Cook advertised his modest house in Brooklyn as the Headquarters of the American Antarctic Expedition.   But when he was unable to secure backing for his own forward-looking plans to explore Antarctica, Cook accepted the position of surgeon on the Belgian Antarctic Expedition, organized by Adrien de Gerlache, which sailed from Antwerp in August 1897.  Cook met the expedition at Rio and sailed with it to the far southern ports of Chile and Argentina.  There he had the opportunity to compare and photograph members of three tribes of primitive Indians who inhabited Tierra del Fuego.    Off Harberton, the Belgica was caught on the rocks as the tide ebbed and was nearly lost.    But she escaped at the last moment and was able to continue her voyage to the Antarctic, where she entered Hughes Gulf and discovered and explored the islands of the Gerlache Strait, making 22 landings. On one of these, Cook participated in the first camping and sledging trip in Antarctica.  Today a plaque at Buls Bay commemorates this event.   During the stay on Brabant Island, Cook climbed to the summit of one of the peaks in the Solvay Range, the highest of which is now named Cook Summit.  During these landings, Cook took a series of excellent photographs of the unknown lands and continued his photographic record of the expedition throughout its duration.

     The Belgica intended to land three men to overwinter at Cape Adair, one of which was to be Cook, and then sail to Australia.  But instead, she became the first ship to winter in the Antarctic pack when she was caught in the ice of the Bellinghausen Sea for a year, a month, and a day. During that time, Cook rose to the medical crisis that developed among the crew, who showed signs of incipient scurvy.  Taking his cue from the antiscorbutic effects of the raw meat diet he had observed among Greenland’s Inuit, he had the crew eat lightly cooked seal and penguin steaks, from which they obtained the nutrients needed to stem the progress of the disease.
    During the polar night of 70 days, he observed other physical and mental effects, which he correctly attributed to the absence of the sun. He devised a crude form of the light therapy that is now used to treat what has come to be known as seasonal affective disorder, alleviating its debilitation by placing the afflicted crewmembers before an open fire.  During the winter Cook joined with Roald Amundsen, who was the second mate, to design equipment.  Cook’s ingenious medical treatments, inventiveness, and prior polar experience won over the reserved Norwegian’s respect, and he became Cook’s friend for life.

     With the return of the sun, Cook devised a plan to free the ship from the ice flow. At first he advocated digging a series of shallow trenches filled with ashes from the boilers to try to melt the ice, then turned to sawing an actual passage for the ship using explosives and ice-saws. After a monumental effort by all hands, the escape passage was finished, only to be closed again by ice pressure.  But it fractured again along a weak line and the ship was able to escape to a lake of open water in the pack.    Eventually, she worked her way to the open sea and returned to South America in March 1899.   There Cook made additional observations among the Onas and persuaded Thomas Bridges, a retired English missionary, to allow him to publish his dictionary of the Yahgan language as part of the expedition’s results.
      Cook returned to the United States in July and published an exclusive account of the voyage in the New York Herald, anticipating the official report by some months and causing some hard feelings in Belgium.  Nevertheless, he, like all of the other officers, was awarded the Order of Leopold I by the King of the Belgians for his services.  He then started to write a book-length account of his experiences, which was published in 1900 as Through the First Antarctic Night. The book showed Cook to have a great deal of literary talent.  It sold well and has continued to be a respected account of the Belgian Antarctic Expedition. 
      In 1901 Cook traveled to Brussels as part of the official commission meeting to arrange publication of the expedition's scientific results.  There he became interested in the problems of mountain climbing through meetings with Sir Martin Conway and Edward Whymper.  When he returned to America, he helped organize the American Alpine Club.
    Herbert Bridgman, Secretary of the Peary Arctic Club, which sponsored Peary’s attempt to reach the North Pole in 1898, asked Cook to accompany the Club’s relief expedition aboard its steamer Erik, as second in command. Upon arrival in Greenland, Cook found Peary suffering from the aftereffects of the loss of seven toes to frostbite and what Cook considered a poor dietary regime.  He advised Peary to return to America to recover his health before making any further attempts at the Pole, but Peary refused and remained an additional year in the Arctic. 
     Cook did not travel outside of the US in 1902, and at first he seemed to have given up exploring.  He married a rich widow, Marie F. Hunt, set up a well-equipped office in Brooklyn, and lectured widely on his Arctic/Antarctic experiences.  But his eye was caught by an account of the expedition of Alfred Brooks, which had reached the foot of what many believed was North America’s highest mountain, Mount McKinley in Alaska.
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