WILLIAM tllLHERT. 



57 



certain orbit of magnetic virtue, as by a surrounding atmosphere. In 

 further experiments loadstones were cut into two parts, the parts being 

 floated on water in Httle vessels to observe their mutual attractions 

 and repulsions. All experiments which Gilbert considered as being 

 original he claimed as his own by affixing an asterisk, large or small 

 according to the importance of the matter, in the margin of the text. 

 He suggest mapping out the lines of magnetic virtue upon the surface 

 of his terrella as the parallels of latitude. The tropics, the arctic circles 



Fig. 2. 



Fig. I. 



|>'!^ 



J>-»=> 



GiLDEKt's Sl'HERICAL LOADSTONE, 



OR Terrella. 



The Terrella and its surrounding field 

 of action. 



and the meridians are marked out upon a geographical chart o^" 

 terrestrial globe. The fact that a magnet of elongated shape — a 

 magnetic rod — is more powerful than one of spherical or cubical or 

 any other shape of equal weight (the horseshoe shape not being dis- 

 covered until many years later) is announced. The screening effect 

 of a sheet of thin iron, and the failure of other metals to screen off 

 magnetic action, are noted. Then comes a series of studies on the 

 effect of capping loadstones with armatures of iron, and on the 

 strengthening of the power of loadstones. Chapter xxxii. of Book II. 

 is a notable one, containing a number of magnetic aphorisms, each 

 tersely summing up some result of experiment or observation. In it the 

 principleofequalityofactionandreactionisillustrated by the experiment 

 of floating a magnet in a little skiff and showing that it attracts itself to 

 a piece of iron, just as the iron, if placed in the skiff, will be attracted 

 to the magnet, thus furnishing an illustration of the principle of action 

 and reaction. Several experiments are also described illustrative of 

 the mutual repulsions of similar poles, north repelling north and south 



