Clearly not written by someone with native English ("subtract is became easy" is particularly memorable), but still not hard to understand, and the content is very interesting although it jumps around a bit --- I like how "physical" these early calculating machines are, as one could easily see the process being carried out.
Indeed, for learning arithmetic and many other areas of mathematics, being able to do calculations physically is better than (optimistically speaking) 90% of what goes as "ed tech" these days. Cuisenaire rods teach you basic counting and get things like 3+7=10 into your subconscious (or, if you have synaesthesia, also green+black=orange). The abacus, back when it was taught, made place-value and things like "to subtract 8 you subtract 10 then add 2" a physical as well as a mental operation which helps with math learning the way phonics helps with reading. Later on, of course, slide rules do the same for logarithms (as is explained in the linked PDF).
I wish we had more of this back in our classrooms.
