Sentence:  "Gnats" and "Flies" are very troubleſome in Houſes [...] Balls made of Horſe-dung and laid in a Room will do the ſame [attract gnats and flies] if they are new made; by which means you may <WSD>whelm</WSD> ſome things over them and keep them there. 
Meaning of the word marked with <WSD> tags:  To throw (something) over a thing so as to cover it.