In the below passage sourced from Joseph Conrad's Youth: A Narrative, Conrad describes a "conical flame with a twisted top". What exactly is it?
Of course they very soon found out in the steamer that the rope was gone. She gave a loud blast of her whistle, her lights were seen sweeping in a wide circle, she came up ranging close alongside, and stopped. We were all in a tight group on the poop looking at her. Every man had saved a little bundle or a bag. Suddenly a conical flame with a twisted top shot up forward and threw upon the black sea a circle of light, with the two vessels side by side and heaving gently in its center.
An ealier passage provides context to the situation:
The speed of the towing had fanned the smoldering destruction. A blue gleam appeared forward, shining below the wreck of the deck. It wavered in patches, it seemed to stir and creep like the light of a glowworm. I saw it first, and told Mahon. ‘Then the game’s up,’ he said. ‘We had better stop this towing, or she will burst out suddenly fore and aft before we can clear out.’
So I suspect that Conrad is describing one of the mast's being consumed with flames, and thus after fire has consumed the totality of the mast, the light produced by it creates a circle occuped by the both the steamer and the Judea(name of Marlow's ship).