Diggers in the Earth

ed. 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, hewill 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 s...