Most authorities give the credit for the invention to Joseph Aspdin, a bricklayer of Leeds, England. He took out a patent on the material and in 1825 set up a large factory. In 1828 Portland cement was used in the Thames tunnel, making the first time that the material figured in any big engineering work. In those days even the most enthusiastic supporters of cement little dreamed that in this modern age it would be the material that would make possible such tremendous victories over the obstacles of nature as the Panama Canal, the tunnels under the rivers that surround New York and the great dams that hold back the waters all over the country. Aspdin, however, is not given the credit for the invention of Portland cement by all authorities, as some claim that Isaac Johnson, also an Englishman, who early in 1912 died at the age of 104, was really the first man to invent a practical, commercial, artificial cement. The advantage in Portland cement is that it can be made of a number of different kinds of earth, to be found in many different parts of the world, and makes a far stronger rock. It sets more slowly than natural or hydraulic cement, but is more satisfactory for use in reinforced concrete work. In the Lehigh Valley, where about two thirds of the Portland cement used in the United States is made, the raw material is a rock, called cement rock, and limestone. In New York State they make Portland cement of limestone and clay; in the Middle West they make it of marl and clay, while in other Western States they make it of chalk and clay. In Europe slag is sometimes used. The artificial product contains lime oxide, silica, alumina, iron oxide, and other minerals in varying quantities, but the necessary ones are silica, alumina, and lime. In making Portland cement the raw material is ground into a fine powder and poured into one end of a long cylindrical kiln which looks like a smokestack lying on its side. Powdered coal is shot into the kiln, where it is kept burning, at a heat of about 2,500 to 3,000 degrees Fahrenheit. After the raw material has been burned thoroughly and is taken from the kiln it looks like little cinders or clinkers about the size of marbles. The cement clinker is then cooled and ground to a powder, after which it is stored away for a little while to season. The first Portland cement ever made in the United States was turned out by David O. Saylor, of Coplay, Pa., in 1875, but the development of the new industry was very slow, as builders and engineers seemed to be blind to the great possibilities of the material that built Imperial Rome. In 1890, nearly twenty years after the process was introduced in America, only 335,500 barrels of Portland cement were manufactured in this country. The country woke up to the situation a few years later, and in 1905 there were manufactured in the United States 35,246,812 barrels of Portland cement. In 1911 the industry turned out the stupendous total of 77,877,236 barrels.