The most famous proof of the law of the lever is the one by Archimedes, where he relies on the concept of center of balance. You can find this proof all over the place on-line. However, there was another proof attributed to Euclid that relies on a concept of torque instead of center of balance.
I've seen sketches of this proof on-line. The proof assumes that if a balance is in equilibrium, then you can move the weights at will along an axis at right angles to the line from the pan to the fulcrum.
Now my question: does anyone know where I can find a translation (with diagrams) or a modern restatement of the proof on-line? I've spent a couple of hours searching and can't find anything.