Sentence:  Mr James M'Kenzie Davidson, M.B., C.M., with the assistance of Processor Alex. Ogston of Aberdeen, has invented and advertised in the Lancet a new instrument which is termed a "<WSD>lithophone</WSD>," and is to enable surgeons to hear a stone in the bladder. 
Meaning of the word marked with <WSD> tags:  A device by which the presence of bladder stones can be audibly detected.