

INTRODUCTORY. 



The magnetical observations proposed in the programme for the Expedition siibmitted to the 

 American Philosophical Society and Academy of Arts and Sciences, and which was adopted by 

 the honorable Secretary of the Navy, contemplated only a determination of the three elements 

 on the term-day of each month, with occasional observations on the same days for horary 

 changes of the declination. After the instruments arrived, it was found that we might multi- 

 ply the number of absolute determinations without adding excessively to our labors ; and, con- 

 sequently, the experiments were made on the 1st and 11th days of each month also. The three 

 years' results carefully compared would afford interesting data for secular changes ; but there 

 was yet another question which the Andes might enable us to elucidate satisfactorily — viz : the 

 variation of the elements, and more particularly of the total force with distance from the centre 

 of the earth. This determined me to place the instruments in charge of Lieut. MacKae, as soon 

 as our use of them in Chile terminated, and instruct him to return home via the Cumbre and 

 Uspallata passes and Buenos Ayres. The observations indicated to him were for elevation, lati- 

 tude, longitude, declination, inclination, and horizontal force of the magnet and meteorological 

 data, for each three thousand feet elevation ascending the western and descending the eastern 

 slopes of the Andes, and for each hundred miles of longitude between the cities of Mendoza and 

 Buenos Ayres. Other information of a geographical and statistical character was specified as 

 greatly interesting to numerous classes of our countrymen. 



We left Chile on the 1st of October, but the snow had not sufficiently melted on the pass to 

 warrant the departure of Lieut. MacRae from Santiago before the 8th of November. The entire 

 journey to Buenos Ayres occupied him about sixty days, twelve of which were employed in ob- 

 servations within the Andes. Accidents on two occasions having caused the breakage of his 

 mountain barometer, and such injuries to his chronometers as might place the longitudes of his 

 stations in doubt, on arriving in the United States, he, with the most laudable zeal, volun- 

 teered to retrace the route at his own cost if a new set of instruments could be supplied. 

 This, as well as the charges for their transportation, was promptly authorized by the honorable 

 Secretary of the Navy ; and Lieut. MacEae re-embarked for South America in August, 1853. 

 He finally returned in the following March, and shortly afterwards submitted the following 

 report of his two expeditions. 



After enumerating the various observations legitimately comprised within the purposes for 

 which the Expedition was solicited of Congress, the programme above referred to goes pn to say: 

 "These nine classes or series of observations embrace as great an amount of labor as it will be 

 prudent for two observers to undertake, and even its accomplishment must, of necessity, leave 

 all reductions until after the return of the Expedition to the United States ; but, (whilst I dis- 

 claim knowledge of almost every branch of natural history,) as so little has been learned of the 

 immediate country we shall probably select, if the collection of specimens at leisure hours, 

 remarks concerning the flowering of plants, the migrations of birds, or other designated phe- 

 nomena, would be of interest from one so iinskilled, the enterprise is embarked upon with full 

 determination to gather every scientific fruit that may offer." 



Immediately after arrival in Chile, it was ascertained that, under the liberal patronage of its 

 government, no one field of its natural history had been uncultivated, and those who would 



