MAGNETIC OBSERVATIONS. 37 



SECTION IV. 



OBSERVATIONS ON TERRESTRIAL MAGNETISM. 



THE observations of magnetic declination and force were made by means of the 

 same instruments a portable declinometer, and a transit theodolite. 



The Declinometer, kindly lent by the U. S. Coast Survey, and marked D. 22, 

 was originally constructed by Jones, of London, but had been altered in many 

 particulars so as to make it more convenient for field use. It was provided with 

 two collir.iator magnets which were hollow cylinders of steel, each 0.70 of an inch 

 in external diameter, and 0.58 of an inch in internal diameter. One of them, 

 marked C. 32, was 3.92 inches long; while the other, marked S. 8, was 3.25 

 inches long. Each of these magnets carried in its south end a lens; and in its 

 north end, at the solar focus of the lens just mentioned, a piece of plane glass on 

 which was cut a scale of equal parts containing one hundred and seventy divisions, 

 each division being equal to 0.00255 of an inch. Both magnets were provided 

 with light sliding brass rings which were intended to be used for keeping them 

 horizontal under great changes of magnetic declination, but the slight play which 

 the magnets had in the stirrup was found quite sufficient for that purpose, and the 

 rings were never employed. The same suspension was used during the whole of the 

 observations. It consisted originally of six parallel fibres of unspun silk, each 

 about nine inches long ; but at Callao one of the fibres was accidentally broken, 

 and after that the remaining five were used. The torsion circle, which formed 

 part of the suspension apparatus, was 0.88 of an inch in diameter, divided to every 

 three degrees, and read by means of a vernier to single degrees. 



The Transit Theodolite, which perhaps might be more correctly called an altitude 

 and azimuth instrument, was provided with a horizontal and a vertical circle, each 

 five inches in diameter, and each reading by means of two opposite verniers to 

 thirty seconds. The telescope had an object-glass with a clear aperture of one 

 inch, and a focal length of about nine inches. It was provided with two eye-pieces ; 

 a direct one magnifying about twenty times, which was employed in almost all the 

 observations ; and a diagonal one of lower power, which was sometimes used for 

 objects near the zenith. Both these eye-pieces had colored glasses for observing 

 the sun. The system of wires in the focus of the object-glass was a simple 

 rectangular cross, one wire being vertical, the other horizontal. 



For the sake of convenience in setting up the instruments, and also for the per- 

 fect security which it affords against changes in the angular value of the divisions 

 of the magnet scales depending upon changes in the distance between them and 



