Sentence:  But the decline of Rome was the natural and inevitable effect of immoderate greatneſs. Proſperity <WSD>ripened</WSD> the principle of decay; the cauſes of deſtruction multiplied with the extent of conqueſt; and as ſoon as time or accident had removed the artificial ſupports, the ſtupendous fabric yielded to the preſſure of its own weight. The ſtory of its ruin is ſimple and obvious; and instead of enquiring "why" the Roman empire was deſtroyed, we ſhould rather be ſurprised that it had ſubſiſted ſo long. 
Meaning of the word marked with <WSD> tags:  To mature; to fit or prepare; to bring to perfection