Twelve years have elapsed since I left Europe to explore the interior of the New Continent. From my earliest days I was excited by studying nature, and was sensitive to the wild beauty of a landscape bristling with mountains and covered in forests. I found that travelling out there compensated for a hard and often agitated life. But pleasure was not the only fruit of my decision to contribute to the progress of the physical sciences. For a long time I had prepared myself for the observations that were the main object of my journey to the torrid zone. I was equipped with instruments that were easy and convenient to use, made by the ablest artists, and I enjoyed the protection of a government that, far from blocking my way, constantly honored me with its confidence. I was supported by a brave and learned friend whose keenness and equanimity never let me down, despite the exhaustion and dangers we faced.