INTRODUCTION. 



The observations (printed on pages 127, 129) require no explanation. They were made a few 

 yards south of the unifilar observations, and on the same spot as those for absolute declination. 

 Prior to October, 1850, and at the six stations specified in the separate table of page 129, they 

 were made by myself; the others by Mr. Phelps. No correction has been applied for defective 

 axle to magnet A 1. 



It was supposed that this structure of the instrument might permit small changes of the in- 

 clination to be observed with considerable accuracy, and hourly or semi-hourly readings were 

 made on certain of the term-days for the purpose of testing the question. The records then 

 made and given on page 130 indicate that our inclinometer was not a reliable instrument for 

 such a purpose. It is proper to mention, however, that the bar-magnets were so weak, the 

 needles could not be charged satisfactorily; and as the latter were lifted just prior to each 

 observation, it is quite possible that their directive force was not wholly sufficient to overcome 

 even the slight friction of the axles on the agate planes. Saturated needles might have given 

 much more accordant results. 



METEOROLOGICAL OBSERVATIONS. 



TOPOGRAPHY OF THE SANTIAGO BASIN. 



The city of Santiago, in latitude 33 26' 26" south, west longitude 70 38' 27", is near the 

 northern focus of an elliptic plain or basin, whose longer axis is in a N.N.W. and S.S.E. direc- 

 tion, and at an elevation of 1,790 feet above the Pacific ocean. The general superficial stratum 

 of the basin consists of sand mixed with rolled pebbles, and fragments of stone disposed in 

 longitudinal layers, through which many hills, principally of porphyry, rise to heights varying 

 from 200 to 1,300 feet. It has inclination in two directions : one from north to south ; the 

 other from east to west ; but, as the latter is the more precipitous, their resultant is from E.N.E. 

 to W.S.W., or perpendicular to the longer axis. One of the eminences that attains a height of 

 200 feet, and is about 450 yards long, (Santa Lucia,) is within the city proper ; a second, of 

 double these dimensions, is just inside the northern boundary ; and a third, distant five or six 

 miles in a northwest direction, has a height of 1,100 feet. 



The base of the nearest Andean range lies nine miles to the east. This, reaching a mean 

 elevation of 10,500 feet, with knolls that run up to 13,000, sweeps off in a general line N.E., 

 leaving a narrow and abruptly inclined valley between it and a lower spur whose origin is a 

 little further to the northward. Eastward of the near range there are three or four others, 

 with intervening valleys and basins, that lie nearly parallel with a meridian of longitude, and 

 whose bases are spread over about 120 miles. The southwestern point of the spur named is 

 more than 700 feet high, and is separated from the city by the Mapocho, an impetuous rivulet, 

 whose snow-waters descend through the valley just mentioned. In its direction the view is cut 

 off by gigantic Cordilleras, perhaps scarcely forty miles away, and not less than 8,000 feet of 

 which, from the summit downwards, is forever buried beneath snow. Though hidden by the 

 nearer chain, Tupungato and Aconcagua, each exceeding 22,000 feet in height, both belong to 

 that group. On the more elevated and shaded portions of the near range, also, snow is visible 

 in patches through the heats of summer. 



