Sentence:  Its course was punctuated by <WSD>tamkins</WSD>, small brick buildings whose name, a variant of the word "tompion," still in use for the plug closing the muzzle of a gun, denoted their purpose: an access point for plugging off sections which required isolation so that repairs could be carried out. 
Meaning of the word marked with <WSD> tags:  A brick housing that protects an access point where pipes beneath London could be plugged for maintenance.