TERRESTRIAL MAGNETISM, ETC. 299 



The paper ring is cut across in thirty-two 

 places, midway between the silk-thread spokes, to 

 prevent it from warping the aluminium rim by the 

 shrinkage it experiences when heated by the sun. 

 Compass cards of the new kind made before this 

 simple piece of engineering was applied to the 



FIG, 40. 



structure, used to be perfectly flat in cloudy 

 weather at sea, and to become warped into a 

 saddle-shape surface when the sun had shone 

 brightly on them for a few minutes. Now with 

 the radical cuts in the paper the compass may 

 be first thoroughly moistened by the steam of a 



