Sentence:  `` Unfortunately '' , says Chief Postal Inspector David H. Stephens , who has prosecuted many <WSD>device</WSD> quacks , `` the ghouls who trade on the hopes of the desperately ill often cannot be successfully prosecuted because the patients who are the chief witnesses die before the case is called up in court '' . 
Meaning of the word marked with <WSD> tags:  an instrumentality invented for a particular purpose