MAGNETICAL OBSERVATIONS, 



INSTRUMENTS AND METHODS OF MAKING THE OBSERVATIONS. 



The following observations were made with a set of magnetic instruments, fur- 

 nished to Baron Von Miiller by the Smithsonian Institution. The set contained a 

 small theodolite, a declinometer, and a unifilar-magnetometer, made by Jones, and 

 a Barrow dip-circle. The last-mentioned instrument I had formerly used in Dr. 

 Kane's Arctic expedition, where it was subjected to some rough usage ; and 

 although it had been carefully adjusted before it was taken out to Mexico, the axis 

 of the needles proved not to be as perfectly cylindrical as was desirable. 



The determinations of absolute declination were made by means of the small 

 theodolite and the declinometer. The azimuth circle of the first named instrument 

 reads to thirty seconds by means of three verniers. It carries also a vertical circle, 

 reading to single minutes. On its transit from Vera-Cruz to Potrero it suffered 

 much, and the clamping-screws and the arms which carry the verniers were bent; 

 this accounts for the great differences in the readings of the single verniers after 

 the first observations. This instrument was used to determine the true [astrono- 

 mical] and the magnetical azimuth of a mark, for the purpose of obtaining the 

 magnetic declination. The first of these operations consisted in observing the mark 

 and the first and second [western and eastern] limb of the sun, and in determining 

 the error of the chronometer, by which the time of the solar observations was 

 noted. 



This error of local time was found generally by double altitudes of the sun, 

 taken either with the theodolite or by means of a pocket-sextant, reading to single 

 minutes, and a mercurial horizon. As the level attached to the vertical circle 

 of the theodolite was not sufficiently sensitive, it was generally thought best to 

 observe also with this instrument the reflected image of the sun. An equal number 

 of observations of the upper and lower limb of the sun were always taken. A 

 pocket-chronometer of Parkinson & Frodsham was used, and its rate on mean time 

 continued always very small. 



The declinometer carried a wooden box with a glass-tube of ten inches in length 



for suspending the magnet. This box was fastened to a table of the same material. 



After bringing the sides of the box nearly into the magnetic meridian and levelling 



it, the torsion was taken out of the silk thread by means of a brass collimater, 



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