MAGNETIC A L OBSERVATIONS. 3 



The iinifilar-raagnetometer was only used for experiments of deflection. The 

 horizontal circle of this instrument read to 20 seconds. It supported a heavy 

 copper box, which quieted the magnet, suspended in it, to a considerable extent. 

 To this box were fastened two arms, of about 1.5 feet in length, with grooves, in 

 which the deflecting magnet rested. One of these arms carries also the telescope 

 and the scale, which was reflected from a small mirror, attached to the magnet in 

 such a manner that its axis was parallel to the plane of the mirror. Tlie deflecting 

 magnet was the same used in the vibration experiments. After the instrument was 

 levelled, and the torsion taken out of the silk thread, a magnet of somewhat smaller 

 dimensions than the deflecting magnet was suspended. This last one was placed in 

 the groove on the eastern arm at the distance of one foot, and the instrument 

 turned until the middle of the scale appeared behind the vertical wire of the tele- 

 scope. Then the circle was read and the magnet placed on the western arm at the 

 same distance and the same pole pointing in the same direction (so that the opposite 

 pole was now turned towards the deflected magnet), when the circle was read ofl' again, 

 and the south pole of the deflecting magnet placed Avhere its north pole was before, 

 without altering the distance. The instrument Avas now turned until the middle of 

 the scale was again behind the vertical wire and the circle-reading noted. Then the 

 magnet was placed on the eastern arm, at the same distance, and without changing 

 the poles, etc. Afterwards the distance was increased to 1.3 feet, and the operation 

 repeated. At the end of each set the temperature of the deflecting magnet was 

 noted. In the observations with the dip-circle the instrument was brought into 

 the meridian by turning the horizontal circle in azimuth until the dipping-needle 

 indicated 90°. Then the circle was read and turned about 180°, when the needle was 

 brought again into a vertical position and the horizontal circle was read once more. 

 By means of these readings the needle was put in the magnetic meridian. The 

 poles of the needles were reversed at each observation. 



OBSERVATIONS AT VERA-CRUZ. 



Description of Station Xo. I.— The instruments were mounted in the centre of a 

 room, situated on the first floor, and in the northern corner of a villa or cottage, 

 called "la Guacca," two hundred yards outside of the walls of Vera-Cruz, and on 

 the south side of this city. The floor was of marble, and no basement below it. 

 There was no iron in the neighborhood, the locks and hinges of the doors, etc., being 

 of brass. On the walls of the city, two hundred yards to the north, were several 

 heavy iron guns. The grounds near the villa consisted of sand and clay. The 

 instruments were elevated about fourteen feet above the level of the sea, which is 

 only five hundred yards distant. 



