580 WEBER ON A TRANSPORTABLE MAGNETOMETER. 



fig. 7j required for the deflection experiments, consists of a 

 copper-plate fitting on the tripod, and carrying the supporters 

 of the deflecting-bar ; each of these is formed of two conver- 

 ging tubes connected at their extremities, from whence proceeds 

 a third tube provided with a graduation, and on this the deflect- 

 ing-bar is to be placed : this tube forms also the reading tele- 

 scope, and has the reading scale attached to it. 



Fig. 8. represents the magnetometer placed on the measuring 

 apparatus, which rests itself upon the tripod : the needle is 

 suspended in a copper case, which acts as a damper in checking 

 the vibrations. The mirror close below the needle is directed to 

 the east. The whole of the eastern side of the copper case can 

 be removed, to give access to the screw to which the suspension 

 is fastened, and by which the inclination of the mirror may be 

 corrected. In the middle of this side is an opening closed by a 

 piece of plane glass, making a small angle with the vertical, 

 in order that the I'eading telescope, which is directed to the 

 mirror behind the glass, may not see a double image of the 

 scale. 



For the measurement of the absolute intensity the deflection 

 experiments alone require to be made in the open air ; the re- 

 mainder may be made in a room if more convenient ; for if the 

 magnetism of the needle, which can be ascertained in a room, 

 be known, the intensity of the earth's magnetism may be calcu- 

 lated from that of the needle, and from the experiments of de- 

 flection made in the open air*. It should be noticed, how- 

 ever, that the determination of the magnetism of the needle in 

 such cases requires a complete measurement of the intensity to 

 be gone through, including both the experiments of deflection 

 and those of vibration, with and without the weights. The 

 magnetism of the needle should also be determined either shortly 

 before, or shortly after, the deflection experiments in the open 

 air, because it is liable to alteration : and the temperature in 

 the room and in the air should be as nearly the same as 

 possible. 



The experiments of deflection in the open air require only a 



• The experiments of vibration might be made in the open air instead of 

 those of deiioction ; but in such case the instrument would afford less cer- 

 tainty and less convenience. 



