Sentence:  Generally, however, the <WSD>stone</WSD> or petra, almost always of 14 lbs., is used, the tod of 28 lbs., and the sack of thirteen stones. 
Meaning of the word marked with <WSD> tags:  A unit of mass equal to 14 pounds. Used to measure the weights of people, animals, cheese, wool, etc. 1 stone ≈ 6.3503 kilograms