Please answer the following question: Given the following context:  In selecting his crew, De Long's priority was men with Arctic experience. For his second-in-command he chose lieutenant Charles W. Chipp, who had served with him on the Little Juniata adventure. Another veteran of the Polaris rescue mission, George W. Melville, was appointed as ship's engineer. Other experienced Arctic hands were William F. C. Nindemann, a Polaris survivor, and the ice pilot William Dunbar, who had many years' experience in whalers.The appointment of the expedition's navigating officer was problematic; John W. Danenhower, a young naval officer from a well-connected Washington family, was recommended to Bennett by the former president Ulysses S. Grant. Such sponsorship won Danenhower his place, despite a history of depression that had seen him briefly incarcerated at the Government Hospital for the Insane. On Bennett's request, Danenhower accompanied De Long on the voyage from Le Havre to San Francisco, during which he confided details of his medical history. The navigator's competent performance persuaded De Long that such troubles were in the past.The ship's surgeon, James Ambler, was assigned to the expedition by the Navy's Bureau of Medicine and Surgery, only because he was next on the list of medical officers available for sea duty. Ambler deduced from Danenhower's medical records that a probable cause of the navigator's medical lapses was syphilis, but Danenhower's influential connections ensured that he kept his place on the expedition.Two others from Jeannette's voyage from Le Havre, carpenter Albert Sweetman and boatswain John Cole, were enlisted, as was the Herald's meteorologist, Jerome Collins. Dubbed "chief scientist," he was in charge of the Edison apparatus and of a rudimentary telephone system that De Long hoped to utilize. The remaining places were filled from a long list of applicants; the cook and steward were recruited by Danenhower from San Francisco's Chinatown.  answer the following question:  What is the first name of the navigator for whose medical lapses Ambler felt the probable cause was syphilis?
A:
John