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

A scene has just passed of such uncommon interest that, although it is highly probable that these papers may never reach you, yet I cannot forbear recording it.

The sentence you are solving goes here.

I mentioned in my last letter the fears I entertained of a mutiny. This morning, as I sat watching the wan countenance of my friend - his eyes half closed and his limbs hanging listlessly - I was roused by half a dozen of the sailors, who demanded admission into the cabin. They entered, and their leader addressed me. He told me that he and his companions had been chosen by the other sailors to come in deputation to me to make me a requisition which, in justice, I could not refuse. We were immured in ice and should probably never escape, but they feared that if, as was possible, the ice should dissipate and a free passage be opened, I should be rash enough to continue my voyage and lead them into fresh dangers, after they might happily have surmounted this. They insisted, therefore, that I should engage with a solemn promise that if the vessel should be freed I would instantly direct my course southwards.