336 HISTORY OF SCIENCE. 



die upper plate of which is pierced with two openings : from the 

 central one rises a glass tube, c D, which is firmly fixed. At the top 

 of this is an arrangement to which the head of the torsion wire is fixed, 

 and by which it may be turned round, while the number of degrees 

 are indicated on a graduated scale. The wires used by Coulomb were 

 sometimes extremely fine. Thus in some of his experiments a degree 

 of torsion corresponded to a force much less than the millionth part 

 of a grain. 



The investigations of Gilbert on the natural magnet, which have 

 been mentioned in a former chapter (page 94), furnished almost all 

 the information on the subject that existed during the seventeenth 



FIG. 163. THE DIPPING-NEEDLE. 



century. Something was, however, added to the knowledge of the^ 

 compass during that period. The declination of the magnetic needle, 

 that is, the angle between its direction and that of the geographical 

 meridian, had, during the sixteenth and seventeenth centuries, been 

 observed and recorded for different places \ and it was known that 

 the declination was liable to a slow change, and that its amount at any 

 given time was different at different places. The discovery of the dip 

 or inclination of the needle was made in 1576 by Robert Norman. 

 He was in the habit of balancing needles on their points before touch- 

 ing them with the magnet, and he found that when they were touched 

 the north-pointing end of the needle always dipped below the horizon. 



