the change of density of the atmosphere in the most elevated regions; or at the very least to an altitude of 25,000 metres (82,000 ft.). Further, the one which attained this height was, at the moment of its disappearance, at a distance of 80 kilometres (49½ miles) from the observers. So remarkable a result is explained by the transparency of the atmosphere in the Arctic Regions, a transparency which, under other circumstances, permitted us to follow distinctly on the snow of a glacier, at a distance of 40 kilometres, the movements of a party of four persons whom I had sent on a mission of exploration in the interior of Spitsbergen."
This translucency of the atmosphere is a well-known character of the Polar Regions. Captain Armitage says, when the Discovery was off Cape Washington, Victoria Land, "the atmosphere was exceedingly clear, as may be imagined from the fact that we could plainly see Coulman Island and Mount Erebus at the same time, although they are 240 miles distant from one another." In Spitsbergen, at sea-level, I have seen the mountains on the south side of Bell Sound from the north end of Prince Charles Foreland quite clearly—a distance of 100 miles; and I could quote many other instances of extraordinary visibility. The only comparison in temperate climates is from mountain tops: from the summit of Ben Nevis I
