As the Sky-Bird came leisurely over the hills surrounding this British seaport of Aden, they could see that the town nestled in the crater of an extinct volcano, as they had read.  All around the low, white buildings spread the rugged hillsides, and in declivities they passed over numbers of the great brick tanks or reservoirs which catch and store the scanty rainfall of the region and thus furnish Aden with its only water supply.