It has been said that concrete is a plastic material, meaning that it is soft and pliable in the sense that clay or putty are plastic. For this reason it is cast in forms or molds. Sometimes it is used in the form of plain concrete, and on other occasions it is reinforced, meaning that iron rods, steel bars or woven wire mesh are imbedded in the concrete. When we speak of a “reinforced” concrete building, imagine a huge wire bird cage encrusted within and without with concrete. Place a block, beam or column of concrete upon the ground and it will bear a tremendous load, meaning that it has great strength in compression. On the other hand, if we were to place a long beam upon supports at either end, leaving the greater length of it suspended and without support, it would carry but a small load compared with concrete in compression. Therefore, in making concrete beams or girders in a building, strong steel bars are embedded in the concrete to take up what are termed the tensile strains. [Illustration: WHAT A CEMENT MILL LOOKS LIKE This is a picture of a cement mill. Millions of dollars are invested in these great mills, which are now located in practically all sections of the country. Material is brought from the quarry to the mills, where it passes through various stages, such as grinding, burning and bagging. Expert chemists are employed to see that the cement is made exactly right. It is a very scientific matter to make a thoroughly good cement. There must be no guess work. Some mills are very large, the plant comprising a number of buildings, and some companies operate several mills in different localities. A single company supplied all of the cement used in the Panama Canal, which great project required more than six million barrels.] [Illustration: This picture shows a quarry in the famous Lehigh cement district. The giant steam shovel or excavator burrows into the hill like some great animal, and when the bucket is full it is dumped into the cars shown on the track, which convey the rock or the raw material to the mill.] [Illustration: WHERE THE MATERIAL IS OBTAINED This is an illustration of a method of excavating and loading marl and clay to be manufactured into Portland cement. The large bucket suspended over the cars does not gouge into the hillside as shown in the preceding picture, but descends like a huge steel hand, the metal parts opening and closing like fingers. The long derrick elevates the bucket and swings it over the train of cars.] [Illustration: This is a view of a powerful rock crusher, which is operated by the electric motor shown at the right. The cement rock is brought from the quarry and dumped into the machine, from which it issues in broken fragments, as shown in the illustration, this being the first or preliminary crushing process.] [Illustration: THE HUGE ROCK GRINDERS