TERRESTRIAL MAGNETISM, ETC. 249 



two hundred years after Gilbert's time. A very 

 small magnetized needle, pivoted so as to be per- 

 fectly free to turn about its centre of gravity any- 

 where in the neighbourhood of a terella, will place 

 its length exactly in the direction of the curves of 

 the diagram through it or beside it, with its poles 

 in the positions marked by the arrow (feather for 

 true north pole, and point for true south). 



Gilbert uses the results of his observations on the 

 direction of a small needle in the neighbourhood of 

 a terella to explain both the horizontal direction 

 indicated by the mariner's compass in different 

 parts of the earth, which had been known for 

 thousands of years, and the " dip," discovered by 

 Robert Norman, sailor and nautical instrument- 

 maker, a quarter of a century before the publication 

 of Gilbert's book. Imagine the terella of the 

 diagrams to be not a terella, but the earth itself, 

 and by looking at the diagrams you will have, from 

 the one showing curved lines of force, a clear idea 

 of the general character of the directional tendency 

 exhibited by a needle anywhere at the earth's sur- 

 face, or which would be exhibited by a needle 



