Horizontal Intensity of the Earth's Magnetism. 



117 



fastened by a quadrangle, the lowest one is held by a nut (not 

 shown in the drawing). The object of the lower rotation is to 



Fig. 1. 



test with accuracy whether the plane of the coils coincides when 

 at rest with the magnetic meridian ; the object of the upper one 

 is the production of this correct azimuth. 



The rectangular hooks already mentioned are connected with the 

 lower disk by the supports b. To the middle disk, on the contrary, 

 the two supports c are fastened, which are seen in the figure as 

 the continuations of b. In the arrangement represented, the 

 middle and lower disks are firmly joined together, inasmuch as 

 over two pairs of horizontal arms, the lower of which are screwed 

 and soldered to b and the upper ones to c, tightly fitting hollow 

 cubes, w, are passed. When the cubes are removed, by turning the 

 lower disk through 90° the lower arms may be brought into the 



