In a few seconds the air was filled with a steady and continuous rumbling sound, like the noise of a distant cataract.

Pursuers and fugitives drew rein instinctively, and came to a dead stand; while the rumbling increased to a roar, and evidently approached them rapidly, though as yet nothing to cause it could be seen, except that there was a dense, dark cloud overspreading the sky to the southward.

The air was oppressively still and hot.
"What can it be ?" inquired Dick, looking at Joe, who was gazing with an expression of wonder, not unmixed with concern, at the southern sky.
"Dun'no', boy.

I've bin more in the woods than in the clearin' in my day, but I niver heerd the likes o' that." "It am like t'ondre," said Henri; "mais it nevair do stop." This was true.

The sound was similar to continuous, uninterrupted thunder.