Felsenthal computer altitude correction airplane circular slide rule 1960 USA

This is a larger US army circular slide rule with 2 cursors, used for calculating altitude corrections for airplanes. They were used when flying over a plateau, taking pressure and temperatures into account. This one was made by Felsenthal for the US military. The manual is printed on the back. It comes in the original box with all the markings. The slide rule is in very good condition with small signs of use and age only, it measures ca. 20 cm or 8 inches in diameter. Check the attached video on how to work your vintage man tool : slide rule operation instructions! And also make sure to check the aerial photography version by Felsenthal as well.