That brings back old memories. A remarkably accurate mechanical vernier device. It assumed that you had a rudimentary knowledge of mathematics first and were useing it as an aid and time saver, unlike the calculators of today, where you only have to know what the symbols are for the function you want to perform.
It would take up to much space here to explain its use, Google it and print the instructions or find an old fart like me to show you how to use it. You will quickly go back to your calculator. On the other hand you won't have to buy batteries ever again.
bearhunter