When darkness fell, they were still flying high, though at reduced speed, as John was afraid that a rate too much over schedule might cause them to overrun their destination before daylight could disclose its outlines to them.  Every half-hour the pilot's helper checked up their position on the chart.  Had this not been done from the very start of the trip, they never could have struck their ports with the accuracy they did, and disaster would have been the result, if not death to the crew.