5/2 WEBER ON A TRANSPORTABLE MAGNETOMETER. 



for 1837, P- 22*, the line connecting the middle of the bar with 

 the middle of the magnetometer needle may form with the mag- 

 netic meridian an angle of 35° 16'. Thus observations of the 

 variations of the declination and of the intensity may be conve- 

 niently combined in this manner by travelling observers. 



§ III. Examples of Observations and Measurements. 

 Measurement of the Absolute Declination. 

 This measurement resolves itself into three parts: 1. The 

 determination of torsion. 2. The azimuthal determination of 

 the magnetic axis. 3. The azimuthal determination of the true 

 meridian. By the azimuth of a direction is here understood 

 the angle formed by two vertical planes, one in the direction in 

 question, and the other in the direction of the optical axis of 

 the telescope of the theodolite, the ahdade being placed on the 

 zero point of the circle. 



1 . Determination of Torsion. 

 This determination consists of the measurement of the force 

 of torsion, and of the angle of torsion. 



Force of Torsion. 

 There belong to the magnetometer two needles, the magnetic 

 and the torsion needle, which may be suspended to the same 

 thread, and which ditfer in the proportion of their magnetic 

 moments (M, m). Designating by T the horizontal part of the 

 earth's magnetic force, the force of torsion is to be compared 

 with the force M T, as w'ell as with the force m T. 



Comparison with the force M T. 



In order to reduce the observations to the same time, the 

 dechnation was observed in the magnetic observatory simulta- 

 neously with both the observations. 



Hence the force of torsion is given in parts of M T 

 _ 57-295 38-36 _ J_ 

 ~ 180°^ ' 2174 ~ T78 ' 

 * Sci. Mem. Part VI. p. 270. 



