130 THE INTELLECTUAL RISE IN ELECTRICITY. 



formation afforded by the early writers, whose works ap- 

 peared shortly after Neckatn's treatises. 



The needle was not in constant use, as it is now, on a 

 ship at sea, nor was it even employed to indicate any par- 

 ticular course. The roughness of the construction of the 

 first compasses made them wholly unsuitable for such pur- 

 pose. When the Pole-star could be seen at night, the pilot 

 steered by it, as usual, and by day he kept along shore. 

 If, however, the sky at night became cloudy, so that the 

 stars were obscured, the needle was brought out and rubbed 

 with the lodestone. This rubbing was repeated every time 

 the needle was used, and is what is meant by (acum super 

 magnetem ponunt) the placing of the needle over the mag- 

 net. This operation was called " inunction." The needle 

 was thrust through a reed or short piece of wood (jaculo 

 suppositam), which supported it floating in a vessel of 

 water. If the needle was left in this receptacle, naturally 

 it would move against the side, and thus be held, by con- 

 tact, in a position not at all coinciding necessarily with 

 the earth's lines of force. Consequently its magnetic 

 quality would become more or less impaired, and that was 

 apparently one reason for the remagnetization prior to 

 every observation. Another reason probably was the re- 

 moval, by the rubbing process, of the rust which would 

 accumulate on the iron needle, the effect of the oxidation 

 being to enlarge the needle and to roughen it, and so to 

 impede its free movement on the water. 



After the needle had been magnetized and carefully 

 floated it was given an oscillating and circular movement, 

 in order to carry it clear of the sides of the vessel and to 

 overcome its own inertia, and also the normal resistance of 

 the water to its motion. This was done by moving the 

 magnet in the vicinity of the needle in a circular direction, 

 the magnet attracting the needle and causing it to follow. 

 After this motion was established the lodestone was with- 

 drawn and the needle allowed to come to rest, and the 

 point on the horizon noted which the north end desig- 

 nated. 



