X INTRODUCTION. 



More than one obstacle intervened to prevent the carrying out of the plan to its full extent 

 by the Astronomical Expedition, nor is it known whether the Toronto observatory or tho boundary 

 Commission took part in the series. The observations made under direction of Professor Bache, 

 and those at Marburg, (in Hesse,) instituted by Dr. C. L. Gerling, have been very considerately 

 furnished, and form a most valuable contribution to the interest of the present volume. 



There was yet another question, for whose solution more than one eminent magnetician had 

 expressed great desire, namely, the changes of intensity with distance from the centre of the 

 earth ; and, as a journey across the Andes would afford stations for the necessary experiments 

 to a height of nearly 13,000 feet above the sea, when the observations of the Astronomical Expe- 

 dition had terminated in Chile, all the requisite instruments were placed in charge of Lieutenant 

 MacKae, who was instructed to return to the United States across the continent, via Mendoza 

 and Buenos Ayres. He was required to repeat the magnetical determinations at every 3,,000 

 feet elevation, both on ascending the western and descending the eastern slope of the great 

 Andean chain, and at every 100 miles of longitude in crossing the pampas between the two 

 cities named. The results attained by this accomplished and most conscientious officer have 

 already been published, Vol. II, pages 69 82. 



Originally the meteorological journal was placed in charge of the junior assistant and Mr. 

 E. K. Smith, who were instructed to make eight records daily, at equal intervals, commencing 

 with 3 A. M. instead of the three periods proposed in the programme. This was continued 

 during nearly a year, but after the return of the former to the United States in September, 

 1850 the whole duty was devolved upon the latter, and, of necessity, the 3 A. M. observations 

 were omitted. On several of the term-days, and during a temporary indisposition of Mr. Smith, 

 we were very cheerfully assisted in both magnetical and meteorological observations by Professor 

 Ignacio Valdivia and Don Gabriel Izquierdo, two of the gentlemen named by the Chile govern- 

 ment as students of practical astronomy. Occasional notes respecting earthquakes or remarkable 

 phenomena, which had escaped the notice of the regular observer, have been inserted by other 

 members of the expedition, and very generally the initials of the name are appended. 



With this brief exposition, I proceed to the account of the several instruments used, and the 

 observations made in Chile. The reader will find other meteorological data in Vol. I, Ap- 

 pendices B and C ; and Vol. II, pp. 76 82. 



ABSOLUTE DECLINATION. 



DESCRIPTION OF THE INSTRUMENTS. 



The absolute declination was ascertained on the term-day of each month by means of a porta- 

 ble declinometer and an altitude-and-azimuth instrument. The latter has circles of five inches 

 diameter, each reading, by two opposite verniers, to 30". The object-glass of its telescope is 

 one inch in diameter, and, for distinct vision of near objects, is fitted in a short cell, movable 

 within the tube by rack and pinion. A small magnet, enclosed in a brass box with a glass top, 

 is fitted on the telescope tube for the purpose of placing it in the magnetic meridian. The in- 

 strument has two eye-pieces one direct, the other diagonal, for objects near the zenith and a 

 rectangular cross of wires at the focus of the object and eye glasses ; a tripod stand ; a lamp for 

 illumination of the wires at night ; and a plummet and adjusting tools, accompany it. 



The declinometer consists of an oaken box six inches long, three and seven-tenths inches 

 broad, and four and three-tenths inches deep, with a heavy base resting on three levelling 

 screws. Its top is of thick glass, and the ends have apertures seven-tenths of an inch wide, 

 fitted with parallel glass. That of the front aperture is inserted in a slide capable of inversal. 



