This was because the age of concrete had dawned on the world and man had learned in those years that by mixing gravel and sand with cement he could make a material cheaper, more easily handled, and far more lasting than wood, brick or some stone. As Edison once said to some of his associates: "I think the age of concrete has started, and I believe I can prove that the most beautiful houses that our architects can conceive can be cast in one operation in iron forms at a cost, which, by comparison with present methods, will be surprising. Then even the poorest man among us will be enabled to own a home of his own--a home that will last for centuries with no cost for insurance or repairs, and be as exchangeable for other property as a United States bond." The technical definition of concrete is as follows: "Concrete is a species of artificial stone formed by mixing cement mortar with broken stone or gravel. Cement is the active element called the _matrix_ and the sand and stone forms the body of the mixture called the _aggregate_." The ingredients are mixed in different proportions for different work. A common proportion is 1 part cement, 2 parts sand, and 5 parts broken stone or gravel. Cement users speak of this as a "1: 2: 5 mixture." Sometimes the gravel is left out and a mixture of 1 part cement to 3 or 4 parts sand is made. The cement binds the mass together and sand fills up any little vacant spaces about the gravel, making what is called a dense mixture. [Illustration: THE SILENT KNIGHT MOTOR Two views of the latest automobile engine. At the top can be seen the sliding sleeves, the inlets and outlets which do away with valves] [Illustration: A PORTABLE ARMY WIRELESS OUTFIT The Signal Service is rapidly increasing its wireless equipment for use on land.] [Illustration: THE WIRELESS IN THE NAVY Practically all of Uncle Sam's warships and Navy Yards now are equipped with wireless, and a regular navy wireless operators' school is maintained at the Brooklyn Navy Yard.]