puffing of locomotives.

Marble is cut out in somewhat the same way as granite, but a valuable machine called a "channeler" is much used. This machine runs back and forth, cutting a channel two inches wide along the ends and back and sometimes the bottom of the block to be taken out.

Marble is so much softer than granite that it is far more easy to work. Cutting it is a simple matter. The saw, which is a smooth flat blade of iron, swings back and forth, while between it and the marble sand and water are fed. It does not exactly cut, but rubs, its way through. The round holes in the tops of washstands are cut by saws like this, only bent in the form of a cylinder and turned round and round, going in a little deeper at each revolution. A queer sort of saw is coming into use. It is a cord made of three steel wires twisted loosely together. This cord is stretched tightly over pulleys and moves very rapidly. Every little ridge of the cord strikes the stone and cuts a little of it away.

There are varieties of marble without end. The purest and daintiest is the white of which statues are carved; but there are black, red, yellow, gray, blue, green, pink, and orange in all shades. Many are beautifully marked. The inner walls of buildings are sometimes covered with thin slabs of marble. These are often carefully split, and the two pieces put up side by side, so that the pattern on one is reversed on the other. Certain kinds of marble contain fossils or remains of coral and other animals that lived hundreds of thousands of years ago. In some marbles there are so many that the stone seems to be almost made of them. When a slab is cut and polished, the fossils are of course cut into; but even then we can sometimes see their shape. One of the most common is the crinoid. This was really an animal, but it looked somewhat like a closed pond lily with a long stem, and people used to call it the stone lily. This stem is made up of little flat rings looking like bits of a pipestem. The stems are often broken up and these bits are scattered through the marble. The animals whose shells help to make marble lived in the ocean, and when they died sank to the bottom. Many of the shells were broken by the beating of the waves, but both broken shells and whole ones became united and hardened into limestone, one kind of which we call marble. Common chalk is another kind. Blackboard crayons are made of this: so are whitewash and whiting for cleaning silver and making putty.

Another stone that builders would be sorry to do without is slate. This, too, was formed at the bottom of the sea. Rivers brought down fine particles of clay, which settled, were covered by other matter, and finally became stone. It was formed in layers, of course, but, queerly enough, it splits at right angles to its bottom line. Just why it does this is not quite certain, but the action is thought to be due to heat and long, slow pressure, which will do wonderful things, as in the case of coal. This splitting is a great convenience for the people who want to use it for roofing and for blackboards. Blocks of slate are loosened by blasting, and are taken to the splitting-shed.

Splitting slate needs care, and a man who is not careful should never try to work in a slate quarry. The splitting begins by one man's dividing the block into pieces about two inches thick and somewhat larger than the slates are to be when finished. The way he does this is to cut a little notch in one end of the block with his "sculpin chisel" and make a groove from this across the block. He must then set his chisel into the groove, strike it with a mallet, and split the slate to the bottom. This sounds easy, but it needs skill. Slate has sometimes its own notions of behavior, and it does not always care to split in a straight line exactly perpendicular to the bottom of the stratum. The man keeps it wet so that he can see the crack more plainly, and if that crack turns back a little to the right, he must turn it to the left by striking the sculpin toward the left, or perhaps by striking a rather heavy blow on the left of the stone itself. Now the chief splitter takes it, and with a broad thin chisel he splits it into plates becoming thinner at each split. The second assistant trims these into the proper shape and size with either a heavy knife or a machine. Slate can be sawed and planed; but whatever is done to it should be done when it first comes from the quarry, for then it is not so likely to break. It would be very much cheaper if so much was not broken and wasted at the quarries and in the splitting. It is said that in Wales sometimes one hundred tons of stone are broken up to get between three and four tons of good slate. Within the last few years the quarrymen have been using channeling machines and getting out the slate in great masses instead of small blocks. This is not so wasteful by any means; but even now there is room for new and helpful inventions.

III

HOUSES OF SAND

If you wanted to build a house, of what should you build it? In a new country, people generally use wood; but after a time wood grows expensive. Moreover, wood catches fire easily; therefore, as a country becomes more thickly settled and people live close together in cities, stone and brick are used. Large cities do not allow the building of wooden houses within a certain distance from the center, and sometimes even the use of wooden shingles is forbidden. Of late years large numbers of "concrete" or "cement" houses have been built. Our grandfathers would have opened their eyes wide at the suggestion of a house built of sand, and would have felt anxious at every rainfall lest their homes should suddenly melt away. Even after thousands of concrete buildings were in use, many people still feared that they would not stand the cold winters and hot summers of the United States; but it has been proved that concrete is a success provided it is properly made.

No one can succeed in any work unless he understands how it should be done. Concrete is made of Portland cement, mixed with sand and water and either broken stone, gravel, cinders, or slag; but if any one thinks that he can mix these together without knowing how and produce good concrete, he will make a bad mistake rather than a good building material.

First, he must buy Portland cement of the best quality. This cement is made of limestone and clay, or marl, chalk, and slag. These are crushed and ground and put into a kiln which is heated up to 2500° or 3000°F.; that is, from twelve to fourteen times as hot as boiling water. The stone fuses sufficiently to form a sort of clinker. After this has cooled, it is ground so fine that the greater part of it will pass through a sieve having 40,000 meshes to the square inch. To every hundred pounds of this powder, about three pounds of gypsum is added. The mixture is then put into the bags in which we see it for sale in the stores. This powder is so greedy for water that it will absorb the moisture from the air around it. Even in the bags, it begins to harden as soon as it gets some moisture; and as soon as it hardens, it is of no use. The moral of that is to keep your cement in a dry place.

The second substance needed in concrete is broken stone or gravel. Of course a hard rock must be selected, such as granite or trap rock. Limestone calcines in a heat exceeding 1000° F., and therefore it cannot be used in fireproof construction. Soft rock, like slate or shale or soft sandstone, will not answer because it is not strong enough. Gravel is always hard. If you look at a cut in a gravel bank, you will usually see strata of sand and then strata of rounded pebbles of different sizes. The sand was