Over other widely scattered portions of the earth, similar evidences are found that the early races were familiar with the constructive use of cement. In Spain and other Old World countries are found abundant examples of the ancient use of concrete in forms that have survived the rav ages of time and the elements. The lookout towers of Ireland, supposed to have been built by the Druids more than a thousand years ago, are made of hydraulic cement concrete, cylindri cal in form, about six feet in diameter, and 100 feet high. Some years ago one of these towers was undermined and overturned. In its fall, the shock of the huge mass was so great that the shaft sank into the ground to the depth of one half ita diameter for its entire length; yet not the slightest fracture occurred—a fact which demonstrates not only the strength but the elas ticity of concrete. Had the shaft been a mono lith (that is, one solid piece) of even the best quality of natural granite, the shock of such a fall would have reduced it to fragments.
A similar story as to the early use of concrete might also be written of the vanished races of the New World. The Peruvian builders in the days of the Incas employed concrete, impelled probably by the necessity of adopting a form of construction that would withstand earthquakes and volcanic tremors. Some of their structures, though built many centuries ago, have endured to the present time. And even in our own North America, the use of this most versatile of plastic materials of construction antedates all historic or prehistoric records written in less enduring form. Twenty miles northeast of the City of Mexico are remains of a former but now van ished civilization, in the shape of pyramids of masonry that were built partly, if not entirely, of concrete. And ethnologists and antiquarians tell us that as far back as eleven thousand years ago, while the slow-working geological forces had not yet moulded the face of this continent to its present contour, the remarkable race of men known as the Mound Builders, along what are now the edges of the Ohio valley, were accus tomed to boil salt water in kettles of artificial stone. And in the manufacture of their pottery, specimens of which are the most enduring me mentos of their intelligence, they used as the cementing material carbonate of lime—a me dium used for a similar purpose to the present day.
In one sense, therefore, concrete can lay no claim to novelty, being a return to principles once well known if less perfectly understood than now, but for a long time lost sight of or for gotten. It is not to be supposed, however, that in quality of material or in methods and appli ances of construction the early workers in con crete attained anything like the high standards of technical accuracy that characterize the man ufacture of cement and its use in the various forms of concrete construction at the present day. The ancient skill which had been devel
oped in the making and working of concrete ap pears for many hundreds of years to have be come a "lost art." As the empires gradually went to pieces through internal decay, and the world stumbled on toward that dark period of utter standstill known as the Middle Ages, when the lamp of progress flickered but faintly in the cells of monasteries, we find that even in the elaborate cathedrals erected during this period the use of hydraulic mortars had given way to that of fat lime and silt mortars. These ingre dients in time became inert powders devoid of strength and hardness, which necessitated con stant watchfulness and costly bracing and re pairs to keep the structures erect.
About the beginning of the eighteenth cen tury, there came a revival in the demand for hydraulic mortars, which was met by supplies of Pozzuolana from Italy, or of the low-grade natural cement known as "trass," from Ger many. In the erection of the Eddystone Light, about 1757, Smeaton developed a form of nat ural hydraulic cement, after experimenting on limestones containing different proportions of clay, and demonstrated the practical importance of chemical analysis in cement making.
With the discovery, however, of true Port land cement, the inauguration of accurate physi cal and chemical tests, and, in particular, the combination of concrete with reinforcing mem ber's, and analysis of the principles of construct ive design, an entirely new era of development was ushered in—one distinctively modern and without analogy or precedent.
Development of the Portland Cement Indus try. Notwithstanding its great powers of endur ance, the concrete employed by the Romans was far inferior to present-day standards. That used in constructing the dome of the Pantheon, for example, was made by mixing broken stone and coarse gravel with a cement consisting of a mixture of slaked lime and volcanic ash. This cement was not a Portland cement, but closely resembles the Pozzuolan cement still produced in small quantities in Italy and other countries of Europe, which is manufactured by mixing volcanic lava (a natural product) and slaked lime, the resulting mass forming a rather weak hydraulic cementing compound without requir ing the agency of heating.