While Mr. Clarence L. Hay and I were on the slopes of Choqquequirau the clouds would occasionally break away and give us tantalizing glimpses of snow-covered mountains. There seemed to be an unknown region, "behind the Ranges," which might contain great possibilities. Our guides could tell us nothing about it. Little was to be found in books. Perhaps Manco's capital was hidden there. For months afterwards the fascination of the unknown drew my thoughts to Choqquequirau and beyond. In the words of Kipling's "Explorer": "... a voice, as bad as Conscience, rang interminable changes On one everlasting Whisper day and night repeated--so: 'Something hidden. Go and find it. Go and look behind the Ranges-- Something lost behind the Ranges. Lost and waiting for you. Go!' " To add to my unrest, during the following summer I read Bandelier's "Titicaca and Koati," which had just appeared. In one of the interesting footnotes was this startling remark: "It is much to be desired that the elevation of the most prominent peaks of the western or coast range of Peru be accurately determined. It is likely ... that Coropuna, in the Peruvian coast range of the Department Arequipa, is the culminating point of the continent. It exceeds 23,000 feet in height, whereas Aconcagua [conceded to be the highest peak in the Western Hemisphere] is but 22,763 feet (6940 meters) above sea level." His estimate was based on a survey made by the civil engineers of the Southern Railways of Peru, using a section of the railroad as a base. My sensations when I read this are difficult to describe. Although I had been studying South American history and geography for more than ten years, I did not remember ever to have heard of Coropuna. On most maps it did not exist. Fortunately, on one of the sheets of Raimondi's large-scale map of Peru, I finally found "Coropuna--6,949 m."--9 meters higher than Aconcagua!--one hundred miles northwest of Arequipa, near the 73d meridian west of Greenwich. Looking up and down the 73d meridian as it crossed Peru from the Amazon Valley to the Pacific Ocean, I saw that it passed very near Choqquequirau, and actually traversed those very lands "behind the Ranges" which had been beckoning to me. The coincidence was intriguing. The desire to go and find that "something hidden" was now reënforced by the temptation to go and see whether Coropuna really was the highest mountain in America. There followed the organization of an expedition whose object was a geographical reconnaissance of Peru along the 73d meridian, from the head of canoe navigation on the Urubamba to tidewater on the Pacific. We achieved more than we expected.