"I was surprised to find the temperature of boiling water at Egas to be but 208 deg. 2', the same within 2' of a degree that it was at a point one day's journey below Tingo Maria, which village is several hundred miles above the last rapids of the Huallaga river; at Santa Cruz, two days above the mouth of the Huallaga, it was 211 deg. 2'; at Nauta, three hundred and five miles below this, it was 211 deg. 3'; at Pebas, one hundred and seventy miles below Nauta, 211 deg. 1'. I was so much surprised at these results that I had put the apparatus away, thinking that its indications were valueless; but I was still more surprised, upon making the experiment at Egas, to find that the temperature of the boiling water had fallen 3 deg. below what it was at Santa Cruz, thus giving to Egas an altitude of fifteen hundred feet above that village, which is situated more than a thousand miles up stream of it. I continued my observations from Egas downward, and found a regular increase in the temperature of the boiling water until our arrival at Para, where it was 211 deg. 5'. "From an after-investigation, I am led to believe that the cause of this phenomenon arises from the fact that the trade-winds are dammed up by the Andes, and that the atmosphere in those parts is, from this cause, compressed, and, consequently, heavier than it is further from the mountains, though over a less elevated portion of the earth. The discovery of this fact has led me to place little reliance in the indications of the barometer for elevation, at the eastern foot of the Andes. It is reasonable, however, to suppose that this cause would no longer operate at Egas, nearly one thousand miles below the mouth of the Huallaga." The report of Lieutenant Gibbon, is also exceedingly instructive. Separating from Lieutenant Herndon at Tarma, upon the Andes, he pursued a southern course, along the eastern slopes of the chain from 11 deg. 30' south, almost to 18 deg. south, at Ohuro, making a journey of about 7 deg. 30' of latitude.