Nipher — Variations in the Earth's Magnetic Field. Ill 



greatly reduced by wrapping the magnets in heavy pad- 

 ded blankets. It was, however, soon eliminated by 

 maintaining all of the control magnets at a temperature 

 of 32° F. This was done by placing each magnet within 

 a piece of heavy rubber tubing. This tubing was of 

 strong fiber, coated within and without with rubber. The 

 ends were plugged and sealed with wax. The tubes with 

 the enclosed magnets were mounted in V-shaped sup- 

 ports within boxes put together by means of copper 

 nails, and calked. The boxes were then filled with frag- 

 ments of ice, packed closely around the rubber tubes. The 

 ends of the boxes rested on the side timbers of the frame 

 so that the blankets could be wrapped around the boxes. 

 The boxes were provided with outflow tubes of brass. 



The maintaining of the control magnets at a fixed tem- 

 perature, diminished the daily swing of the magnetic 

 needle in a very appreciable degree. It did not appar- 

 ently affect the character of the changes due to wind- 

 gusts and cloud shadows. It did, however, serve to re- 

 move all possible doubt from the conclusions. In this 

 series of observations, it was not the object to make 

 precise measurements of the quantities involved. It 

 was a search for fundamental phenomena. For precise 

 measurements the two sets of magnets might each be 

 replaced by two coils as in the Helmholtz-Gaugain gal- 

 vanometer. These might be mounted on a table capable 

 of rotating around a vertical axis coincident with the 

 suspension fiber. Two telescopes with scale mounted 

 90° apart upon the table would serve to properly adjust 

 the table when both circuits were open, and to deflect 

 the needle 90°. The current in the deflecting coils would 

 be increased until the telescope with axis east and west 

 is directed upon the reading corresponding to the mag- 

 netic axis of the needle, as determined by the other tele- 

 scope, the torsion head being turned 90°. 



The current and the constant for the coils being known, 

 the value of H is determined at that instant. The re- 

 sultant field could then be decreased to any desired 



