246 POPULAR LECTURES AND ADDRESSES. 



in the form of a monument, ornamented with com- 

 memorative illustrations of Gilbert's theory and 

 experiments, and a fantastic indication of the 

 earliest European mariner's compass, a floated 

 lode-stone, but floating in a bowl on the sea and 

 left behind by the ship sailing away from it. 



In the upper left-hand corner is to be seen 

 Gilbert's terella and orbis virtutis. The terella is a 

 little globe of lode-stone, which he made to illustrate 

 his idea that the earth is a great globular magnet. 

 Terellas have been made for the illustration of mag- 

 netic principles by the philosophical instrument- 

 makers ever since Gilbert's time, and specimens arc 

 to be found probably in every old collection oi 

 physical lecture apparatus. The orbis virtutis is 

 simply Gilbert's expression for what Faraday called 

 the field of force, that is to say, the space round a 

 magnet, in which magnetic force is sensibly exerted 

 on another magnet, as, for instance, a small needle, 

 properly placed for the test. Gilbert's word virtue 

 expresses even more clearly than Faraday's word 

 force the idea urged so finely by Faraday, and 

 proved so validly by his magneto-optic experiment, 



