Dear Sir,—I had the pleasure to receive your letter of Aug. 3rd by the last monthly steamer from the north, and greatly lamented I could not answer it by the mail, which left Dec. 30th. Leaving the United States on the 16th of August, want of a proper amount of fire surface in the boilers of the steam-ship, caused my arrival at Chagres only on the day (27th) when the mail for Chile left. A month was thus unavoidably lost; for in anticipation of a passage through without delay, all instruments, except an aneroid barometer and thermometer, had been despatched round Cape Horn. With these such observations were made, until arrival here, as their construction permitted. From the indications of the aneroid there is a region extending from 200 miles to the S. S. W. of San Domingo to about 1° of north latitude on this side of the continent, where the pressure rarely exceeds 29·850 in., nor was the barometer but once in that whole distance as high as 29·900 in. At Panama the mean is 29·795 in. from observations at 9 a.m., 3 p.m., and 9 p.m., with a mean diurnal fall from the first to the second hours of ·08 in. The temperature for the same hours was 81°·0 with a range of 2°·9, and almost constantly saturated with moisture, though rain fell no more frequently than often occurs during the same period in the United States. As evidence of the hygrometric condition of the atmosphere, it was found impossible to dry clothing in my room after several days’ open exposure, and they were finally exposed to the direct rays of the sun. Leather moulds in forty-eight hours. The light wind experienced was almost constantly from the northward and westward during the day, and variable at night. I think Lieut.-Col. Emery made observations for declination and dip en route for California, but nothing is known to me of the results, and I must await our return to give you data on these points. Should nothing intervene to change present intentions, I contemplate making observations at each of the fifteen ports where the steamer touches between Valparaiso and Panama. Nothing of note occurred during our passage to Chile. There was time to glance at Buonaventura, Guyaquil, Payta, Huanchaco (part of Truxillo), Casma, five days at Lima, Pisco, Islay, Arica, Iquique, Coleiga, Copiapo and Coquimbo,—a multitude of little towns unimportant in themselves, and mentioned only to indicate the points where I hope to obtain observations of the magnetic elements. Reaching Santiago on the 27th of October, I was convinced in a brief time that no other part of Chile would so well answer the purposes of the expedition, and the Government here having acted promptly and with most commendable liberality on all points, there was no hesitation in selecting this city as my station. You know it is situated on a plain varying in width from twelve to forty miles, which, commencing just north of 33°, with a slight interruption in 34½°, extends to the Gulf of Onend in 41½°. The sea range of the Cordilleras, from which Santiago is distant from four to five leagues, has an elevation of 3000 to 4000 feet above the ocean, whilst the main chain to the eastward varies from 10,000 to 17,000 or 18,000 feet, and is distant about six leagues at the base. Interrupting the eastern horizon as they do, the interference with observations on the planet Venus in the morning twilight rendered so near an approach objectionable; but there was no locality in the vicinity of a proper residence free from this obstacle, and no place in the interior offering the facilities possessed by Santiago. If I mistake not, in one of my former letters I stated that the coast was impracticable, on account of very frequent fog and mist; and this was the opinion of the most observant residents here. There were two positions offered for our use by the Government,—a hill (Santa Lucia) in the eastern part of the city, with such rooms in the Castle, about halfway up, as might be needed, and the plain in the southern suburbs. The former has an elevation of some 200 feet, whilst the latter is half submerged during the rainy season, and almost inaccessible to pedestrians. Many reasons inclined me to prefer Santa Lucia, could its rocky crest be leveled, and this the Government at once undertook.