Sentence:  Could it but for a longer Space / Lengthen the Bliſs it let's us taſte, / Who would not doat on't? But alas! / The Joy's too exquiſite to laſt. / Two white "Herculean" Pillars prop / The tufted Gin, the tempting Snare; / When they divide, then in we pop, / Before we well know where we are. / Then that for this, and <WSD>tit for tat</WSD>; / But when the pleaſing Minute's flown, / As uſeleſs it returns the Bait, / And both look fooliſh when 'tis done. 
Meaning of the word marked with <WSD> tags:  Equivalent retribution; an act of returning exactly what one gets; an eye for an eye.