OF THE EXPEDITION. xli 



were almost unknown, and of themselves merited a special journey; I therefore determined to 

 place suitahle instruments in his charge, and to direct him to proceed home via the Uspallata 

 pass of the Andes, and the pampas of the Argentine republic. In addition to the magnetical 

 instruments, he was supplied with three pocket-chronometers ; a sextant and an artificial hori- 

 zon; a Bunten's and an aneroid barometer; thermometers, &c. ; and his orders were as follows: 



" It being considered of much interest to science that a series of magnetical and other obser- 

 vations should be made at various heights and across this continent, you will take charge of 

 the instruments selected for the purpose, and proceed to the United States, via the Uspallata 

 pass and Buenos Ayres. The successful prosecution of your expedition being dependent on the 

 transitable condition of the Andes, for which no definite period can be assigned, the time of 

 departure from Santiago is left to your discretion. On arriving at Buenos Ayres or Montevideo, 

 should you ascertain that the United States can be reached more exptiditiously by so doing, you 

 are authorized to proceed to England in the mail steamer, in which case you will convey the 

 instruments to Woolwich and repeat the observations there also, for the purpose of ascertaining 

 what changes, if any, have taken place in the magnets since they were first examined by Col. 

 Sabine, R. A. And as there are facilities for determining the temperature co-efficients of the 

 magnets at Woolwich which may not be so readily accessible at Washington, should you visit 

 England, it is desirable that these experiments should not be omitted. 



" In the journey across the Andes and pampas of Buenos Ayres, the objects to which your 

 attention is especially directed are : 1st. The law of the decrease of magnetic intensity with 

 height. 2d. The measurement of zenith distances of celestial objects exceeding 90, with a 

 view to improving tables of refraction. 3d. Geographical information. 4th. Meteorology. 



" The height of the Uspallata pass being about 14,000 feet above the level of the sea, it is 

 desirable that all the magnetical elements be determined at five nearly equi-distant elevations, 

 both ascending to the Cumbre and descending towards Mendoza. At the summit, the observa- 

 tions for total intensity should be repeated, and at not less than two intervals of the deflecting 

 magnet. Here, also, you may have the best opportunity for observing zenith distances. 

 Crossing the pampas, the magnetical observations should be made, as nearly as practicable, at 

 every hundred miles of longitude, or, assuming Mendoza to be in longitude 69 west of Green- 

 wich, and Buenos Ayres in 58^, there will be six stations between these two cities. 



' ' Though it is not to be expected that you will have time to execute an elaborate map of any 

 great breadth of country, your chronometers and astronomical instruments will enable you to 

 locate with far greater accuracy than is now known, not only the stations you may select, but 

 also the towns, rivers, and lakes near which you may pass. Whatever information you collect 

 cannot fail to be of the highest interest to geography, and your attention is particularly invited 

 to the lakes and rivers lying between Mendoza and Rio Quinto, just beyond San Luis. Have these 

 rivers and lakes outlets in the Parana, or are their waters lost in the sands of the pampas? It 

 is possible that manuscript maps will be found at some of the cities you may visit, of which it 

 may be permitted to make copies. 



"Nine A. M., and 3 p. M., being near the hours of maximum and minimum atmospheric pres- 

 sure, should be selected for meteorological observations whenever you are stationary. These, 

 better than any others, will enable us, when calculating the elevations of your stations, to deter- 

 mine the corrections applicable to observations at other epochs. The temperature of dry and 

 wet thermometers, the direction and estimated strength of winds, and character of clouds, will 

 occupy your attention also. To these, many notes may be added respecting the hours at which 

 the winds begin in the morning; the electrical state of the higher atmosphere; whether there 

 are strata of clouds moving in different directions above the Andes facts that will throw light 

 on the meteorology of a region almost if not quite as little known to us as is its magnetical 

 condition. 



"Baron Humboldt has desired to be informed whether the snow on the mountains is of a 

 'glowing red' color about sunset and sunrise, as he had often observed it on the Alps and the 

 G 



