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He is, according to legend, supposed to have traveled to China and returned with a knowledge of Chinese mathematical achievements and the suan-pan, a Chinese abacus. The suan-pan was probably introduced to Japan much earlier.
In any case, Mori was apparently a skilled manipulator of the suan-pan, known as the soroban in Japan .
The modern era (Showa) sorobans are approximately the same size as slide rules making them very portable, even for the pocket.
If you are a regular visitor to ISRM you will have noticed the other new gallery on 'slide rule calculators' which was inspired by the 40th anniversary of the introduction of the HP-35 electronic slide rule, or scientific calculator.
This was mostly because of lack of time and resources, but as I have come to learn about, and appreciate, the manufacturing art and long history of abaci in various cultures.
I have become a great fan of 'bead math' or 'counting beads' and am just learning the intracacies of their use (and usefulness).
He taught the soroban arithmetic to many pupils, and may have written a text on the soroban, now lost." Two of Shigeyoshi's students wrote extant works which discussed the use of the soroban.
(I am sorry I cannot do multiplication nor division by using soroban, but only addition and substraction) Sometimes they use the left part of soroban to store the result tentatively.".
Suanpan arithmetic was still being taught in school in Hong Kong as recently as the late 1960s, and in Republic of China into the 1990s.
However, when hand held calculators became readily available, school children's willingness to learn the use of the suanpan decreased dramatically.
A skilled abacus operator can work on addition and subtraction problems at the speed of a person equipped with a hand calculator (multiplication and division are slower). In fact, the oldest surviving abacus was used in 300 B. Another possible source of the suanpan is Chinese counting rods, which operated with a decimal system but lacked the concept of zero as a place holder.
The zero was probably introduced to the Chinese in the Tang Dynasty (618-907 AD) when travel in the Indian Ocean and the Middle East would have provided direct contact with India, allowing them to acquire the concept of zero and the decimal point from Indian merchants and mathematicians.