Pulleys.

Visible, physical, simple math... and magical when scrawny little me can lift Big Thing.

Hydraulics / Pneumatics are not as old and aren't as open to inspection while operating, usually; so the magic seems stronger there.

Today's digital systems are still built atop quantization of analog effects; the edge cases and errors (and the techniques used to avoid and correct those errors) might actually be magic, if you want to get into metaphysical wanking about definitions. The lovely maths of signalling systems even resemble those of pulleys, some ways: it's the same magic.