French engineer (1841–1925) best known for his pioneering work in aviation. Rare ALS in French, signed “Ct Ader,” one page both sides, 4.75 x 6.5, Villa Labourdette letterhead, September 25, 1920. An important letter where he reaffirms his early claims to military aviation that were then being challenged, addressed Jacques Mortane, a writer and early champion of aviation. In fine condition. RR Auction COA.
