between one and two inches thick. A wooden house must first be covered with two thicknesses of roofing-paper, then by wire lathing. The concrete will squeeze through the lathing and set. Stucco work is nothing new, and if it is well done, it is lasting.

Concrete has been used for many purposes besides building, and the number of purposes increases rapidly. For blackboards, refrigerator linings, and railroad ties it has been found available, and for poles or posts of all sizes it has already proved itself a success. It has even been suggested as an excellent material for boats, if reinforced; and minute directions are given by one writer for making a concrete rowboat. To do this, the wooden boat to be copied is hung up just above the ground, and clay built around it, leaving a space between boat and clay as thick as the concrete boat is to be. The wooden boat is covered with paper and greased, then the concrete is poured into the space between the boat and the clay mould; and when it hardens and the wooden boat is removed, there is a boat of stone—or so the directions declare; but I think most people would prefer one of wood. However it may be with rowboats, concrete is taking an important place in the construction of battleships, a backing for armor being made of it instead of teakwood. The Arizona is built in this way.

Concrete that is carelessly made is very poor stuff, and dangerous to use, for it is not at all reliable and may give out at any time; but concrete that is made of the best materials and properly put together is an exceedingly valuable article.

IV

BRICKS, THEIR FAULTS AND THEIR VIRTUES

The simplest way to make a brick is to fill a mould with soft clay, then take it out and let it stiffen, and then put it in the sun to dry. This is the way in which the "adobe" bricks of Central America are made. They answer very well in countries where there is little rain; but one or two heavy downpours would be likely to melt a house built of such material.

Clay is a kind of earth containing mostly alumina and silica or sand, that can be mixed with water, moulded into any shape, retain that shape after it is dry, and become hard by being burned. If you want to make a china cup, you must have a fine sort of clay called "kaolin," which is pure white when it is fired and is not very common; but if you want to make bricks, it will not be at all difficult to find a suitable clay bank. And yet the clay, even for bricks, must be of the right kind. If it contains too much silica (sand), the brick will not mould well; if too much alumina it will be weak; if too much iron, it will lose its shape in burning; if too much lime, it will be flesh-colored when it is burned.

If you want to find out whether a building-brick is of good quality, there are some tests that a boy or girl can apply as well as any one. First, look the brick over and note whether it is straight and true, and whether the edges and corners are sharp. Strike it, and see whether it gives a clear, ringing sound. Then weigh it and soak it in water for twenty-four hours. Weigh it again, and if it is more than one fifth heavier than it was before soaking, it is not of the first quality.

After the clay has been dug, it must be "tempered," that is, mixed with water and about one third or one fourth as much sand as clay, and left overnight in a "soak pit," a square pit about five feet deep. In the morning the workmen shovel the mass over and feed it into the machines for forming the bricks. The mixing is better done, however, in a "ring pit." This is a circular pit twenty-five or thirty feet in diameter, three feet deep, and lined with boards or brick. A big iron wheel works from the center to the edge and back again for several hours, through and through the clay. A method even better than this is to put the clay and sand and water into a great trough, in which there is a long shaft bristling with knives. The shaft revolves, mixes the clay, and pushes it along to the end of the trough. This is called "pugging," and the whole thing—trough, shaft, and knives—is a "pug mill."

In the old days bricks were always made by hand. The moulder stood in front of a wet table whereon lay a heap of soft clay. He either wet or sanded his mould to keep it from sticking. Meanwhile, his assistant had cut a piece of clay and rolled it and patted it into the shape of the mould. In making bricks, there can be no patching; the mould must be filled at one stroke, or else there will be folds in the brick. To make a good brick, the moulder lifts the clay up above his head and throws it into the mould with all his force. Then he presses it into the corners with his thumbs, scrapes off with a strip of wood any extra clay, or cuts it off with a wire, smooths the surface of the brick, puts mould and brick upon a board, jerks the mould up and proceeds to make another brick.

IN A NEW JERSEY BRICK MILL

Copyright by Underwood and Underwood.

This man is moulding a fire-brick to its final shape.

No matter how expert a moulder may be, brick-making by hand is slow work, and in most places machines are used. In what is called the "soft-mud" process, the clay is pushed on by the pug mill to the end of the trough. There stands a mould for six bricks. A plunger forces the clay into it, the mould is emptied, and in a single hour five thousand bricks can be made. By what is called the "stiff-mud" process, the stiff clay is put into a machine with an opening the size of the end or side of a brick. The machine forces the clay through this opening, cuts it off at the proper moment; and so makes bricks by the thousand without either mould or moulder. A third way of making brick is by what is called the "dry process." The clay is pulverized and filled into moulds the length and breadth of a brick, but much deeper, and