National Geographic : 1924 Jul
Photograph from Post Office Department TRANSFERRING AIR MAIL AT RENO, NEVADA Relays are used to speed up the air mail service, just as frequent changes of engines are made on the fast mail trains. The doors ofthis hangar open in 30 seconds. The air mail fields are used by army and civilian flyers, and thus they havecontributed materially tothe progress of American aviation. Ask an aviator what is the greatest factor in aviation development and he probably will reply, "More flying fields."