Very gradually the brilliancy of the flashes, as well as their frequency and duration, increased, until, by two bells, the glimmer of some of them endured for perhaps as long as three seconds, during which the entire sky, with its enormous, fantastic cloud-shapes, from horizon to zenith, was lit up with a faint sulphurous blue glare, strongly suggestive of the idea that we were afloat in the heart of an enormous cavern, momentarily illuminated by the burning of a port-fire.
