INTRODUCTION. XV 



The circle reading for the true meridian being the same in both positions of the telescope, in 

 u proof that the line of collimation of the latter had been properly adjusted. Had the zero di- 

 vision of the collimator magnet been observed, the difference between this true meridian and the 

 circle reading, when the telescope was directed to that zero, ( 180,) is the angular value of 

 the absolute declination at that time. But as division 50.0 was bisected by the vertical tele- 

 scope wire instead of 40.7, which was found to be the zero, a correction for 9.3 divisions, or 

 20' 13". 6, becomes necessary. The alt-azimuth instrument being to the north of the magnet, 

 and the numbers on the scale of the latter erect as in the present instance increasing num- 

 bers denote increasing easterly declination. The angle which was observed is, therefore, too 

 small, and the circle reading for the magnetic meridian requires to be increased. Thus for 

 10A. 35m. 545.2 sid. time, equivalent to Oh. 37w. 12.9 mean time Santiago 



O ' " 



Mean of azimuth circle verniers + 180 = 323 53 37.0 



Correct for zero -j- 20 13.6 



Magnetic meridian 324 13 50.6 



True meridian 309 14 22.0 



Absolution declination EAST 14 59 28.6 



The observations at Santiago, commencing with October, 1850, were made by Mr. Phelps 

 and all of them have been calculated by him ; the others were made by myself. 



The remarkable differences at occasional consecutive dates between calculations of the abso- 

 lute declination, made it necessary that every step of the attained results should be minutely 

 scrutinized. After the most careful examination, no good reason could be found for their re- 

 jection. The final result given is deduced from several measures which have been computed 

 independently, and are most accordant with each other ; and thus they must be considered en- 

 titled to equal confidence with those apparently less discrepant. 



The computations were made by Assistant Daniel Major, and the revisions by Lieutenant 

 Phelps and myself. 



DIURNAL VARIATION. 



After completing the experiments for absolute elements, the instruments were re-conveyed 

 to our residence, nearer Santa Lucia, and the unifilar magnetometer was mounted under the 

 corridor of an inner court, where changes of the declination were observed throughout the term- 

 day, commencing at 10 p. M., mean time Gottingen. This instrument will be found described 

 in the introduction to the observations for determining the absolute horizontal force. 



It was always set up on the same spot, and the plane of detorsion of the suspension thread 

 was made to coincide with the magnetic meridian, when the central division of the scale was 

 bisected by the wire of the reading telescope. The verniers to the azimuth circle were read 

 then, and at the close of observation, to ascertain whether the instrument had remained un- 

 changed. The observed changes from the magnetic meridian, published in the following pages, 

 are given in scale divisions whose value was found by reading the azimuth circle verniers when 

 the central division was bisected by the telescope wire, then quickly turning the brass plate 

 carrying the box with the magnet, by means of the tangent screw, until another point of the 

 scale came under the wire and reading the verniers again. As no change of declination will 

 probably have occurred in the instant required for this, the angular value of one division is 

 equal to the arc described, divided by the number of such divisions between the two points seen 



