about eleven o'clock in the morning
The wind being easterly the rate was not interfered with at all , and as the thermometer only falls a degree centigrade for every seventy meters of elevation the temperature was not insupportable . And so , in chatting and thinking and waiting for the engineer , Uncle Prudent and Phil Evans walked about beneath the forest of screws , whose gyratory movement gave their arms the appearance of semi-diaphanous disks . The State of Illinois was left by its northern frontier in less than two hours and a half ; and they crossed the Father of Waters , the Mississippi , whose double-decked steam-boats seemed no bigger than canoes . Then the `` Albatross '' flew over Iowa after having sighted Iowa City about eleven o'clock in the morning . A few chains of hills , `` bluffs '' as they are called , curved across the face of the country trending from the south to the northwest , whose moderate height necessitated no rise in the course of the aeronef . Soon the bluffs gave place to the large plains of western Iowa and Nebraska -- immense prairies extending all the way to the foot of the Rocky Mountains . Here and there were many rios , affluents or minor affluents of the Missouri .