Cryptogram from Frankenstein; or, the modern prometheus, chapter 24

"Farewell! I leave you, and in you the last of humankind whom these eyes will ever behold. Farewell, Frankenstein! If thou wert yet alive and yet cherished a desire of revenge against me, it would be better satiated in my life than in my destruction. But it was not so; thou didst seek my extinction, that I might not cause greater wretchedness; and if yet, in some mode unknown to me, thou hadst not ceased to think and feel, thou wouldst not desire against me a vengeance greater than that which I feel. Blasted as thou wert, my agony was still superior to thine, for the bitter sting of remorse will not cease to rankle in my wounds until death shall close them for ever.

The sentence you are solving goes here.

He sprang from the cabin-window as he said this, upon the ice raft which lay close to the vessel. He was soon borne away by the waves and lost in darkness and distance.