once an ancient sea beach; the pebbles were dashed up on it by waves or storms or some change of currents. They were at first only broken bits of rock, but after being rolled about for a few thousand years in the ocean and on the shore, the corners were all rounded. Soft rock would have been ground to powder by such treatment. Sometimes, if there is to be no great strain on the concrete, cinders or pieces of brick may be used instead of stone; and for some purposes they answer very well.

The third substance used in concrete is sand; but it must be the right kind of sand, having both fine and coarse grains. These grains need to be sharp, or the cement will not stick to them well. They must also be clean, that is, free from dirt. If you rub sand between your hands, and it soils them, then there is clay or loam with it, and it must not be used in making concrete unless it is thoroughly washed. Another way of testing it is to put it into a glass jar partly full of water and shake it. Then let it settle. If there is soil in the sand, it will appear as a stratum of mud on top of the sand.

The water with which these three substances are to be mixed must be clean and must contain no acid and no strong alkali. As a general rule, there must be twice as much broken stone as sand. When people first make concrete, they often expect too much of their materials. A good rule for the strongest sort of cement, strong enough for floors on which heavy machines are to stand, is one fourth of a barrel of cement, half a barrel of sand, and one barrel of gravel or broken stone. Apparently this would make one and three fourths barrels; but in reality it makes only about one barrel, because the sand fills in the spaces between the gravel, and the cement fills in the spaces between the grains of sand.

There are many sorts of machines on the market for mixing the materials; but small quantities can just as well be mixed by hand. The "mixing-bowl" is a platform, and on this the sand is laid. Then comes the cement; and these two must be shoveled together several times. While this is being done, the broken stone or gravel must be wet, and now it is put on top of the sand and cement and well shoveled together, with just enough water added so that the mass will almost bear the weight of a man.

Concrete is impatient to be hardening, and if it is not put into the right place, it will begin promptly to harden in the wrong place, and nothing can be done with it afterwards. If it is to be made in blocks, the moulds must be ready and the concrete put into them at once and well tamped down. For such uses as beams and the sides of tanks where great strength is needed, the cement is often "reinforced," that is, rods of iron or steel are embedded in it. For floors, a sheet of woven wire is often stretched out and embedded. At first only solid blocks, made to imitate rough stone, were used for houses, but the hollow block soon took their place. This is cheaper; houses built this way are warmer in winter and cooler in summer; and it prevents moisture from working through the walls. Many cities have regulations about the use of hollow blocks, all the more strict because concrete is comparatively new as a building material. In Philadelphia the blocks must be composed of at least one barrel of Portland cement to five barrels of crushed rock or gravel. They must be three weeks old or more before being used; the lintels and sills of the doors must be reinforced; and every block must be marked, so that if the building should not prove to be of proper strength, the maker may be known. There would seem, however, to be little question of the quality of the blocks, for samples must pass the tests of the Bureau of Building Inspection.

Even better than the hollow block is the method of making the four walls of a house at once by building double walls of boards and pouring in the concrete. When this has hardened, the boards are removed, and whatever sort of finish the owner prefers is given to the walls. They can be treated by spatter-work, pebble dash, or in other ways before the cement is fully set, or by bush hammering and tool work after the cement has hardened. Coloring matter can be mixed with the cement in the first place; and if the owner decides to change the color after the house is completed, he can paint it with a thin cement of coloring matter mixed with plaster of Paris.

A concrete house has several advantages. In the first place, it will not burn. Neither will granite, but granite will fall to pieces in a hot fire. Granite is made of quartz, mica, and feldspar, as has been said before. These three do not expand alike in heat; and therefore great flakes of the stone split off, so that it really seems to melt away. A well-made concrete is not affected by fire. It will not burn, and it will not carry heat to make other things burn. For a concrete house no paint is needed and less fuel will be required to keep it warm. If the floors are made with even a very little slant, "house-cleaning" consists of removing the furniture and turning on the hose. Water-tank, sink, washtubs, and bathtubs can be cast in concrete and given a smooth finish. Wooden floors can be laid over the concrete, or a border of wood can be put around each room for tacking down carpets or rugs. A concrete house may be as ornamental as the owner chooses, for columns and cornices and mouldings can easily be made of concrete; and if they are cast in sand, as iron is, they will have a finish like sandstone.

It is somewhat troublesome to lay concrete in very cold weather, because of the danger of freezing and cracking. Sometimes the materials are heated, and after the concrete is in place, straw or sand or sawdust is spread over it. These will keep it warm for several hours, and so give the concrete a chance to "set." Sometimes a canvas house is built over the work. When a concrete dam was to be built in the Province of Quebec and the mercury was 20° below zero, the contractors built a canvas house over one portion of the dam and set up iron stoves in it. When this part was completed, they took down the house and built it up again over another portion of the dam. Sometimes salt is used. Salt water is heavier than fresh water and will not freeze so easily. Therefore salt put into the water used in making the concrete will enable it to endure more cold without freezing; but not more than one pound of salt to twelve gallons of water should be used.

BUILDING A CONCRETE ROAD

Courtesy Alpha Portland Cement Co.

The concrete mixer travels along the prepared roadbed, and after it follow the workmen with levelers and stamps.

Concrete objects to being frozen before it is "set," but it is exceedingly accommodating about working under water. It must, of course, be carried in some way through the water to its proper place without being washed away, but this is easily done. Sometimes it is let down in great buckets closed at the top, but with a hinged bottom that will open when the bucket strikes the rock or soil where the material is to be left. Sometimes it is poured down through a tube. Sometimes it is dropped in sacks made of cloth. This cloth must be coarse, so that enough of the concrete will ooze through it to unite the bag and its contents with what is below it and make a solid mass. Sometimes even paper bags have been successfully used. The concrete, made rather dry, is poured into the bags and they are slid