WEBER ON A TRANSPORTABLE MAGNETOMETER. 581 



solid foundation, on which the tripod may be placed and le- 

 velled ; the measuring apparatus, resting on it, carries the de- 

 flecting bar, the telescope, and the scale, each in its due posi- 

 tion relatively to the others ; and the whole system can be 

 turned upon the tripod mthout their displacement. The copper 

 case of the magnetometer fits into the depression a b, fig. 8, by 

 which its position is fixed relatively to all the other parts. The 

 whole instrument is then turned on the tripod until the middle 

 of the scale is seen in the reading telescope, and it is then ready 

 for the deflection experiments. 



The vernier of the deflecting bar being placed on the zero 

 point of the graduation of the measuring apparatus, the deflec- 

 tion of the needle is observed. The deflecting bar is then re- 

 versed, and the observation repeated. The bar is then removed 

 to the end of the measuring apparatus, and the vernier set to 

 lOOO'""' of the graduated scale, when the deflected position of 

 the needle is again observed before and after the reversal of the 

 bar. Let the four obsen^ed deflections be called m, m', n, rl, — 

 the absolute intensity of the magnetism of the needle, previously 

 observed in a room, M, — and the arc- value of a division of the 

 scale, determined also in a room (the torsion being taken into 

 account), «, — then the absolute horizontal intensity of the earth's 

 magnetism %vill be 



I = ^ M 



500^ • tan v' 

 where v = \ arc-tang, i {ni — w' + w — nl) «. 



This simple formula may be employed, because the small 

 dimensions of the needle and bar, relatively to their distance 

 apart, renders the next member (having the fifth power of the 

 distance in the denominator) insensible. 



Fig. 10 represents the theodolite used in observing the de- 

 clination and its variations ; it is provided with a verification 

 telescope, having a small scale at the end: a larger scale is 

 placed above the theodoUte, perpendicular to the optical axis of 

 the principal telescope. 



The observation of the absolute declination may be divided 

 into those parts which must be made in the open air, and those 

 which may be made in a room. Fig. 11. represents in A the 



VOL. II. PART VIII. 2 Q 



