This my friends is a proportion wheel.
It was used in the time before computers to calculate scaling of graphic design elements such as images, height and width of design elements and more. How it works is very simple: just measure the design element you would like to scale (up or down); use one of these measurements (width or height) for the inside wheel; rotate the inner wheel measurement to the desired scaled size on the outer, larger wheel; in the window, the arrow will tell you what percentage it needed to be enlarged. Say you calculated the width, to check the height, find that measurement on the inner wheel with the scaled percentage still selected and ‘whala’ you have your scaled size!