To the Romans, however, we are indebted for some of the best examples of ancient concrete work. They used this material in their wonderful city for buildings, bridges, sewers, aqueducts, water mains, and in fact in a great many of the ways that we have seen it is used to-day. The great Coliseum and the Pantheon at Rome are relics of the skill of the ancient architects in the use of concrete. Although many historians think that the secret of making cementious building material was lost from the fall of Rome until the middle of the eighteenth century, there are ruins of ancient castles which stood in mediæval times in Europe which indicate at least some use of concrete. The real discoverer of natural cement in our modern times though, was John Smeaton, who will be remembered by the readers of "The Boy's Second Book of Inventions" as the man who built the first rock lighthouse at Eddystone, England, in 1756. In his great work he discovered a kind of limestone with which he could make a cement that would set, or harden, under water. His discovery was hailed as the recovery of the secret of the ancient Romans of making hydraulic cement. It was so called because it would harden under water. In 1796, Joseph Parker, another Englishman, made what he called Roman cement. Several others followed, and in 1818 natural cement was first made in the United States by Canvass White near Fayetteville, N. Y. The material was made from natural rock and was used in the construction of the Erie Canal. All of these early cements are called natural cements by engineers nowadays, because they were made from natural rock. It was only necessary to find a clayey limestone which contained a certain percentage of iron oxide and two other minerals known as silica and alumina. The limestone was crushed to a convenient size and was burned in a kiln. The heat turned the stuff into cinders which, when ground to a fine powder and mixed with water, would make a cement that would harden under either air or water very quickly, and last for practically all time. Just for the sake of those who have studied chemistry we will say that in this process the heat drives off the carbon dioxide in the limestone, and the lime, combining with the silica alumina and iron oxide, forms a mass containing mineral properties called silicates, aluminates, and ferrites of lime. These properties mixed with the water make natural cement. In the United States, natural cement was called Rosendale cement, because it was first made commercially in a town of New York State by that name. The supply of natural cement, however, is limited, because the proper kind of limestone is only found in a few places. Consequently, when an artificial mortar called Portland cement was invented in 1824, the world took a step forward that could not be measured in those days.