An unusual subject has been chosen for this week’s cover. The photograph shows a set of immense gearwheels, whose size gives a good indication of the scale of modern engineering. These wheels, of cast steel, were designed for a large crane and were made in a German steelworks. The largest wheel in the photograph has a diameter of more than 16 feet. The double helical, or herringbone teeth were cut after the wheels had been cast. This process is described in the chapter Gears and Gear Cutting in this week’s Part.
CONCRETE MIXING PLANT used during the building of the Boulder Dam, one of the largest concrete structures ever built. Altogether, including subsidiary works, about 4,000,000 cubic yards of concrete were required. This amount involved the consumption of 755,360 tons of cement and the use of many mixing plants. Compressed air was used to force cement from the silos through pipes to the containers of each mixing plant. In one instance the pipe was 5,600 feet long (over a mile), and the cement was delivered to the container at the rate of 76 tons an hour.