Final answer:
Percy risks air travel from Los Angeles to New York due to the importance of airplanes in revealing their hidden territory. The advancement in air travel technology, as shown by Charles Lindbergh's historic flight, also allays general skepticism about the viability of long-distance air travel.
Step-by-step explanation:
The passage suggests that Percy risks traveling by air from Los Angeles to New York despite the potential dangers for a couple of reasons. Firstly, the Wright brothers' skepticism about long-distance air travel was proven wrong when Charles Lindbergh successfully flew solo across the Atlantic. This historical background indicates a shift in attitude towards the feasibility of air travel. Secondly, Percy's discussion about aeroplanes being the only way to discover their concealed location hints at the significance and risk associated with air travel for them. Their territory has been deliberately hidden and any aircraft flying overhead poses a threat of revealing their secrets, which they protect with anti-aircraft measures. This fear of discovery might necessitate Percy to risk traveling by air to control or understand this threat closely.