What is: two-stone-bow(二石弓)
stone(石) is an unit used to measure weight in ancient China.
stone-power(石之力): Similar as horsepower, in ancient China, the power of an bow is measured in terms of stone-power. In different dynasties, one stone-power may be different. In all, one stone-power may equal the power of 30-60 kilograms.
The calculation of bow power: A bow is fixed on the wall, and then a measure weight is attached onto the bowstring. When the bow is fully drawn, the weight of the measure weight suspended on the bowstring is the bow power.