This is a view of the electric motors operating the grinding machines which reduce the raw material to a very fine powder. There are various types of mills or grinders, to which the material comes after going through the rock crusher. They grind it in preparation for the kilns.] [Illustration: The kiln is a very important feature of the cement plant. The finely ground raw material must be calcined or burned before it becomes Portland cement. These kilns range from 60 to 240 feet in length. They are slightly inclined and revolve upon rollers. The finely ground material enters the kiln at the upper end and travels throughout its length as the kiln slowly revolves. Powdered coal dust is fed into the kiln at the lower end, where it is ignited and generates intense heat. When the finely ground raw material comes into contact with the heat, which reaches 2800 degrees F., it is transformed into what is known as clinker, which issues from the lower end of the kiln and is passed on to other machinery, which grinds it into impalpable powder or Portland cement.] [Illustration: HOW CONCRETE IS MIXED This is an ingenious machine which bags and weighs the cement. The bags are suspended as shown, and when filled and weighed by the machine are placed in barrels and shipped to their destination. Every device of this kind that will save time and labor cheapens the cost of manufacture.] [Illustration: In mixing cement, sand and stone together in order that concrete may be obtained, it is customary to use, if the operation is a large one, what are known as mechanical mixers. These are large iron cylinders into which the three materials are put and water added. The cylinder or iron drum revolves until the contents are thoroughly mixed, when they issue from the mixer through a chute or spout. A mixer of this type is shown on a succeeding page describing the making of a concrete road. This picture shows mixing concrete by hand. The sand and cement are first thoroughly mixed in the dry state and subsequently the stone and water are added. Concrete should be thoroughly mixed in order that every grain of sand may be entirely coated with cement, and then these two combined make a rich mortar, which should surround entirely every piece of stone.] [Illustration: HOW CONCRETE BUILDINGS ARE MADE This picture shows how concrete houses or walls are built through the use of what are known as forms. In building a wall we have an inside and outside form, as shown in the picture, between which the concrete is placed. After it hardens the forms are removed. In some operations, such as the construction of a large factory building or great bridge, there is such a vast array of timber construction as to make the scene quite impressive, especially when bridge arches of great span and height are under construction.]