Odhner Model 30 Mechanical calculator

  Willgodt Odhner, based in St. Petersburg, made his first mechanical calculator prototype in 1875. It utilizes a pinwheel mechanism, not unlike a mechanical speedometer, and it will add, subtract, multiply, divide and calculate square roots by pushing various buttons and turning the crank in different directions. 

  Odhner passed away in 1905, his son continued the business, moving it to Sweden during the Russian Revolution. The company passed through more hands, machines were developed further, new models released and were in common use till the early 1970s. 

 I'd love to take one apart and play with it (probably ruin it) but fortunately there are people online who have done detailed studies of these mechanical miracles.

 How to use one here.

History and lots more here.

Thank you, Iris!