Ntphei' — Local Magnetic Storms. 65 



rubber, against which the wire was lightly pressed by 

 small flexible bundles of hair. The wire is then slightly 

 lifted when longitudinally displaced. 



The discharge terminals were in the form of rings 

 which encircled the wire near its ends, the ends being 

 bent downwards outside of the ring terminals. 



The air in the room must be very quiet, in order to 

 prevent its effect upon the suspending fibers of silk. 

 Such circulation as exists in a room heated by warm air 

 interferes with the result. If the wire is placed in a tube 

 as in Fig. 1 referred to above, the tube may be divided 

 in half, and a pointer consisting of a fine fiber of glass, 

 or a silk fiber, may be attached to the wire between the 

 tubes. This serves as a mark upon which the telescope 

 may be set, for observing the displacement. The sup- 

 port for the glass tubes must of course be provided with 

 leveling screws. 



EXPLANATIONS OP THE PLATES. \ 



Plate XIX represents the amplitude and extreme scale readings of 

 a magnetic needle during a wind storm, the effect of the earth's field 

 being partly compensated by bar magnets. The resultant field was 

 about 1/5 of H. The general direction of the wind was at right angles 

 to that of the needle. 



Plate XX, conditions as in Plate XIX, excepting that the wind direc- 

 tion was parallel to that of the needle. The effect on the needle is 

 ascribed to wind-gusts which locally varied the permeability of the 

 air, at fairly regular intervals. 



Issued April 5, 1913. 



