348 



POPULAR SCIENCE MONTHLY. 



they said they were, on the trackless ocean by their only trusty guide. 

 Columbus calmed their fears by saying that the needle did not turn to 

 the polar star, but to some fixed and invariable point near it. This 

 explanation, born of inspiration, quieted the sailors, who marveled 

 much at the Admiral's great astronomical knowledge. Gilbert rightly 

 states that the declination changes with place, but he slips into error 

 when he says : "As the needle hath ever inclined towards east or towards 

 west, so even now does the arc of variation continue to be the same in 

 whatever place or region, be it sea or continent; so, too, will it be for- 

 ever unchanging." 



We know, however, that for any given place this angle is continu- 

 ously, though slowly, changing. Some of these changes require cen- 

 turies for the completion of their cycle, and are therefore called 

 'secular'; others require but a year, and are termed 'annual'; whilst 

 others run their course in the space of a day and are known as 'diurnal.' 

 Though these periodic changes in the declination have been established 

 by careful and prolonged observations, we can not say that they are yet 

 satisfactorily accounted for. 



The dip of the needle was also familiar to Gilbert, having been first 

 observed in 1576 by Robert Korman. Our philosopher illustrates this 

 phenomenon by balancing a piece of steel so that it remains exactly 



horizontal when unmagnetized, 

 and by observing that the moment 

 it is stroked by a magnet its north- 

 seeking end dips 'as low as the 

 fulcrum on which it is supported 

 permits.' Gilbert moves a needle 

 over his terella, and finds (1) that 

 the dip is 0° on the equator, (2) 

 that it gradually increases with the 

 latitude being 90° at either pole. 



He extends this experiment 

 from the terrella to the earth itself, 

 and even devises an instrument for 

 determining the latitude of any 

 place on land or on sea in the 

 thickest weather and in the darkest night, 'without the help of sunne, 

 moone, or starre.' In this, however, he was wTong. For he assumed 

 the isoclinic lines to be circles running parallel to the magnetic equator, 

 which he erroneously supposed to coincide with the geogrpahical 

 equator. 



Gilbert recognized that the earth exerts on a freely movable needle 

 a force that gives it direction and not a motion of translation. He illus- 

 trates this by fioating a needle on a cork and observing that it points 



'E=0=J> 



<=G:Sj 



<J=o=4 



P=e^> 



Fig. 3. Gilbert's Terella showing the Be- 

 havior OF A Dipping-needle at its 

 Poles, Equator and other 

 Intervening Places. 



