A binary adding machine created with D. Moews' program - see thing:2425746 - "A Slide Adder". The program is available for download at http://djm.cc/slide-adder-0.03.zip. A feature of the program is that one can choose the base of the number system; here base 2 was used. The machine has six digits and overflows if sums greater that 63 occur. However it might be useful for instruction. An exercise is to add 7, (binary 111), 9 times. If properly done the result will be 111111, (decimal 63).