GILBERT OF COLCHESTER. 



349 



N. and S., remaining all the while at any place in the vessel of water 

 in which it may be put. His words, though quaint, are exact: "It re- 

 volves on its iron center, and is not borne towards the rim of the vessel." 

 He knew nothing of the mechanical couple in play; but he knew the 

 fact ; and with the instinct of a true philosopher, tested it in a variety of 

 ways. With a most Imninous insight into terrestrial magnetic phe- 

 nomena, he observed that near the poles a compass needle tending, as 

 it does, to dip greatly, must necessarily experience only a feeble hori- 

 zontal directive force. To this he adds that 'at the poles there is no 

 direction,' meaning thereby that a properly balanced compass needle 



Fig. 4. Gilbert's Method of showing Magnetic Dip. 



would remain indifferently in any azimuth in which it might be placed. 

 We express the same by saying that the horizontal component of the 

 earth's force vanishes at the poles. 



Gilbert dwells at length on the inductive action of the earth. We 

 have seen him hammering heated bars of iron and then allowing them 

 to cool while lying in the magnetic meridian. He notes that they be- 

 come magnetized, and does not fail to point out the polarity of each 

 end. He likewise attributes to the influence of the earth the magnetic 

 condition acquired by iron bars that have for a long time lain fixed in 

 the north and south position as bars often are fixed in buildings and in 

 windows, and he ingenuously adds : for great is the effect of long-con- 



